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Transcription

ISSN 1829-5819
urnal
ournal
Jurnal Sumber Daya Geologi / Journal of Geological Resources
J
Sumber Daya Geologi
of Geological Resources
Volume 20 / Nomor 3 / Juni 2010
Jurnal
Sumber Daya Geologi
Journal
Vol. 20
No. 3
Hlm.
133 - 176
Bandung
Juni
2010
Terakreditasi sebagai Majalah Ilmiah
berdasarkan Keputusan Kepala LIPI
No.178/AU/P2MBI/08/2009
PUSAT SURVEI GEOLOGI
Diterbitkan berkala enam kali setahun oleh/Published periodically six times annually by:
Pusat Survei Geologi/Centre for Geological Survey
Gambar Sampul:
Folded metalimestone in studied area (Foto: Hamdan Z. Abidin)
CALL FOR PAPER :
Redaksi menerima makalah ilmiah yang berkaitan dengan kegeologian (Geo-Resources,
Geo-Environment, Geo-Hazards, Geo-Sciences) untuk dapat diterbitkan di Jurnal Sumber Daya
Geologi ini. Makalah yang diterbitkan akan diberikan honor sesuai ketentuan yang
berlaku.
Vol. 20, No. 3, J u n i 2010
urnal
ournal
J
ISSN 1829-5819
Sumber Daya Geologi
KATA PENGANTAR
Penasihat
Kepala Badan Geologi
Pembaca yang budiman,
Puji syukur kami panjatkan kehadirat Tuhan YME atas berkat rakhmat
dan hidayahnya, sehingga kami dapat menerbitkan Jurnal Sumber Daya Geologi
bulan Juni tahun 2010. Jurnal Sumber Daya Geologi kali ini memuat dua makalah
Geo-Resources dan dua makalah Geo-Sciences.
Makalah Geo-Resources yang pertama membahas tentang
karakteristik granit Arai dan hubungannya dengan mineralisasi cebakan
endapan bijih besi, Zn, Cu, Pb, dan emas. Batuan granit Arai, yang merupakan
granit busur vulkanik diinterpretasikan menjadi sumber panas untuk terjadinya
mineralisasi di daerah ini. Granit Arai ini menerobos batuan karbonatan yang
menghasilkan cebakan tipe skarn. Makalah Geo-Resources yang kedua
mengetengahkan tema tentang karakteristik batubara dan potensi coalbed
methane di daerah Muara Lakitan. Sejumlah analisis kimia dilakukan untuk
mengetahui kualitas dan tingkat kematangan batubara di wilayah ini.
Makalah Geo-Science pertama mengutarakan topik bahasan
mengenai delineasi cekungan busur muka Simelue yang membahas mengenai
batas-batas wilayah Cekungan Simeulue, struktur tinggian dan
kedalaman/deposentrum cekungan Simeulue. Makalah ini dilengkapi dengan
model penampang bawah permukaan dari data gaya berat yang selanjutnya
dibuat model Cekungan Simeulue. Makalah kedua membahas tentang stratigrafi
dan sedimentologi Formasi Merawu di Banjarnegara berdasarkan hasil
pengukuran stratigrafi runtunan batuan Neogen di sepanjang Kali Tulis di Desa
Sokaraja. Tulisan ini mampu memisahkan Formasi Merawu menjadi dua
anggota, yaitu anggota bawah batulumpur dan anggota atas batupasir.
Akhirul kata, kami Dewan Redaksi mengucapkan selamat membaca
dan semoga mendapatkan manfaat dari tulisan yang tersaji.
Salam,
Penanggung Jawab
Kepala Pusat Survei Geologi
Dewan Redaksi
Ketua
Prof. (Ris.) Dr. Ir. Udi Hartono
(Geologi Ekonomi-Petrologi Batuan Beku)
Anggota
Dr. Hermes Panggabean, M.Sc. (Energi Fosil Konvensional)
Ir. Asdani Soehaimi, Dipl.Seis. (Seismotektonik)
Ir. Sidarto, M.Si. (Struktur Geologi)
Ir. Subagio, M.Si. (Geofisika Terapan)
Ir. Syaiful Bachri, M.Sc. (Stratigrafi-Tektonik)
Dr. A.A. Polhoupessy (Paleontologi)
Redaksi Pelaksana
Isnu Hajar, ST, MT.
Imam Setiadi, S.Si, MT.
M.H. Hermiyanto Z, ST, MT.
Sukahar Eka A. Saputra, ST.
Penyunting Ilmiah Edisi Ini
Prof. (Ris.) Dr. Ir. Udi Hartono
(Geologi Ekonomi-Petrologi Batuan Beku)
Dr. Hermes Panggabean, M.Sc. (Energi Fosil Konvensional)
Ir. Syaiful Bachri, M.Sc. (Stratigrafi-Tektonik)
Ir. Asdani Soehaimi, Dipl.Seis. (Seismotektonik)
Ir. Sidarto, M.Si. (Struktur Geologi)
Ir. Subagio, M.Si. (Geofisika Terapan)
Dr. A.A. Polhoupessy (Paleontologi)
Dewan Redaksi
Penyunting Bahasa
Dra. Nenen Adriyani, M.A.
Dewan Penerbit
Ketua
Anggota
Alamat Redaksi
Pusat Survei Geologi,
Jl. Diponegoro 57, Bandung, 40122 Telp. (022) 7203205 Fax. (022) 7202669
E-mail : [email protected]
[email protected]
http://www.grdc.esdm.go.id
Ir. Ipranta, M.Sc.
Ir. Kusdji Darwin Kusumah
Dra. Nenen Adriyani, M.A.
Cipto Handoko
Hari Daya Satya, A.Md.
Vol. 20, No. 3, J u n i 2010
urnal
ournal
J
ISSN 1829-5819
Sumber Daya Geologi
Daftar isi / Contents
Geo-Resources
133- 146
Chracteristics of the Arai Granite Associated with the Iron Ore and Zn-Cu-Pb Deposits in Musi Rawas
Regency, South Sumatra
Hamdan Z. Abidin
147 - 157
Coalbed Methane Potential and Coal Characteristics in Muara Lakitan Area, South Sumatra
M.H. Hermianto and R. Setiawan
Geo-Sciences
159 - 167
Delineasi Cekungan Busur Muka Simeulue Berdasarkan Data Anomali Gaya Berat
L.D. Santi, I. Setiadi, H. Panggabean
169 - 176
Stratigrafi dan Sedimentologi Endapan Dataran Pasang-Surut di Kali Tulis, Banjarnegara
S. Bachri, E. Slameto, dan I. Nurdiana
Geo-Resources
CHARACTERISTICS OF THE ARAI GRANITE ASSOCIATED WITH THE IRON ORE AND
Zn-Cu-Pb DEPOSITS IN MUSI RAWAS REGENCY, SOUTH SUMATERA
Hamdan Z. Abidin
Centre for Geological Survey,
Jl. Diponegoro 57, Bandung - 40122
Abstract
The Arai Granite exposed in the Jangkat District, Musi Rawas Regency, South Sumatra. This rock which is in the form of a
stock, is assigned to be Cretaceous in age. Petrographical identification shows that this rock is dominated by quartz,
feldspar (plagioclase and orthoclase), biotite with minor hornblende, pyroxene and secondary muscovite of
holocrystalline- equigranular textures. On the basis of A/CNK ratio (<1.1), the Arai granite belongs to metaluminous type
of calc-alkali composition (K2O/Na2O = 0.9-1.06). The Plot of trace elements indicates that this rock belongs to I-type and
falls within VAG/SYNCOLG. This granite is intimately associated with subduction of Indian Ocean and Eurasian Plates. It is
characterized by strong depletion of Nb, P and Ti significantly. The Arai granite intrudes the Rawas and Peneta Formations
of older ages so that lithology of both formations experienced contact metamorphism (marble and hornfels) and
mineralization. The presence of iron ore, Zn-Cu-Pb and gold deposits is closely associated with limestone replacement
within those formations. Therefore, these deposits are classified into skarn style.
Keywords: Arai granite, pluton, Rawas Formation, Peneta Formation, mineralisation, skarn
Sari
Granit Arai tersingkap di Kecamatan Jangkat, Kabupaten Musi Rawas, Sumatra Selatan. Batuan yang berumur Kapur
ini muncul sebagai stok. Secara petrografis, batuan ini didominasi oleh kuarsa, felspar (plagioklas dan ortoklas), biotit
dan sedikit horenblenda, piroksen dan muskovit sekunder dengan tekstur holokristalin-ekuigranular. Berdasarkan
perbandingan A/CNK (<1.1), batuan granit Arai ini termasuk jenis metaluminus yang berkomposisi kalk-alkali
(K2O/Na2O = 0.9-1.06). Plot unsur jejak termasuk granit tipe-I dan berasal dari granit busur vulkanik (VAG) dan granit
tumbukan/orogenik (SYNCOLG/ORG). Granit Arai diduga berhubungan erat dengan subduksi Lempeng Samudra Hindia
terhadap Lempeng Asia. Hal ini ditandai oleh penurunan unsur Nb, P and Ti secara mencolok. Batuan granit ini
menerobos Formasi Rawas dan Peneta yang berumur lebih tua, sehingga litologi formasi ini sebagian mengalami
malihan kontak (marmer dan batutanduk) dan mineralisasi. Adanya mineralisasi berupa cebakan bijih besi, Zn-Cu-Pb
dan emas di daerah ini diduga berkaitan erat dengan “replacement” batugamping yang terdapat dalam formasi
tersebut. Dengan demikian, cebakan tersebut digolongkan ke dalam jenis “skarn”.
Kata kunci: granit arai, pluton, Formasi Rawas, Formasi Peneta, mineralisasi, skarn
Introduction
The Arai granite is one of the granite body occupying
the Jangkat District, Musi Rawas Regency, South
Sumatra (Figure 1 and 2). The body is exposed as a
window within the older sediment sequence of
Peneta and Rawas Formations. The presence of the
Arai Granite is important because it could be as a heat
source for ore deposit in the area. Several ore deposits
such as iron ore, Zn-Cu-Pb and gold are present in the
area.
Consequently, many investigators visit the area in
order to evaluate the occurrence of those deposits
Naskah diterima :
Revisi terakhir :
24 Maret 2010
25 Juni 2010
(Van Bemmelen, 1949; Hamilton, 1979; Hartono,
2002; Kusnama et al.,1994). Suwarna et al. (1993)
has carried out a geological mapping in the area.
British Geological Survey (BGS) in cooperation with
Directorate of Mineral Resources (DMR) as well as
the Geological Research and Development Centre
has collected stream and pan concentrate samples in
order to compile a geochemical map of south
Sumatra (Machali et al., 1997).
During 1980s, DMR (Indonesia) joint cooperation
with JICA (Japan) (JICA, 1987) carried out a detailed
investigation on geology, mineralization and has
established some drilling tests in order to evaluate the
resource potential in the area. A detailed and
systematic drilling to evaluate the deposit is being
JSDG Vol. 20 No. 3 Juni 2010
133
Geo-Resources
carried out by PT. Galtam, Indonesia (Prayogo,
2009). The main target of the company is to explore
base metal deposits, mainly Zinc with minor lead
(Pb), copper (Cu) and silver (Ag).
101
0
N
40
Medan
250 km
m
Su
A geology team from the Centre for Geological Survey
(CGS) (Harahap et al., 2009) undertook field work in
the area to collect samples (volcanic and intrusive
rocks, mineralized outcrops, and ore minerals) in
order to study the geochemical characters of the
rocks as well as to evaluate mineral occurrences in
the area.
an
atr
Sibolga
20
Base metal prospects
ult
Fa
S
A
Padang
M
-2
0
ren
aT
atr
This paper is to study basic petrology and
geochemical characters (major, trace elements and
REE) of the Arai granite which is intimately
associated with ore deposit, mainly iron ore and ZnCu-Pb deposits in the area.
A
m
Su
TR
m
ste
Sy
U
00
studied area
-40
ch
Bengkulu
-60
Sampling and analytical methods
Sampling and fieldwork have been carried out during
a research project under the Centre for Geological
Survey (CGS) in 2009. Several samples of granite
and other rocks have been collected in the area of
study. However, in the office, these samples are again
screened in order to delineate both altered and
weathered materials. Samples have been selected for
thin sections and geochemical analysis. All sample
treatments were conducted in the Geo-Lab of the
CGS. The chemical analyses including major and
trace elements were analyzed using X-Ray
Florescence (XRF) while ICP-MS type X-7 Thermo
was used to analyze REE. Result of analyses is
tabulated in Table 1.
Figure 1. Map showing the location of the studied area.
To Jambi
Sungai jauh
Pantai
Surolangun
Pangkalan
Teladas
IRON ORE PROSPECT
Karanganyar
Jangkat
Karangwaru
Pantai
TUBOH PROSPECT
Muara Kutu
- 2°45
Muararupit
N
Batugajah
5 km
To Lubuklinggau/
Bengkulu
102O45
102 O30
Figure 2. Access road to the Iron ore and Tuboh Prospects.
Geology
Regional Tectonics
Sumatra forms a complex tectonic setting (Katili,
1969; 1973; Hamilton,1979; Curray et al.,1979;
Daly et al., 1991; Taponier et al., 1982; Kusnama et
al., 1994; Barber et al., 2005). A subduction
process between the Indian Ocean Plate from the
west and the Sundaland basement in the east, took
place from Mesozoic to Caenozoic times.
Consequently, a mixing rock originated from oceanic
and continental areas within both volcano and
magmatic zones were emplaced (Aspden et al.,
1982).
Tectonically, the studied area falls within the West
Sumatra (Cathaysian) Complex (Figure 3)
134
(Hutchison, 1994). It is superimposed with the
magmatic arc of the Bukit Barisan Range of the
Southern Sumatra within the Sumatran Fault Zone
where Indian Oceanic Plate is currently being
obliquely subducted beneath the Sundaland
continental plate (Hamilton, 1979). This oblique
subduction has resulted in the formation of dextral
transcurrent fault zones of Sumatran Fault System
(SFS) or Semangko Zone, parallel to the plate margin
(Katili, 1969). It links to a series of transform faults
associated with spreading on the Andaman Sea
(Curray et al., 1979). The SFZ can be traced over a
distance of approximately 1650 km from the
Semangko Bay in South Sumatra to Aceh Valley in the
north (Bemmelen, 1949). Dextral displacements of
approximately 130 km along the SFZ have taken
Geo-Resources
place since the Tertiary and have continued up
to present. This movement has led to
development of complexly superposed volcanic
and magmatic arcs (granite/ andesite) as well as
mineralization. The subsequent dextral fault
zone is of more importance in localizing
mineralization in the area.
Table 1. Result of Analytical Data of the Arai Granit, Jangkat Area
Regional Geology
The studied area that forms flat to undulated
hilly country is occupied by several formations
(Figure 4). The oldest rock cropped out in the
area is Peneta Formation. The age of this
formation is assigned to be Cretaceous
–Jurassic (Suwarna et al., 1993). It is
comprised of slate, shale, siltstone and
sandstone and limestone intercalation. In
general, these rocks have been
metamorphosed, collectively termed as “meta”
(meta sandstone and meta siltstone, marble
and hornfels). This formation is interfingering
with the Rawas Formation consisting of
turbidite, pebbly wake, sandstone, siltstone,
limestone, grewake, argillite, diabas and basalt.
Both formations are well distributed in the
studied area and become as a host rock for
mineralization.
The Peneta and Rawas Formations were intruded by
Cretaceous Arai Granite consisting of granite and
aplite. The granite is exposed in the Iron Ore prospect
while the aplite formed as a dyke and cut the volcanic
rocks. The older rocks are unconformably overlain by
the Air Benakat and Muara Enim Formations.
Prospect geology
In the prospect area, a detailed geology has been
mapped by JICA (1987) (Figure 5a). The oldest
sequence found in the area is meta-sediments and
limestone. The meta sediments consist of sandstone,
siltstone and andesite lava, slate, and phyllite (Figure
5b, c). Sandstone is light grey, meta, fine grained,
well-bedded and folded. Limestone is light grey,
meta, luticeous, thin bedded and strongly folded.
Due to contact aureole metamorphism, some
limestones have been changed into marble (Figure
5d) while siltstone changed into hornfels (Figure 5e).
These rocks are exposed both in the iron ore deposit
and in the Tuboh prospects.
This meta sediment/limestone sequence is
unconformably overlain by interbedded sandstone,
shale, slate, basalt and pyroclastics. As a whole, the
oldest formations were unconformably overlain by
younger sediments.
The lithology of older meta sediment sequences are
very similar to those of Kluet and Kuantan Formations
of Carboniferous age in the north Sumatra (Aspden et
al., 1982). Kuantan and Kluet formations have been
proved to contain base metal deposit (Cu-Pb-Zn).
Within the Kluet Formation, the well known Sedex
deposit has been discovered while within the Kuantan
Formation, the skarn Latong deposit which is similar
to the Tuboh deposit has been identified (Noya et al.,
2002).
The metasediments were intruded by granite, aplite
and andesite. Granite is in the form of blocks or small
outcrops, exposed in the Jangkat and the Tuboh
prospect. Granite is light grey, medium grained, hard
and compact (Figure 5f). It is mixed within the iron ore
deposit. The granite body could be a heat source for
the formation of ore deposit that hosted within the
metasediments. Aplite and andesite are in the form of
dykes cutting the metasediment.
135
Geo-Resources
S
-2°
STUDY
AREA
Figure 3. Tectonic map of Sumatra (Hutchison, 1994).
136
-4°
Geo-Resources
Figure 4. Regional geological map of the studied area (Suwarna et al., 1993).
137
Geo-Resources
102o17’E
2o36’S
O
2 36’S
102o44’E
un
im
ur
ut
K
S.
S. M
efiki
L
S.
ban
S. Re
IRON ORE
PROSPECT
LEGEND
Andesite
Dacitic lava, pyroclastics
Granite
Sst, shale, slates,
piroclastics
Limestone
2o50’S
Slate, phyllite, andesite,
dacitic tuff.
TUBOH
PROSPECT
Syncline/
Anticline
Fault/
Lineament
n
2o50’S
Alluvial
bo
u
.S
S
o
102o44’E
102 17’E
Figure 5a. Prospect geology of the studied area (JICA, 1987).
Figure 5b. Meta sandstone in the studied area.
138
Figure 5c. Folded metalimestone in the studied area
Geo-Resources
all metal prices including the iron ore. Zn-Cu-Pb
prospect refers to as the Tuboh Prospect (Figure 6b)
which is located in the eastern part of the iron
prospect. The Tuboh deposit is polymetallic minerals
(Zn, Cu, Pb, Py, and Ag). Besides this, it is also found
oxidized hematite/magnetite, goethite and oxidized
copper ore as malachite and azurite) within the Tuboh
deposit. These ores are associated with NE-SW
trending structures. The placer gold is mined by the
local people using mechanic technology and gold
panning during the dry season in the Rawas River
(Figure 6c).
Figure 5d. Marble in the Tuboh prospect.
Petrography
Figure 5e. Hornfels outcrop in the Tuboh prospect.
A total of five fresh granite samples has been
petrographically identified. In general, granites are
light grey, medium to coarse grained, and show
holocrystalline/granular textures (Figure 7a). The
primary minerals are quartz, plagioclase, orthoclase,
biotite with minor hornblende, pyroxene and
secondary muscovite (Figure 7b). Quartz is subhedral
to euhedral and represents as free from alteration.
Plagioclase is colourless, subhedral, bladed and
represents the most abundant phenocrysts phase. It
is generally displays simple oscillatory zoning and
twining (Figure 7a). Apatite inclusions are common
found within plagioclase. In contrast, orthoclase
forms as megaphenocryst, fractured, and show
microperthitic textures (Figure 7b). Biotite is typical
bladed crystal and showing strong pleochroic colour.
Hornblende is found as bladed crystal while pyroxene
forms as small discrete crystals. Both hornblende and
pyroxene are minor constituents. Secondary
muscovite due alteration is found within feldspar.
Geochemistry
Major Element
Figure 5f. Granite expore in the Iron ore deposit.
Mineralization
As mentioned earlier that the studied area is attractive
for the mining company due to the presence of several
ore deposits (iron ore, Zn-Cu-Pb and placer gold). The
iron ore prospect owned by PT MAJU, is located about
1 km northwest of Jangkat Village. Iron ores as
hematite and magnetite have been exploited (Figure
6a). However, it is now terminated due to the drop of
The result of major oxide analysis is shown in Tabel 1.
The content of SiO2 is quite constant, ranging from
70-71 wt.%. This is also followed by low content of
CaO (1.6-2.22 wt.%), Fe2O3 (3.15-3.65 wt.%), MgO
(0.49-0.56 wt.%) while K2O is slightly higher (3.634.27 wt.%). In order to plot the oxide mineral within
diagrams, it is firstly calculated to 100% total without
LOI while Fe2O3 is calculated for FeO total using
division of 1.111. Plot of the Alkali Index versus SiO2
(Figure 8a) indicates that this granite composition
falls within calc-alkaline. It is consistent with the ACF
diagram (Wright, 1969; Miller, 1985) (Figure 8b),
where they fall within “mataluminous granite”
139
Geo-Resources
a
_!
b
!
c
!
d
!
e
!
f
g
!
h
!
i
!
j
!
k
!
!
l
!
1
!
Plg
2
3
4
5
Qtz
Bio
-
Plg
era
Qtz
Qtz
6
-
Plg
era
7
8
Bio
9
0 _
Plg
era
Figure 6a. Iron ore mining by PT. Maju, Jangkat.
Figure 7a. Microphotograph of granite (09HZ53PK) showing primary
minerals of plagioclase (Plg), quartz (Qtz) and biotite (Bio).
Plagioclase show zoning textures.
a
_!
b
!
c
!
d
!
e
f
!
!
g
!
h
!
i
!
j
!
k
!
l
!
!
1
2
-
Ort
era
3
4
-
Bio
Qtz
z
Bio
Bio
5
6
Bio
7
8
9
Figure 6b. Local mining in Tuboh prospect.
Plg
era
Ort
0 _
Qtz
Plg
Figure 7b. Microphotograph of granite (09HZ53PK) showing primary
minerals of plagioclase (Plg), orthoclase (Ort), quartz (Qtz)
and biotite (Bio). Orthoclase shows microperthitic texture.
the rock is possibly related to subduction process.
Moreover, plot of Rb- (Y+Nb) (Pearce et al., 1984)
(Figure 9b), belongs to volcanic arc granite (VAG)
while plot of Y-Nb (Fig 9c), falls within VAGSYNCOLG. This is also respectively confirmed by
SiO2-Rb and SiO2-Y plots (Figure 9d, 9e).
Figure 6c. Local gold panning in Rawas River.
Trace elements
In order to obtain the trace element contents, five
selected samples have also been analyzed (Table 1).
This rock indicates a slightly depletion of Nb and
strong depletion of P and Ti in the spider diagram
(Figure 9a). However, mobile elements such as K and
Sr, Rb, Th significantly increase. This means that
140
Rare Earth Elements
The result of analysis of samples for Rare Earth
Elements (REE) is shown in Table 1. Chondritic
normalized plot of rare earth element data is shown in
Figure 10. The figure shows a flat pattern with a
slightly enriched in LREE, drop of Eu and decrease in
HREE. The drop of Eu suggested due to plagioclase
fractionation or the presence of garnet within the
rock.
Geo-Resources
Discussion
The Arai granite in the studied area belongs to calcalkaline (Figure 8a) and falls into I-type
(metaluminous) (Figure 8b). Mole ratios of Al2O3/
Na2O + K2O + CaO (A/CNK) indicate the value of
<1.1 which also confirms that the Arai Granite
belongs to I-type granite (Chappell and White, 1974;
Hanson, 1978; Takahasi et al., 1980; Chappel et al.
(1987). The presence of biotite and hornblende
within this rock also supports the characteristic
features of I-type granite for the Arai granite.
Major and trace element characteristics (Figure9)
indicate the rocks arc originated from magma in an
orogenic environment. This is confirmed by the
regional geology that the genetic origin of the Arai
granite in the area is intimately related to tectonic
development of Sumatra due to the oblique collision
(Taponier et al., 1982) of Australian Plate and
Eurasian Plate with respect to the Indian Ocean Plate.
Amiruddin (1998) suggests that the granite was
formed during collisions.
Magma source of the Arai granite which mostly
belongs to calc-alkaline (Figure 8a) due to an
anaxtesis/partial melting of lower crust (i.e.,
amphibole/biotite break down in the presence of
quartz and feldspar). It is possibly coincident with
initiation of SE–directed subduction of Indian Ocean
Plate with respect to the Eurasian plates (Hamilton,
1979; Watanabe and Izawa, 2002). This is also
confirmed by the ratio of K2O/Na2O (0.9-1.06)
suggesting that the rocks are calc-alkaline (Vogt and
Flower 1989). The presence biotite and hornblende
suggest that the source of materials of the Arai
Granite were possibly derived from an original
igneous source of basalt (gabbro), andesite (diorite)
within the continental crust and belongs to the
contact aureole granite (Chappel et al., 1974;
Takahasi et al., 1980).
In Sumatra as a whole, many deposits (Au, base
metals, Fe) are formed due to the influence of the
granite intrusions (Sukirno, 2006). The presence of
Pb-Fe deposit in Lokop District, East Aceh Regency is
due to the effect of Lokop Granite (Abidin and
Harahap, 2006). The occurrence of gold deposit in
Bonjol area, East Pasaman Regency, West Sumatra,
is also triggered by the granite intrusions (Abidin and
Harahap, 2007). Again, the Latong skarn Pb deposit
in Latong River, Madina Regency, North Sumatra was
associated with granite intrusions (Noya et al.,
2002). Also, the presence of Pb-Fe in Abai District
(Solok Selatan) and iron ore in Surian area, Alahan
Panjang District, Solok Regency, West Sumatra are
also related to granite intrusion (Abidin, 2005; 2006;
Abidin and Baharuddin, 2008).
Granite, in general, can be divided into two main
types i.e., I-type (magnetite series) and S-Type
(ilmenite series) (Chappel and White, 1974; Chappel
et al., 1974; Ishihara, 1977; Takahashi et al., 1980;
Kutsukabe, 1988; Andrew, 2009). The I-type granite
which is referred to magnetite series is formed by the
melting of igneous rocks while S-type granite is
produced by the partial melting of sedimentary rocks.
The type of ore deposits associated with granotoids
can also be related to these classifications.
Molibdenum (Mo) and base metals (Fe, Cu, Pb, Zn),
precious metals (Au) and porphyry copper are the
product of I-type granite of magnetite series (Takahasi
et al., 1980). On the other hand, tin deposits
(greisen-type) occur characteristically in ilmenite
series of S-type granites (Smirnov, 1976). Similarly,
the Arai Granite is also I-type of magnetite series in
character. Therefore, the Arai granite could also
produce such base metals and gold. For example,
gold mineralization in the Kerinci Regency, Jambi is
associated with I-type granite (Abidin and Suyono,
2004; Abidin and Suwarti, 2005).
As discussed earlier that the area of study belongs to
Rawas Cluster” (Machali et al., 1987) (Figure 11). In
this cluster, geologically it is occupied by Peneta and
Rawas Formations which consist of interbedded
claystone, sandstone and intercalation of limestone.
The emplacement of the Arai granite (Cretaceous in
age) has generated a metamorphic contact with
limestone and changed lithology of both formations.
As a result, most lithology of both formations become
“meta” referred as to meta sandstone, meta siltstone
etc. A strong thermal metamorphic effect of the
granite intrusion to both formations has resulted
marble and hornfels. At the same time, it is also
followed by a metasomatic process. The process
transformed the existing minerals into totally/partially
new mineral by replacement of their chemical
constituent (Lapidus, 1987). In this regard, the
limestone has been changed partially/totally by new
minerals such Fe, Cu, Pb, Zn, Py, Au etc. As a result,
iron ore and Zn-Cu-Pb as well as gold were formed in
the area of study. The iron ore prospect is found
together with the granite body while base metal is
present in the eastern part of the other granite body.
141
Geo-Resources
SiO2 (wt. %)
80
70
Nb
ALKALINE
1000
CALC-ALKALINE
60
PERALKALINE
50
100
WPG
(Al2O3+CaO+(Na2O+K2O)
(Al2O3+CaO-(Na2O+K2O)
5
4
3
2
1
Nb
6 7 8 91011
VAG+SYN-COLG
10
ORG
Figure 8a. Alkali index of the Arai granite (Wright, 1969).
1
A (AL-Na+K)
1
Muscovite
10
Y
100
1000
Figure 9c. The Arai granite within Y vs Nb (Pearce et al., 1984).
Peraluminous
Plagioclase
Cordierite
1000
um
all
et
M
SIN-COLG
us
ino
Biotite
100
Hornblende
C (Ca)
F (Fe+Mg)
VAG
Figure 8b. ACF diagram showing the distribution of the Arai granite
(Wright, 1969) .
SiO2
10
60.00
70.00
80.00
SiO2
1000
Figure 9d. The Arai granite within Rb vs (Y+Nb) (Pearce et al.,
1984).
100
Rocks/chondrite
Rb
09H0Z50Pk
10
09H0Z51Pk
Y
09H0Z52Pk
09H0Z53Pk
1
09H0Z54Pk
1000
0.1
0.01
Ba
Rb
Th
K
Nb
Ta
La
Ce
Sr
Nd
P
Sm
Zr
Hf
Ti
Y
Yb
100
WPG+ORG
Y
Figure 9a. Spider diagram of the Arai granite.
Y+Nb
VAG+COLG+ORG
10
60.00
1000
70.00
80.00
SiO2
Figure 9e. The Arai granite within Rb vs (Y+Nb) (Pearce et al.,
1984).
WPG
100
VAG
Y+Nb
ORG
10
1
10
142
100
Y
1000
Figure 9b. The Arai granite within Y vs (Y+Nb)
(Pearce et al., 1984).
Geo-Resources
1000.00
Rocks/chondrite
100.00
09H0Z50Pk
09H0Z51Pk
09H0Z52Pk
09H0Z53Pk
10.00
09H0Z54Pk
1.00
La
Ce
Nd
Sm
Eu
Gd
Dy
Er
Yb
Figure 10. Chondrite REE normalized plot from the Arai
granite.
1O
2O
STUDIED AREA
Figure 11. Mineralization zones of South Sumatra (Machali et al., 1987).
The presence of such mineralization (iron ore and
base metal) which is associated with the Arai granite
could be classified into skarn type. This is confirmed
by the marble and hornfels within the deposit. The
occurrence of the iron ore together with granite body
may be classified into indoskarn while those
associated marble and hornfelsic rocks may
classified into exoskarn.
Conclusions
On the basis of petrologic and geochemical
characteristics, the Arai granite is
metaluminous I-type calc-alkaline affinitiy.
Tectonically, the Arai granite is Volcanic Arc
Granite (VAG) or SYNCOLG/ORG, which is
intimately associated with compression due to
oblique collision between Australian - Eurasian
143
Geo-Resources
Plates and the Indian Ocean Plate. The Arai granite
which is assigned to be Cretaceous in age intruded
the older formations (Peneta and Rawas). Therefore,
this granite is expected to be a heat source for
mineralization in the studied area.
Mineralization which is associated with the Arai
granite is iron ore, Zn-Cu-Pb-Py-Fe-Ag and Au
deposits. The deposits host within the Peneta and
Rawas Formation. The iron ore deposit has been
mined for magnetite and hematite ores by PT. MAJU
but for the time being it is terminated. It is classified
into indo-skarn type. In contrast, Zn-Cu-Pb-Py-Fe-Ag
of the Tuboh deposit which is classified into exo-
skarn is also mined for Zn ore by the locals while
placer gold is mined by the locals in the Rawas River.
Acknowledgements
The writer thanks to the head of the Centre for
Geological Survey who permits to publish data from
the Research Project. Ir. Amiruddin MSc who read
the early draft to improve the manuscript was kindly
appreciated. Thank also goes to the GSI-Geollab
that prepared all analytical data. All Sumatran
magmatic research personel groups are
acknowledged.
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146
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COALBED METHANE POTENTIAL AND COAL CHARACTERISTICS IN MUARA LAKITAN
AREA, SOUTH SUMATRA
M.H. Hermiyanto and R. Setiawan
Jl. Diponegoro No. 57, Bandung - 40122
Abstract
A research on Coal Bed Methane (CBM) of the Muaraenim Formation has been conducted in the Muara Lakitan area.
Megascopically, the coal lithotype varies from dull to bright banded, with black – brownish black and brownish to black
streaks, brittle – friable, dull-greasy luster, even-uneven, dirty on fingers, with resin patch and striation, dirt bands
(clay/mud layers), pyrite striation, and pore structures. The coal quality, gained from geochemical analysis, indicates that
its ash content ranges between 1.22% and 2.47%, total sulphur content is from 0.15% to 0.3 %, and the volatile matter
of 38.02% - 40.81%. The coal is dominated by vitrinite (73.6 – 85.8 %), with minor amount of exinite (1.4 – 4.0 %),
inertinite (4.2 – 21 % ) and mineral matter (2.4 – 8.2 %). Vitrinite reflectance, varies from 0.44% to 0.45 %, tends to
indicate a sub-bituminous to high volatile bituminous-A coal rank. Kaolinite clays are the most prominent mineral matter
within all coal samples analyzed, although the clay textures show irregular shapes. Iron oxides are also present in several
samples. Microcleats found within the coals are mostly open, and are rarely filled by clay minerals. Based on
Barbara/Winter diagram, the methane gas content in the studied area ranges from 0.57 m3/t – 1.70 m3/t = 20.44 scf/t –
60.96 scf/t. The total reserve of gas within six coal seams in the studied area is 15.524,28 scf.
Keywords : Coal Bed Methane (CBM), Muaraenim Formation, Muara Lakitan
Sari
Penelitian gas metana batubara (GMB) Formasi Muaraenim telah dilakukan di daerah Muara Lakitan. Secara
megaskopis, batubara mempunyai ciri yang bervariasi dari “dull” sampai “bright banded”, cerat hitam-hitam
kecoklatan dan hitam – kecoklatan, getas - rapuh, kilap buram-berminyak, pecahan rata - tak rata, mengotori jari,
mengandung resin, adanya lapisan lumpur atau lempung, pirit, dan struktur pori. Berdasarkan analisis geokimia,
kualitas batubara mengindikasikan bahwa kandungan abu berkisar antara 1,22 dan 2,47%, sulfur total dari 0,15 –
0,3%, dan volatile matter dari 38,02 – 40,81%. Batubara didominasi oleh vitrinit (73,6 – 85,8%), eksinit (1,4 –
4,0%),inertinit (4,2 – 21%) dan bahan mineral (2,4 – 8,2%). Reflektansi vitrinit, dari 0,44% sampai 0,45%,
cenderung termasuk dalam tingkat subbituminous sampai high volatile bituminous-A. Kaolinit merupakan mineral
lempung yang dominan di semua percontoh batubara, dengan tekstur lempung yang berbentuk tidak beraturan. Oksida
besi juga hadir di beberapa percontoh batuan. Mikrokleat ditemukan dalam batubara sebagian besar terbuka, dan
jarang yang terisi oleh mineral lempung. Berdasarkan diagram ”Barbara/Winter”, kandungan gas metana di daerah
penelitian berkisar dari 0,57m3/t – 1,70m3/t = 20,44 scf/t – 60,96 scf/t. Total cadangan gas pada enam lapisan
batubara di daerah penelitian adalah 15.524,28 scf.
Kata kunci : Gas metana batubara (GMB), Formasi Muaraenim, Muara Lakitan
Introduction
Administratively, studied area is a part of the Muara
Lakitan Subregency, Musi Rawas Regency, South
Sumatra Province (Figure 1) that is located
approximately 15 km to the northwest of Muara
Lakitan. The study has been focused in the Bare
Santos coalfields, which is presumed to be potential
coalbed methane resources. A research on geological
condition and coal characteristics has significantly
Naskah diterima :
Revisi terakhir :
4 Maret 2010
25 Juni 2010
enhanced the opportunity for profitable exploitation
of the CBM resource in the region.
Coal Bed Methane (CBM) is an economic source of
gas methane that is generated and stored in coal
beds. Methane, both primary biogenic and
thermogenic types, in coal is a result of coalification.
However, in some cases, a post-coalification biogenic
activity occured. Calcification is a process by which
peat is transformed into coal during progressive
burial, involving the expulsion of volatiles, mainly
methane, water, and carbon dioxide.
147
Geo-Resources
106° E
104°E
S
Muara Lakitan
Lubuklinggau
S
0
SYMBOLS
Fault
Anticline
Thrust fault
Syncline
Studied area
40
80
120 km
N
Figure 1. Regional locality map of the CBM study in the Muara Lakitan Area.
Late-stage or secondary biogenic methane is
generated by bacterial activities within groundwater
systems. Most coals at shallow depth are aquifers,
due to the presence of a well-developed cleat
(fracture) system. The late-stage biogenic methane is
significant and reaching maximum phase at the subbituminous level or lower rank coal. As a result, subbituminous coals have comprised dominant target of
coalbed methane exploration. Thereby, low rank
coals, which exist at shallow depths and crop out
significantly, may contain mainly late-stage biogenic
(secondary biogenic) methane.
The aim of the study is to collect information obtained
from coal and its coal measures, both from field and
laboratory analysis. The result of the analysis is
important for a better understanding on the coal
characteristics relating to CBM potential,
predominantly, within the Tertiary coal measures.
The main objective of the study is to evaluate the
CBM potential of low rank Tertiary coals in Muara
Lakitan area, in order to define future exploration
objectives in regions, that contain rich CBM
resources. An overall objectives is to advance our
understanding of geological processes in the
sedimentary basins in the Muara Lakitan area, in
particular with the formation of the Tertiary coalbed
methane resources.
148
Specific objectives are: (a). to determine quantity and
quality of CBM generated from the Muaraenim coals,
and exploration implications of CBM as a source for
new alternative energy, (b). to determine and analyze
the coal deposits and their coalification proceses, (c).
to evaluate source rock characteristics of the coals
and identify the major CBM source area. The results
of the study, as the primary objective of the project,
could provide information for companies regarding to
the occurrence, including quality and quantity of
CBM, which will be used as alternative and additional
energy resources, and in turn would give contribution
in energy sector.
During the fieldwork, the base camp was located at
Muara Lakitan, whilst the subcamp was situated
inside Pelita Jaya Village (Figure 1). Field activities
were concentrated in several areas with suitable
geological conditions for the CBM potency. This
activity was carried out in 2007 under the Coal Bed
M e t h a n e D e v e l o p m e n t Pr o j e c t ( P r o y e k
Pengembangan Coal Bed Methane), a program of the
Research and Development Centre For Oil and Gas
Technology (Pusat Penelitian dan Pengembangan
Teknologi Minyak dan Gas Bumi ) “LEMIGAS”.
Geo-Resources
Special geologic field investigations and laboratory
techniques were conducted, in order to achieve the
aims of the study. The fieldwork investigations
including detailed determination, observations, and
measurement on cleat, lithotype, position, and
characteristics of the coals within the measures, were
performed in selected areas occupied by the relatively
complete coal seams (Figure 2).
Geological Setting and Stratigraphy
Geology
or interfingers the Gumai Formation and composes of
marl, claystone, shale, and silty shale, with
occasionally thin limestone and sandstone. The
Gumai Formation was deposited in a deeper open
marine environment and underlies comformably
litoral to shallow marine Airbenakat Formation,
which comprises sandy and marly claystone,
numerous sandstones with glauconite, sometimes
calcareous. Deposition of the Talangakar and
Airbenakat Formations occurred during OligoMiocene time.
The geological setting of the South Sumatra Basin
was described in several published and unpublished
reports. This basin is located in the southern part of
Sumatra Island, and de Coster (1974) suggest as a
back-arc basin bounded by the Barisan Mountain in
the southwest and by the pre-Tertiary of the Sunda
Shelf to the northeast. The South Sumatra Basin was
formed during east-west extension that took place
during pre-Tertiary and early Tertiary times (Daly et
al., 1987). The tectonic history and stratigraphy of
this basin have been described by de Coster (1974),
Darman and Sidi (2000).
The Late Miocene-Pliocene Muaraenim Formation,
comformably overlying the Airbenakat Formation, is
divided into member “a” (interstratified sandstone
and brownish claystone with principal coal seams),
and member “b” (greenish blue claystone with
numerous ligniteous coal seams). Both members
were deposited in a brackish environment. The
youngest unit is Kasai Formation consisting of gravel,
tuffaceous sands and clays, volcanic concretion,
pumice, and tuff. The formation comformably
overlies the Muaraenim Formation and has PlioPleistocene age. The deposition of the Kasai
Formation coincided with a volcanic and magmatic
activities. This activities formed some igneous
intrusions which intruded the coal measures in the
Bukit Asam coalfields, particularly in Bukit Kendi, Air
Laya, Muara Tiga Besar, and West Bangko areas.
Regional Stratigraphy
Coal Characteristics
The oldest rock in the South Sumatra Basin is preTertiary basement, which comprise various igneous
and meta sediments. The Eocene–Oligocene Lahat
Formation consists of purple green and red brown
tuff, tufaceous clay, andesite, brecia and
conglomerate, unconformably overlies the basement.
The Lahat Formation is unconformably overlain by
Oligocene – Miocene Talangakar Formation that
compose of medium-to coarse-grained sandstones
and coal seams in the lower part; and calcareous grey
shale and sandstone with coal seams in the upper
part. The thickness of the Talangakar Formation is
approximately up to 900 m. Locally, the Talangakar
Formation was deposited in a terrestrial to paralic
environment, rest unconformably on top of preTertiary Basement. Moreover, the Talangakar
Formation is conformably overlain by the shallow
marine calcareous shale and limestone of Baturaja
Formation. The formation is conformably overlain by
Lithology
The studied area is in a small intra-montane basin or
presumably the centre of the South Sumatra Basin. In
general, morphology of the studied area comprises
gentle low hill, rolling country, and rugged
mountainous areas.
The coal seam horizon occupies the upper portion of
the lower part of the coal-bearing measures. These
sediments or coal measures are located in a small
subbasin. The coal seams distributed in NW – SE
direction, and parallel to the Barisan Range. The coal
deposits are found in Bara Santosa Coalfields, the
Muara Lakitan Regency, South Sumatra Province.
Coal seams
In the field, a potential coal deposit was recognized in
the Muara Lakitan area. Its caloric values vary from
4900 to 5100 cal/g. The coal seams in the area occur
in the unclear geologic condition; due to fault
disturbances taking place within the area are hardly
observed. Based on the core samples present,
thickness of each subseam is more than 75 cm.
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Geo-Resources
103º15'E
103º14'E
103º16'E
0
750m
N
-2º49'S
W
E
S
-2º50'S
Legend
Qtk
: Kasai
Formation
Tmpm : Muaraenim
Formation
Figure 2. Sample location of the CBM Study in the
Muaralakitan area.
Sample code
Coal seam
STRATIGRAPHIC COLUMN OF COAL SEAMS
IN MUARA LAKITAN AREA, SOUTH SUMATRA
SCALE
SYMBOL
GRAIN SIZE
5
10
15
20 m
SEAM
THICKNE SS
SAMP LE
AG E
FORMATION
0
DESCRIPTION
Meter
0
SEAM 300
Black, dull-bright, brittle, dull-greasy luster, uneven-even, dirty to fingers,
containing resin patch , dirt bands (clay/mud layers).
SEAM 400
Black, dull-bright, partly banded bright, brittle, dull-greasy luster, uneven,
dirty the fingers, containing resin patch.
10
20
LATE MIOCENE
MUARAENIM FORMATION
30
SEAM 600
SEAM 700
Black-brownish black, dull-bright, brittle, dull-greasy luster, uneven-even,
dirty the fingers, contain resin patch, dirt bands (clay/mud layers), and
silicified wood
fragment
.
Black, dull-bright, brittle, dull-greasy luster, uneven-even, dirty to fingers,
containing resin patch, dirt bands (clay/mud layers).
40
50
SEAM 800
Black, dull-bright, brittle, dull-greasy luster, even-uneven, dirty to fingers,
containing resin patch and striation, dirt bands (clay/mud layers) and
occasionally fragments of silicified wood.
SEAM 900
Black brownish black, dull-bright, brittle friable, dull-greasy luster, evenuneven, dirty to fingers, containing resin patch and striation, dirt bands
(clay/mud layers), pyrite striation, occasionally pore structure.
N 295 E/ 12
60
70
80
90
SEAM 1000
Black, dull-bright, banded bright in the lower part, brittle, dull-greasy luster,
uneven, dirty to fingers, containing resin patch and striation, pyrite.
SEAM 1100
Black, dull-bright, partly banded bright, brittle, dull-greasy luster,uneven,
dirty to fingers, containing resin patch, pyrite.
100
110
120
130
Figure 3. Schematic stratigraphic column of coal seam at Muara Lakitan, South Sumatra.
150
Geo-Resources
The coal samples (11samples) from Muaralakitan,
South Sumatra analyses have been labeled as
07RL201A, 07RL202, 07RL203, 07RL204A,
07RL205A, 07RL206A, 07RL207A, 07MH51A,
07MH52B, 07MH53B and 07MH54B. All samples
are purely coal.
Megascopically, the coal lithotype is dull - bright
banded, black – brownish black, brown to black
streak, brittle – friable, dull-greasy luster, evenuneven, dirty, containing resin patch and striation,
dirt bands (clay/mud layers), pyrite striation, and
pore structure (Photo 1).
Photo 1. Field feature of coal seam cropping out at Pagar Gunung, Muara
Lakitan, Sumatra Selatan.
Coal Quality
The coal quality, gained from geochemical analysis,
indicates that its ash content is 1.22 – 2.47%, total
sulphur content is 0.15 – 0.3 %, and volatile matter
is 38.02% - 40.81% (Table 1).
Based on ash and total sulphur contents, the mineral
matter contained in the coal is low to high level.
Furthermore, organic petrographic analysis shows
that the coal is dominated by vitrinite (73.6 –
85.8%), with minor amount of exinite (1.4 – 4.0 %),
inertinite (4.2 – 21 %) and mineral matter (2.4 –
8.2%) (Photos 2, 3, 4 and Table 2). Vitrinite
reflectance having a value of 0.44 – 0.45%, tends to
indicate a subbituminous to high volatile bituminousA coal rank.
Photo 2. Microphotograph of telocollinite associated with
semifusinite and sclerotinite within the coal seam in the
Muara Lakitan region. Sample: 07 RL 207A.
Coal Cleats and Coalbed Methane Content
A field study on cleats from coal exposures in the
Muara Lakitan area demonstrates that the dip
direction of coal face cleats varies from N160oE/80o
to N330oE/50o, space ranges between 0.2 cm to 19
cm, aperture of 1 to 8 mm, frequency of 0.239 cm -1
to 1.69 cm -1, and density of 0.0099/cm to
0.21/cm (Photos 5 and 6).
The coal seams having a volatile matter content of
38.02% - 40.81% show that predicted calculated
methane content of the coal seam is 0.57 m3/t –1.70
m3/t. It is obtained by plotting the volatile matter
contents on Barbara/Winter diagram as shown in
Figure 4. This methane content variation indicates
that in-situ coal has low to moderate methane or
coalbed gas content. The volatile matter
characteristic indicates that in-situ coal has low to
moderate methane content. However, it is not a
pessimistic methane value excepted from the coal,
because the coals collected from outcrops.
Photo 3. Microphotograph of semifusinite associated with tellocolinite
and pyrite, within a coal sample, from Muara Lakitan region.
Sample: 07 MH 52B.
Photo 4. Microphotograph of pyrite recognized in coal of the Muara
Lakitan area. Sample: 06 MH 47A.
151
Geo-Resources
Photo 5. Coal outcrop showing cleated dull banded lithotype, cropping
out at the Pagar Gunung, Muara Lakitan area.
Photo 6. Coal outcrop showing cleated dull banded lithotype, cropping
out at the Simpang Kulit, Muara Lakitan area.
Table 1. Proximate Geochemistry of Coal Samples Taken from the Muara Lakitan Area, Sumatra Selatan
No.
Sample marks
Moister in Air
Dried Sample
(%, adb)
Ash
(%, adb)
Volatile
Matter
(%, adb)
Fixed Carbon
(%, adb)
Calorific
Value
(Cal/g.
Adb)
Total
Sulphur
(%, adb)
1
07 RL 201A
20.57
2.47
40.81
36.15
5151
0.23
2
07 RL 204A
21.49
2.08
36.54
37.89
4975
0.19
3
07 RL 205B
23.65
1.49
38.31
36.55
5023
0.15
4
07 RL 206A
22.87
1.48
39.00
36.65
5187
0.30
5
07 RL 207A
25.47
1.22
38.02
35.29
4930
0.15
6
07 MH 52B
21.33
1.43
39.69
37.55
5092
0.22
Table 2. Organic Petrology Analysis of Coal Samples Taken from Muara Lakitan, Musi Rawas
152
Geo-Resources
In general, cleat intensity is related to maturity of coal
rank, higher coal rank is more developed cleat
intensity. Commonly, the coal rank in the studied area
is low (Rv < 0.5%), that it is due to cleats intensity.
Coals permeability is moderate, although the coals
are fairly well cleated, and they have very low
porosity. This condition tends to indicate moderate
methane desorption capacity. Another substantial
factor is desorption rate influenced by both rank and
coal. With increasing rank, the effective diffusivity
coefficient decreases. In higher rank coal, gas-release
rate is slower than it is in lower rank coal.
Additionally, the mineral matter acting as a simple
influence to decrease the methane adsorption
capacity indicates that the mineral matter content
had the strongest effect on the adsorption capacity.
The mineral matter content of the coal studied is low
to moderate level. Therefore, it is presumed that the
adsorption capacity of the coal is relatively moderate.
The higher moisture content ranges from 20.57 to
25.47%, indicating that methane adsorption of the
coals will be slightly high. On the other hand,
methane sorption will be moderate to high. Based on
the coal adsorption capacity, coalbed methane
content derived from the Muara Lakitan area
expected to be at least a moderate level. It is indicated
by the presence of bright to bright banded lithotype,
maceral composition dominated by vitrinite; low
moisture content, moderate to slightly high volatile
matter, moderate to high vitrinite reflectance, and low
to medium ash content.
SEM Analysis Results
Each sample of a total 11 (eleven) coal samples from
Muara Lakitan area was examined carefully under the
SEM method. Summary of the SEM results on
microcleat characters and measurements of each
coal sample are listed in Table 3. Maceral identified
under SEM comprises predominantly telocollinite,
followed by desmocollinite. Liptinite maceral are
typically sporinite, resinite and exsudatinite. Droplet
oil is also visible in some samples (Photo 7). Inertinite
consist of semifusinite in one coal sample
(07MH51A). Other samples have inertinite maceral.
Kaolinite is the most prominent mineral matter within
all coal samples, and it has an irregular shape. Iron
oxides are also present in several samples. Detailed
observation and examination on microcleat occured
within coal samples recorded, including frequency
A
B
C
D
E
F
G
H
I
J
K
L
9
K
Sp
8
7
Sp
6
K
Do
5
K
4
3
2
1
Photo 7. Sample no. 07 RL 204A, Microphotograph SEM, showing
droplet oil (Do) surrounded by maceral sporinite (Sp) and
clay mineral of kaolinite (K).
A
B
C
D
E
F
G
H
I
J
K
L
9
8
I
Sp
7
K
I
6
K
K
5
4
3
K
K
2
1
Photo 8. Sample no 07 RL 203, Microphotograph SEM, showing
microcleat maceral sporinite 20%(Sp) and clay mineral of
kaolinite 50%(K) and illite 30%(I); Magnification 250X.
density, length, aperture and the type of microcleat
(face or butts). Butts microcleat appears to be most
abundant compared to face microcleat. Microcleats
found within the coals are mostly open aperture with
very rare filled by clay minerals (Photo 8).
The density of microcleat ranges from 0.02 micron
square/freq microcleat to 0.08 micron square/freq
microcleat. Three coal samples (07RL202,
07RL203 and 07MH52B) have low-density value of
microcleat ranging from 0.02 to 0.05. It means that
those three coal samples are categorized as poor for
CBM reservoirs. While the other eight samples
(07RL201A, 07RL204A, 07RL205A, 07RL206A,
07RL207A, 07MH51A, 07MH53B and 07MH54B)
have high-density values (0.06 to 0.08), may be
categorized as favorable for CBM reservoir. Many
microcleat are connected to each other. Thus, they
facilitate for the pathway of gas migration and
adsorption.
153
Geo-Resources
CBM Potential and Content
This section attempts to evaluate CBM potential in
the Muara Lakitan area based on the field works and
laboratory data. Physical properties (of type,
porosity/ permeability, and rank) and thickness of
coal, structural geology, and cleats assessed in the
previous section and only the important result will be
extracted for the purpose of Muara Lakitan CBM
resource assessment. The Muara Lakitan area,
located in the Sumatra back - arc region, possesses
many favorable and risks for CBM development.
Favorable attributes include slightly thick coals in the
Mio-Plio Muaraenim Formation, low ash and sulfur
content, low to moderate inherent moisture and
volatile matter content, low rank coal (subbituminous B to A grade), and well-developed cleat.
Negative attributes include poor data control, poor
sorption isotherm data, structural complexity,
probably extremely high CO2 gas content, and
relatively narrow prospective area for CBM play.
Gas In-Place Resources
Considering the availability of the Muara Lakitan field
and laboratory data set required for calculating CBM
resources, the calculation of gas in-place potential in
the area conducted. Parameters used to calculate the
gas in-place potential of the Muara Lakitan consist of
theoretical gas content based on Barbara and Winter
diagram, and Lost Gas during drilling (Q1) (Figure 4)
plus gas desorption during transportation (Q2) and
residue gas (Q3). Thereby, the parameter is the
theoretical gas content calculation based on the
Barbara/Winter diagram. In order to calculate the
theoretical gas in-place potential of the Muara Lakitan
area, the required important parameter is the volatile
matter content of the coal. The gas in-place
potential/content of each selected coal seams shown
that methane gas content is from 0.57 m3/t – 1.70
m3/t = 20.44 scf/t – 60.96 scf/t.
The graphics of Volatile Mater versus Methane
content according to Barbara-Winter are shown in
Figure 4. The methane content within the coal seam
in the Muara Lakitan and its surrounding areas is as
follows: According to Barbara-Winter Diagram, the
content of methane gas ranges from 20.44 scf/t –
60.96 scf/t. Gas in-place which is supported by the
Q1, Q2 and Q3 values, as well as laboratory result has
been calculated (Table 4). The calculation follows the
formula proposed several studies (Asian Development
Bank/Migas, 2003) with some modifications as
written below:
Gas in place = (1 - Ash content) x (1 – Moisture Content) x
Density x Adsorption
The total reserve of gas in reservoir in the investigated
area (six coal seam): 15.524,28 scf.
Table 3. SEM Observation of Micro Cleat Content of the Coal from Muara Lakitan
No.
1
2
3
4
5
6
7
8
9
10
11
154
Sample No.
07RL201A
07RL202
07RL203
07RL204A
07RL205A
07RL206A
07RL207A
07MH51A
07MH52B
07MH53B
07MH54B
Microcleat
type
Length
(micron)
Width of Aperture
(micron)
Density
( 100 micron2/
FreqCleat
Butts (80%)
Face (20%);
110; 20; 40; 50;
30; 40; 20;20
0.8; 0.3; 0.3; 0.8;
0.7; 0.7; 0.5; 0.5
0.06
Butts (80%)
Face (20%);
250; 200; 220;
150
0.4; 0.4; 0.5; 0.5
0.02
Butts (90%);
Face (10%)
Butts (90%);
Face (10%)
750; 400; 200;
600; 600; 500
300; 250; 1000;
150; 400; 200
0.9; 0.4; 0.8; 0.7;
0.7; 0.7
0.8; 0.8; 0.9; 0.5;
0.7; 0.8
0.05
Butts (90%);
Face (10%)
Face (60%);
Butts (40%)
300; 100; 75;
300; 150
300; 200; 150;
200; 100; 350;
75
400; 100; 500;
150; 100
150; 200; 100;
100; 250; 50
500; 300; 150;
0.6; 0.6; 0.5; 0.6;
0.5
0.4; 0.6; 0.4; 0.3;
0.3;0.6; 0.3
0.06
0.7; 0.6; 0.6; 0.5;
0.5
0.8; 0.7; 0.7; 0.8;
0.7; 0.7
0.6; 0.6; 0.5
0.07
700; 250; 600;
200; 200; 250;
100; 100;
700; 400; 500;
200; 250;
1.20; 0.8; 1.20;
0.8; 0.8; 0.9; 0.6;
0.6
0.8; 0.6; 1.0; 1.0;
0.8
0.08
Butts (90%);
Face (10%)
Face (60%);
Butts (40%)
Butts (90%);
Face (10%)
Butts (90%);
Face (10%)
Butts (90%);
Face (10%)
0.08
0.08
0.07
0.04
0.08
Remark
Open aperture
(90%); some filled
by clays
Closed aperture
(100%) filled by
clays.
Open aperture
(100%)
Open aperture
(100%)
Open aperture
(100%)
Open aperture
(100%)
Open aperture
(100%)
Open aperture
(100%)
Open aperture
(100%)
Open aperture
(100%)
Open aperture
(100%)
Geo-Resources
Table 4. Gas Content Calculation of the Six Coal Seam Taken from Muara Lakitan area (For sample location see Figure 3)
No.
No. Sample
Geochemical content
Adsorption value
Density
1 - Ash
1 - Moisture
1 - CO2 content
Gas In Place
m2/t
Scf
1
07 RL 201
82.4059
0.9753
0.794
0.976
1.3
80.967
2903.49
2
07 RL 204
74.1285
0.9851
0.765
0.979
1.3
71.097
2549.55
3
07 RL 205B
67.5145
0.9852
0.763
0.981
1.3
64.723
2320.96
4
07 RL 206
65.7020
0.9852
0.771
0.98
1.3
63.581
2280.01
5
07 RL 207A
73.3795
0.9878
0.745
0.974
1.3
68.376
2451.96
6
07 MH 52B
84.9056
0.9857
0.787
0.983
1.3
84.169
3018.31
METHANE CONTENT,m /t OF PURE COAL SUBSTANCE
15,524.28
Geological Risk Assessment for CBM Potential
24
Most of parameters for assessing CBM potential in the
Muara Lakitan area, including coal thickness, rank,
ash content, moisture content, and gas content, have
been identified. However, the calculation of CBM
resource using theoretical 'gas in-place formula' will
remain a tentative estimation due to the unavailability
of accurate in-situ drill hole data. This condition
leads to some geological risk as follows :
20
1
3
3
16
12
2
8
4
Y=-
0,277
4
x+1
1,76
1. over estimate on the adsorption value of a given
coal seam due to limited data.
B
B
D
C
F
E
E
C
D
F
A
A
0
10
14
18
22
26
30
34
38
42
VOLATILE MATTER, %
LEGEND :
1.
2.
3.
4.
ACCORDING TO SCHULZ
ACCORDING TO WINTER
ACCORDING TO STUFFKEN EXPERIMENTAL MINE
ACCORDING TO BARBARA
Code Samp. Coal Seam
07 RL 201A
07 RL 204A
07 RL 205B
07 RL 206A
07 RL 207A
07 MH 52B
a
b
c
d
e
f
Volatile Matter
40,81
36,54
38,31
39,00
38,02
39,69
Methane Content
(4)
0,57
1,70
1,21
1,01
1,29
0,87
(2)
0
0
0
0
0
0
Figure 4. Coalfields consist of theoretical gas content based on Barbara
& Winter diagram, and lost gas during drilling (Q1) in Muara
Lakitan Area.
Maceral and Chemical Analysis Relationship
The graph (Figure 5) shows that vitrinite and ash
contents have negative relationships. On the other
hand, vitrinite and moisture have increase trend
(Figure 6) but vitrinite versus volatile matter have a
negative style (Figure 7). Inertinite versus ash was
decreases or in negative pattern (Figure 8). After that,
inertinite versus volatile matter were interpreting
positive model (Figure 9). Methane gas content
(CH4) versus vitrinite and versus inertinite present
that CH4 versus vitrinite have a positive trend (Figure
10) but CH4 versus inertinite is negative correlation
(Figure 11).
2. over estimate the density of fracture/cleat within
the coal seams, which will create the difficulties in
realizing CBM gas molecules from coal
micropores.
3. the low CBM content produced is interpreted to be
due to unfavorable groundwater condition (both
chemically and physically) for microbial
development.
Discussions
Increasing exploration of the coalbed gas type is due
to the growing recognition of CBM sources. A notable
predictable CBM expectation occurring in the Muara
Lakitan coalfields is derived from the coal
characteristics of the coal measures studied. The coal
characteristics in the areas studied enhance
significantly the opportunity for profitable exploitation
of the CBM resource. Coal type, rank,
porosity/permeability, the presence or absence of
seals, stratigraphic or structural traps, local pressure
variations, and basin hydrodynamics are factors
controlling the distributions of gas contents in coal
beds.
Gas content measurement depends on several
factors, such as sampling procedures, sample type,
155
Geo-Resources
coal properties, and analytical methods and
qualities. The gas storage capacity of coal beds
assumed to correlate with coal rank. There is a
relationship between gas content and depth for each
rank coal category. Furthermore, sorption capacity
increases with progressive coalification.
The investigated coal seams for CBM purpose located
in the Muara Lakitan, South Sumatra based on the
vitrinite reflectance, categorized as a subbituminous
to high volatile bituminous-A coal rank. Furthermore,
commonly, the coal seams are characterized by low
ash and moderate sulfur contents. Due to the level of
vitrinite reflectance values of coal tending to
thermally immature (Rv: 0.44 – 0.45%), the
expected gas present is suggested to be of biogenic
origin. The coalbed gas level category is indicated by
the presence of dull to bright banded lithotype;
maceral composition dominated by vitrinite with
minor content of exinite and inertinite; moderate
moisture content; moderate to slightly high volatile
matter; low to medium vitrinite reflectance, and low
ash content.
A coal rank. Methane content of the coal seam is
0.57 m3/t – 1.70 m3/t = 20.44 scf/t – 60.96 scf/t.
Coal Cleats of each coal field is as followed: the dip
direction of coal face cleat varies from N160°E/80° to
N330°E/50°; space ranges between 0.2 cm to 19
cm, averaged cm; aperture of 1 to 8 mm, frequency is
0.239 cm -1 to 1.69 cm -1, and density of 0.0099 cm
to 0.21/cm . Coal bed methane content of the coal
seam, based on the Barbara-Winter Diagram, ranges
from 0.57 m3/t – 1.70 m3/t = 20.44 scf/t – 60.96
scf/t. This character indicates an in-situ coal have a
low to moderate methane content. Gas in-place
reserved in six coal seams supported by the Q1; Q2
and Q3 calculations show a calculated varieties value
15.524,28 scf.
The coal bed gas level category is indicated by the
presence of dull to bright banded lithotype; maceral
composition dominated by vitrinite with minor
content of exinite and inertinite; moderate moisture
content; moderate to slightly high volatile matter; low
to medium vitrinite reflectance, and low ash content.
The SEM analysis displays that the coal is dominated
by vitrinite maceral, with minor exinite and inertinite.
The microcleat occurs in rare to medium density, and
shows an opened texture.
90
80
Vitrinite
It can be summarized, that coalbed gas in-place
contents derived from the Muara Lakitan area, coal
seams expected to be low - moderate level. However,
in the Muara Lakitan area, based on Barbara-Winter
diagram, the content of methane (CH4) within
coalbed gas ranges from 20.44 scf/t – 60.96 scf/t,
whilst according the formula, the total reserve of gas
in reservoir in the investigated area (six coal seam) is
15.524,28 scf.
70
60
50
40
30
1,0
2,0
3,0
Ash
Vitrinite vs Ash
Linear (Vitrinite vs Ash)
Figure 5. Relationship between percentages vitrinite and ash of coal
from Muara Lakitan, Musi Rawas, South Sumatra.
Conclusions
90
156
Vitrinite
80
The coal quality, gained from geochemical analysis,
indicates that its ash content ranges between 1.22 –
2.47 %, total sulphur content is from 0.15 – 0.3 %,
and volatile matter of 38.02% - 40.81%. The
dominant maceral is vitrinite (73.6 – 85.8 %), with
minor amount of exinite (1.4 – 4.0 %), inertinite (4.2
– 21 %) and mineral matter (2.4 – 8.2 %). Vitrinite
reflectance having a value of 0.44 – 0.45 %, tends to
indicate a subbituminous to high volatile bituminous-
70
60
50
40
30
20,0
25,0
30,0
Moisture
Vitrinite vs Moisture
Linear (Vitrinite vs Moisture)
Figure 6. Relationship between percentages vitrinite and moisture of
coal from Muara Lakitan, Musi Rawas, South Sumatra.
Geo-Resources
30
25
Inertinite
20
15
10
5
0
90
-5
80
Vitrinite
35,0
37,0
39,0
70
41,0
43,0
45,0
Volatile Matter (VM)
60
Inertinite vs VM
Linear (Inertinite vs VM)
50
40
30
30,0
32,0
34,0
36,0
38,0
40,0
42,0
Volatile Matter (VM)
Series1
Figure 9. Relationship between percentages inertinite and volatile
matter of coal from Muara Lakitan, Musi Rawas, South
Sumatra.
Linear (Series1)
88,0
30
86,0
84,0
Vitrinite
Figure 7. Relationship between percentages vitrinite and volatile
matter of coal from Muara Lakitan, Musi Rawas, South
Sumatra.
80,0
78,0
76,0
25
74,0
20
Inertinite
82,0
72,0
15
0,00
0,25
0,50
0,75
1,00
1,25
1,50
1,75
2,00
10
CH4
5
Vitrinite Vs CH4
0
Linear (Vitrinite Vs CH4)
-5
0,0
1,0
2,0
3,0
4,0
Ash
Inertinite vs Ash
Linear (Inertinite vs Ash)
Figure 10. Relationship between percentages vitrinite and CH4 of coal
Figure 8. Relationship between percentages inertinite and ash of coal
from Muara Lakitan,Musi Rawas, South Sumatra.
25,0
Inertinite
20,0
Acknowledgments
15,0
10,0
5,0
The authors thank the Head of Geological Survey
Institute and Head of Research Group on Basin
Dynamics for supporting to publish this paper. The
authors are greatly indebted to Dr. Nana Suwarna,
and Ivan Sofyan Suwardi as partners during
fieldwork.
0,0
0,00
0,20
0,40
0,60
0,80
1,00
1,20
1,40
1,60
1,80
CH4
Inertinite Vs CH4
Linear (Inertinite Vs CH4)
Figure 11. Relationship between percentages inertinite and CH4 of coal
References
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Sector Development Plan (Part B).
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Indonesia Petroleum Association. Proceedings of the 16th Annual Convention, Jakarta, 1, 399 – 426.
Darman H., dan F. Hasan Sidi, 2000. An Outline of The Geology Indonesia: Indonesian Association of Geologists,
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De Coster, G.L. (1974): The Geology of the Central and South Sumatra Basins. Proceedings of Indonesian
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157
Geo-Sciences
DELINEASI CEKUNGAN BUSUR MUKA SIMEULUE BERDASARKAN DATA ANOMALI
GAYA BERAT
L.D.Santi, I. Setiadi, H. Panggabean
Pusat Survei Geologi
Jl. Diponegoro No 57 Bandung - 40122
Sari
Penelitian cekungan busur muka di barat Pulau Sumatra telah banyak dilakukan untuk mengetahui potensinya sebagai
cekungan pembawa hidrokarbon. Cekungan Simeulue sebagai bagian dari rangkaian cekungan busur muka, baru-baru ini
banyak disinggung sebagai hasil publikasi terbaru dari penelitian team BGR-Jerman yang memperlihatkan adanya
akumulasi sedimen yang relatif tebal. Meskipun banyak informasi baru dan juga spekulasi yang timbul dari hasil
penelitian tersebut, namun batas – batas Cekungan Simeulue sendiri belum diketahui secara pasti. Delineasi Cekungan
Simeulue pada penelitian ini dilakukan dengan menggunakan data anomali gaya berat yang mampu mencakup wilayah
yang lebih luas, sehingga penentuan batas pelamparan sedimen dapat dilakukan dengan lebih pasti. Hasil pembuatan
profil gaya berat menggunakan metode forward modelling memperlihatkan geometri cekungan yang melampar dengan
panjang maksimum 418 km, bagian utara dan selatannya dibatasi oleh tinggian yang memisahkannya dengan cekungan
– cekungan lain yang berdekatan.
Kata kunci : Delineasi cekungan, Anomaly gaya berat
Abstract
The study of hydrocarbon potential at the fore-arc basins of west offshore Sumatra has been done by many writers.
Recently, Simeulue basin, has been put at the spotlight as the Germany's BGR research team published several latest
seismic lines showing great sediment accumulation in this basin. Although many new information and speculation
emerge by the result of that research, until now the exact basin boundary itself has never been discussed. Delineation
process of Simeulue basin in this study is done by applying gravity anomaly data that covers a wide area enough to
determine the apparent boundary of sediment distribution within the basin. A gravity anomaly profile is produced using
forward modeling method, showing the geometry of the basin, with maximum length of 418 km. The northward and
southward of this basin is bounded by high topographies that separate Simeulue basin with the other adjacent basins.
Keywords : Basin delineation, Gravity anomaly
Pendahuluan
Cekungan Simeulue merupakan bagian dari
rangkaian cekungan busur muka di depan zona
subduksi Sumatra yang memanjang dengan arah
barat laut – tenggara. Kegiatan penelitian dan
pembuatan profil seismik telah dilakukan oleh tim
peneliti BGR, tahun 2006, yang bekerja sama
dengan BPPT. Hasilnya telah mengungkapkan
bahwa cekungan ini memiliki sedimen tebal dan
diduga berpotensi sebagai cekungan pembawa
hidrokarbon.
Penelitian terdahulu di daerah ini belum
mengungkapkan dengan tepat nama resmi maupun
batas cekungan yang dianggap sebagai bagian dari
Naskah diterima :
Revisi terakhir :
15 Maret 2010
25 Juni
2010
Cekungan Simeulue. Secara umum cekungan ini
terletak di antara 2°LU – 4.5°LU dan 95°BT – 98°BT
yang berada di sebelah barat Pulau Sumatra. Barber
dan Crow, (dalam Barber et al., 2005) menyebut
cekungan ini sebagai Cekungan Meulaboh,
sedangkan Rose, 1983, dan Syam drr., 2007,
menyebutkan sebagai bagian dari Cekungan Sibolga.
Penelitian-penelitian tersebut umumnya memakai
metode korelasi seismik dan data log. Namun, profil
seismik dan sebaran titik sumur pemboran yang ada
di daerah ini kurang memadai untuk delineasi
cekungan. Panjang profil seismik yang terbatas
hanya memberikan sebagian citra cekungan. Oleh
karena itu dalam penelitian ini digunakan data
anomali gaya berat dengan cakupan data yang lebih
regional untuk memperkirakan geometri dan batasbatas cekungan secara keseluruhan.
159
Geo-Sciences
Tujuan penelitian ini adalah untuk mempelajari
bangun struktur kerak bumi yang melandasi
Cekungan Simeulue dan batas-batas pelamparannya
terhadap cekungan lainnya yang berdekatan.
Telford drr. (1990). Model gaya berat yang dihasilkan
merupakan hasil susunan poligon-poligon dari
kelompok-kelompok rapat massa batuan sebagai
berikut :
Metode
1. Kelompok rapat massa 1.03 gr/cc adalah massa
air laut di sekitar Pulau Simeulue perairan
sekitarnya.
Data anomali gaya berat diperoleh melalui dua cara,
yaitu anomali gaya berat di wilayah darat (onshore)
yang merupakan kompilasi dari peta-peta anomali
gaya berat yang diterbitkan oleh Pusat Survei
Geologi/ Pusat Penelitian dan Pengembangan
Geologi dengan skala 1:250.000 (Nainggolan dan
Tasno, 2000; Lelitoly drr., 2000; Sjarif drr., 2000;
Indragiri dan Setiadi, 2007; Mirnanda drr., 2007;
Nasution dan Indragiri, 2007; Setiadi dan Mirnanda,
2007; Setiadi dan Nasution, 2007. Sementara data
anomali gaya berat di perairan (offshore) diperoleh
dari data anomali gaya berat bebas udara (Free Air
Anomaly). Dari hasil kompilasi diperoleh peta
anomali gaya berat dengan interval kontur 12.5
mgal. Variasi nilai anomali gaya berat yang
terpampang merupakan cerminan dari perubahan
ketebalan sedimen di dalam cekungan, kompensasi
isostasi dari akar pegunungan atau deposentrum
yang lebih tua, ataupun karena variasi litologi di
daerah tertentu.
Tiga lintasan anomali gaya berat dibuat melintang
barat laut – tenggara, tiga lintasan lainnya dibuat
melintang barat daya - timur laut. Lintasan – lintasan
tersebut dibuat saling berpotongan dengan tujuan
untuk memperoleh bidang saling berhubungan
dengan titik-titik pengontrol satu lintasan terhadap
lintasan lainnya sebagai kerangka dasar untuk
membangun geometri cekungan. Penampang
anomali Bouger pada masing – masing lintasan
diperoleh dengan mendigitasi nilai kontur di
s e p a n j a n g l i n t a s a n. Fo r w a r d M o d e l l i n g
menggunakan perangkat lunak (software) geofisika
dilakukan terhadap keenam penampang untuk
memperoleh profil geometri bawah permukaan.
Metode ini menggunakan data anomali Bouger
sebagai pengendali utama. Penampang seismik dari
data BGR (BGR 135, BGR 136, BGR 137, dan BGR
139) digunakan pula sebagai panduan bagi
penentuan bentuk geometri poligon dari kelompokkelompok rapat massa batuan.
Dalam analisis dan pemodelan untuk membuat profil
bawah permukaan, digunakan nilai rapat masa dari
160
2. Kelompok rapat massa 3,07 gr/cc merupakan
rapat massa dari mantel bagian atas.
3. Kelompok rapat massa 2,77 gr/cc merupakan
rapat massa kelompok material yang menyusun
kerak samudra (basaltic layer). Rapat massa ini
diperoleh berdasarkan rata-rata rapat massa
material penyusun kerak samudra secara umum,
yang terdiri atas batu-batuan ultrabasa, batubatuan malihan derajad tinggi, dan sedimensedimen pelagos.
4. Kelompok rapat massa 2,67 gr/cc adalah
kelompok material penyusun kerak benua berupa
batuan granitik.
5. Kompleks akresi yang tersusun atas percampuran
batuan malihan, batuan vulkanik, sedimen halus,
dan sedimen pelagos diberi harga rapat massa
2,60 gr/cc.
6. Nilai dari kelompok outer arc ridge dianggap sama
dengan nilai rapat massa Pulau Simeulue, yaitu
2,50 gr/cc. Pulau ini tersusun atas bantuan
malihan derajat rendah aneka bahan (filit,
batusabak, metasedimen), baturijang, gabro,
batugamping, konglomerat aneka bahan,
batupasir, napal dan batulempung (Endharto dan
Sukido, 1995) .
7. Kelompok rapat massa 2,15 gr/cc diberikan untuk
rata-rata batuan sedimen pengisi cekungan. Nilai
rapat massa batuan sedimen umumnya adalah
2,20 gr/cc. Namun sedimen di cekungan muka
busur umumnya banyak dipengaruhi oleh
aktivitas vulkanisme yang bersifat menurunkan
nilai rapat massa batuan (Setyanta dan Setiadi,
2007). Oleh karena itu, nilai 2,15 gr/cc dianggap
lebih mewakili sifat pelamparan batuan sedimen
dalam cekungan Simeulue.
8. Kelompok rapat massa 2,40 gr/cc diberikan untuk
batuan penyusun Pulau Sumatra yang terdiri atas
kerak granitik yang telah terfragmentasi, batuan
malihan Pre-Tersier, batuan vulkanik, dan batuan
sedimen.
Geo-Sciences
°
°
°
°
°
°
°
H
CE
K.A
CE
Area Penelitian
Si
r
sa
Ban
ee
Bat
r naik
Se
i
E
wa
LU
ta
ar
Ses
U
ME
en
M
Sesa
m
ste
SI
r
sa
K.
Se
CE
°
Su
yak
ra
at
m
°
P. Banyak
KE
NG
SI
K.
CE
L
gS
lun
Pa
i
ks
du
ub
°
P. Batu
Gambar 1. Struktur regional Pulau Sumatra dengan perkiraan pelamparan cekungan Simeulue (dimodifikasi dari Barber et al., 2005).
Delineasi Cekungan dari Profil Gaya Berat
Bagian barat daya peta merupakan wilayah perairan
yang didominasi oleh nilai anomali positif yang
berkisar antara + 50 mgal hingga +280 mgal. Ke
arah timur laut nilai anomali menunjukkan
penurunan hingga mencapai -170 mgal. Tiga profil
gaya berat yang dibuat dari lintasan tegak lurus
terhadap arah jurus jalur anomali (lintasan GH, IJ,
dan KL, Gambar 2) menunjukkan bahwa rata- rata
nilai anomali positif tinggi dihasilkan oleh posisi
Moho yang relatif dangkal di zona subduksi, di bawah
fragmen kerak benua dan sedimen penutup tipis.
Penurunan nilai gaya berat semakin besar ke arah
timur laut seiring bertambahnya sudut dan
kedalaman penunjaman, menebalnya kerak benua,
dan menebalnya sedimen penutup di atasnya.
Tiga profil gaya berat lainnya dibuat sejajar dengan
jalur subduksi (lintasan AB, CD, dan EF, Gambar 2).
Tujuan profil tersebut adalah untuk memperoleh arah
sumbu panjang cekungan busur muka. Nilai anomali
pada ketiga lintasan umumnya berkisar antara +55
mgal hingga -20 mgal. Hasil pembuatan profil AB,
CD, dan EF menunjukkan adanya perubahan
ketebalan kerak benua dan kedalaman Moho yang
membesar seiring makin menjauhnya dari jalur
tunjaman (trench) (Gambar 3). Namun demikian,
ketebalan kerak relatif konstan di sepanjang profil itu
sendiri, menandakan bahwa kerak tidak mengalami
penipisan (thinning) atau penarikan (stretching)
seperti umumnya terjadi pada daerah rifting di busur
belakang. Dengan adanya fakta ini maka
pembentukan cekungan di daerah busur muka
Simeulue diperkirakan berasal dari gaya lepasan
(release tension) sebagai pengaruh gaya-gaya
regional yang bekerja di daerah ini.
Hasil pembuatan profil gaya berat dan peta ketebalan
sedimen (Gambar 5) menunjukkan bahwa cekungan
Simeulue memanjang dengan arah barat laut –
tenggara, mencapai panjang maksimum 418 km,
sedangkan lebar cekungan sebesar 140 km ke arah
utara, dan menyempit ke arah selatan sebesar 110 –
115 km. Cekungan ini dibatasi pada bagian utaranya
oleh tinggian Meulaboh (Gambar 2), yang
memisahkan Cekungan Simeulue dengan Cekungan
Aceh. Pada bagian selatan cekungan ini dibatasi oleh
Sesar Banyak, yang menciptakan tinggian yang
memisahkannya dengan Cekungan Singkel.
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Geo-Sciences
Penjelasan terperinci setiap profil yang dibuat adalah
sebagai berikut :
Lintasan AB
Lintasan AB dibuat dengan arah barat laut –
tenggara, sejajar sumbu panjang cekungan sedimen,
dengan panjang total 418 km. Lintasan ini diapit
oleh lintasan CD dan EF. Hasil pembuatan profil AB
menunjukkan kedalaman Moho 35 km. Kerak benua
pada lintasan AB menunjukkan ketebalan konstan
sebesar 25 km. Di atas, kerak benua menunjukkan
adanya perkembangan dua deposentrum yang
dipisahkan oleh suatu tinggian antara jarak 226 –
322 km di sepanjang lintasan. deposentrum
pertama, disebut sebagai D1, melampar sepanjang
250 km, dengan ketebalan rata-rata sedimen
sebesar 5,1 km. deposentrum kedua, disebut
sebagai D2 yang terbentuk sejauh 91 km, dengan
ketebalan sedimen mencapai 7,9 km. Ketebalan
sedimen pada D2 merupakan ketebalan sedimen
terbesar di Cekungan Simeulue.
Lintasan CD
Lintasan CD yang relatif sejajar di sebelah barat dari
lintasan AB, merupakan lintasan paling barat
terhadap posisi jalur subduksi. Lintasan ini memiliki
panjang 300 km, namun demikian lintasan ini hanya
melingkupi jarak 202 km dari bagian deposentrum
D1, sedangkan sisa lintasan ini melewati tinggian
yang meluas di arah barat (Gambar 2 dan Gambar 3).
Kedalaman Moho pada lintasan ini adalah 30 km,
sedangkan kerak benua mencapai ketebalan sekitar
23 km. Ketebalan sedimen rata-rata dari D1 pada
lintasan ini sebesar 3,8 km, dengan ketebalan
maksimum mencapai 4,9 km.
Lintasan EF
Lintasan EF memiliki panjang total 306 km, berada
paling timur dari ketiga lintasan yang sejajar dengan
jalur subduksi. Pada lintasan ini Moho terletak pada
kedalaman 45 km, dan ketebalan kerak benua
hingga alas cekungan setebal 34 km. Pada lintasan
ini tinggian yang memisahkan dua deposentrum
hanya melampar sepanjang 12 km, membatasi
deposentrum D1 sepanjang 175 km dan
deposentrum D2 147 km. Makin ke timur
pelamparan tinggian makin menyempit hingga
akhirnya dua deposentrum bergabung menjadi satu
(Gambar 2). deposentrum D1 pada lintasan ini
memiliki ketebalan rata-rata sedimen sebesar 4,5
162
km, dengan ketebalan terbesar mencapai 5,9 km.
deposentrum D2 memiliki rata-rata ketebalan 4,8
km, dengan ketebalan sedimen terbesar hingga 6,4
km.
Lintasan GH
Lintasan GH dibuat dengan arah barat daya – timur
laut, dengan panjang total 213 km. Lintasan ini tegak
lurus terhadap jalur penunjaman, sehingga profil
lintasan ini dapat menggambarkan perubahan
geometri bawah permukaan dari kompleks akresi
hingga busur magmatik. Lintasan GH memotong
hampir tegak lurus lintasan AB, CD, dan EF, dan
merupakan lintasan yang dibuat paling selatan di
daerah penelitian.
Profil lintasan GH menunjukkan adanya perubahan
sudut penunjaman dari arah barat ke timur. Sudut
penunjaman di bagian barat (mendekat ke arah
trench) memiliki besar sudut penunjaman 16° hingga
20°. Ke arah timur (menjauhi trench) sudut
penunjaman makin besar, dan mencapai sudut 27°.
Pada lintasan ini geometri cekungan sedimen pada
sumbu barat daya – timur laut (sumbu pendek)
memiliki lebar hingga 115 km. Sedimen menebal ke
arah timur laut, dengan ketebalan terbesarnya
mencapai 6,9 km. Diduga cekungan masih
melampar hingga daratan. Ketebalan sedimen pada
poligon Pulau Sumatra yang dilewati lintasan ini
mencapai 23 km. Ketebalan yang relatif besar ini
merupakan hasil kompensasi isostasi saat profil gaya
berat melewati pegunungan.
Lintasan IJ
Lintasan IJ terletak di sebelah utara lintasan GH, dan
memiliki panjang total 222 km. Pada lintasan ini
sudut penunjaman lebih mendatar dibandingkan
pada lintasan GH, yaitu 12° hingga 16° di bagian
yang mendekati trench, dan mencapai 20° ke arah
busur magmatik.
Geometri cekungan sedimen pada lintasan ini
lebarnya mencapai 125 km, dengan ketebalan ratarata sedimen sebesar 4,4 km. Bagian profil yang
melewati pulau Sumatra pada lintasan ini
menunjukkan ketebalan sedimen 6,5 km. Tebal
sedimen Pulau Sumatra yang jauh lebih tipis
dibandingkan terhadap profil GH ini disebabkan tidak
terpengaruhnya profil oleh kompensasi isostasi,
karena profil tidak melewati akar pegunungan.
Geo-Sciences
Lintasan KL
Kesimpulan
Lintasan KL berada paling utara di area penelitian,
dengan panjang total 220 km. Lintasan ini
memperlihatkan bahwa geometri cekungan makin
melebar ke arah utara, mencapai 140 km. Ketebalan
maksimum sedimen yang terlintasi mencapai 5,3
km, menipis ke arah daratan Pulau Sumatra. Profil
bagian Pulau Sumatra yang dilewati oleh lintasan KL
hanya memiliki ketebalan sedimen sebesar 1,5 – 3
km. Tipisnya sedimen yang terbentuk di bagian utara
ini diduga karena erosi yang lebih intensif, sebagai
akibat lebih terangkatnya bagian utara Pulau
Sumatra saat fase kompresional berlangsung di
daerah ini.
Cekungan Simeulue merupakan cekungan busur
muka yang memiliki geometri cukup besar. Cekungan
ini memiliki sumbu panjang maksimum mencapai
418 km dan sumbu lebar maksimum 140 km.
Geometri cekungan ini melebar ke arah utara (140
km) dan menyempit ke arah selatan (115 km).
Tinggian yang terbentuk dengan arah timur laut –
barat daya di depan Pulau Simeulue telah
menyebabkan terbentuknya dua deposentrum dalam
cekungan ini. deposentrum D1 memiliki sumbu
panjang maksimum 250 km, sumbu lebar
maksimum 140 km dan ketebalan sedimen terbesar
5,9 km. deposentrum D2 memiliki sumbu panjang
maksimum 91 km, sumbu lebar maksimum 60 km,
dan ketebalan sedimen terbesar 7,9 km.
Geometri subduksi pada lintasan paling utara ini juga
memperlihatkan makin mendangkal, dengan sudut
penunjaman 12° hingga 15°.
Penelitian yang lebih terperinci mengenai potensi
cekungan perlu dilakukan, bukan hanya pada bagian
utara cekungan yang menjadi bagian D1, tetapi juga
pada deposentrum D2 yang lebih kecil dengan tebal
sedimen yang paling besar.
mgal
U
h
an Ace
Cekung
°
km
Skala
0
50
100
labo
Meu
batas cekungan Simeulue
°
gh
h Hi
A
°
L
E
J
C
°
Ce
D1
°
ku
ng
an
K
H
Si
m
eu
le
u
°
I
basement high/
carbonate plarform
Sim
eu
lue
°
F
D2
D
G
°
°
°
°
°
B
°
°
°
°
°
Gambar 2. Peta anomali gaya berat daerah Simeulue dan sekitarnya (Sumber : kumpulan peta anomali gaya berat terbitan P3G/ PSG periode 2000 –
2007 dan Free Air Gravity).
163
Geo-Sciences
mGal
55.0
45.0
35.0
25.0
15.0
5.0
-5.0
-15.0
-25.0
C
D
= calc
= obs
-20
20
100
60
140
180
220
260
300
J a r a k (km)
D1 (202 km)
5.0
IJ
KL
2.15 gr/cc
4.6 km
SE
GH
.0
4.9 km
5.0 km
-5.0
-10.0
basement high/
carbonate platform (?)
2.67 gr/cc
-15.0
continental crust
-20.0
2.77 gr/cc
oceanic crust
3.07 gr/cc
upper mantle
-25.0
Moho
Kedalaman (km)
NW
-30.0
-35.0
C
D
-40.0
mGal
40.0
A
20.0
.0
-20.0
B
-40.0
= cal
-60.0
= obs
-80.0
0
-50
50
100
150
200
250
300
350
400
450
J a r a k (km)
D2 (91 km)
D1 (250 km)
KL
IJ
2.15 gr/cc
5.0 km
5.0
SE
GH
.0
5.3 km
7.9 km
-5.0
basement high/
carbonate platform (?)
-10.0
-15.0
2.67 gr/cc
continental crust
-20.0
-25.0
Kedalaman (km)
NW
-30.0
2.77 gr/cc
oceanic crust
3.07 gr/cc
upper mantle
Moho
-35.0
A
B
-40.0
mGal
E
= calc
= obs
-20
F
60
20
100
140
180
220
260
-45.0
45.0
35.0
25.0
15.0
5.0
-5.0
-15.0
-25.0
-35.0
300
J a r a k (km)
NW
D2 (147 km)
IJ
KL
4.6 km
5.9 km
5.0
SE
GH
2.15 gr/cc
6.4 km
-5.0
-15.0
2.67 gr/cc
continental crust
2.77 gr/cc
oceanic crust
3.07 gr/cc
upper mantle
-25.0
-35.0
Moho
E
Kedalaman (km)
D1 (175 km)
-45.0
F
-55.0
Gambar 3. Profil geometri bawah permukaan dari hasil pemodelan pada lintasan-lintasan gaya berat AB, CD, dan EF (lokasi lintasan dari barat ke timur
pada gambar 2).
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Geo-Sciences
mGal
100
60
20
-20
-60
= calc
= obs
-100
H
-140
-20
0
20
60
100
140
180
220
J a r a k (km)
115 km
SW
CD
2.6
2.60 gr/cc
accretionary wedge
2.50 gr/cc
Simeulue Is.
AB
2.15
2.5
NE
EF
.0
2.4
2.40 gr/cc
2.15 gr/cc
6.9 km
-10.0
2.67
2.77
G
3.07
continental crust
Kedalaman (km)
2.67 gr/cc
2.77
-20.0
gr/c
c
oce
gr/c3.07
c
anic
-30.0
crus
t
upp
er m
antl
-40.0
Mo
e
ho
-50.0
-60.0
H
mGal
110
I
90
70
50
30
= calc
= obs
10
J
-10
0
-20
20
60
100
140
180
220
J a r a k (km)
125 km
0.0
4.5 km
4.8 km 2.15 gr/cc
5.0
NE
EF
AB
CD
2.50 gr/cc
accretionary wedge
2.40 gr/cc
-5.0
-15.0
2.77 g
r/cc
3.07 g
I
r/cc
upper
2.67 gr/cc
ocean
continental crust
-25.0
ic cru
mantl
e
st
-35.0
Kedalaman (km)
SW
Moh
o
-45.0
-55.0
J
mGal
100
60
20
-20
-60
= calc
= obs
-100
H
-140
-20
0
20
60
100
140
180
220
J a r a k (km)
115 km
SW
CD
2.6
2.60 gr/cc
accretionary wedge
2.50 gr/cc
Simeulue Is.
AB
2.15
2.5
NE
EF
2.15 gr/cc
6.9 km
.0
2.4
2.40 gr/cc
G
2.77
3.07
continental crust
-20.0
gr/c
c
gr/c3.07
c
oce
anic
-30.0
crus
t
upp
er m
antl
e
Kedalaman (km)
-10.0
2.67
2.67 gr/cc
2.77
-40.0
Mo
ho
-50.0
-60.0
H
Gambar 4. Profil geometri bawah permukaan dari hasil pemodelan pada lintasan-lintasan gaya berat GH, IJ dan KL (lokasi lintasan dari utara ke selatan
pada gambar 2).
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Geo-Sciences
Gambar 5. Peta pola penyebaran ketebalan sedimen dalam cekungan Simeulue.
Ucapan terima kasih
Penulis mengucapkan terima kasih kepada Kepala
Pusat Survei Geologi yang telah memfasilitasi
kegiatan penelitian yang kami lakukan. Penulis juga
berterima kasih pada tim geofisika Pusat Survei
Geologi yang telah menyediakan data gaya berat
yang digunakan dalam penelitian ini. Ucapan terima
kasih ditujukan pula untuk tim penelitian cekungan
Simeulue dari Lemigas yang telah banyak
membantu dalam penyediaan data pendukung serta
dalam diskusi mengenai Cekungan Simeulue.
Acuan
Barber, A.J., Crow, M.J., Milsom, J.S. (ed.), 2005. Sumatra, Geology, Resources, and Tectonic Evolution.
Geological Society Memoir No. 31, Geological Society, London.
Syam Beiruny, Permana Wahyu, Perdana Aulia, Tiggi Choanji, 2007. The petroleum system of Sibolga basin
based on correlation seismic and well log data, Proceedings of Indonesian Petroleum Association Annual
Convention.Thirty-First Annual Convention and Exhibition , May 2007
BGR Research Team, 2006. Geo-Risk Potential Along The Active Convergence Zone Between The Eastern
Eurasian and Indo Australian Plates of Indonesia, Cruise Report Sonne Cruise SO-186-2 SeaCause II,
Unpublished Report.
Endharto, M. dan Sukido, 1995. Peta Geologi lembar Sinabang, Sumatra. Skala 1:250.000. Pusat Penelitian
dan Pengembangan Geologi, Bandung.
Indragiri, N.M., Setiadi, I., 2007. Peta Anomali Bouger Lembar Calang, Sumatra. Skala 1:250.000. Pusat
Survei Geologi, Bandung
Lelitoly, D., Ermawan, T., Buyung, N., 2000. Peta Anomali Bouger Lembar Pematangsiantar, Sumatra. Skala 1
: 250.000. Pusat Penelitian dan Pengembangan Geologi, Bandung
166
Geo-Sciences
Mirnanda, E., Hayat, D.Z., Setiadi, I., Indragiri, N.M., 2007. Peta Anomali Bouger Lembar Takengon, Sumatra.
Skala 1: 250.000. Pusat Survei Geologi, Bandung
Nainggolan, D.A., Tasno, D.P., 2000. Peta Anomali Bouger Lembar Sidikalang, Sumatra, Pusat Survei Geologi.
Skala 1: 250.000. Pusat Penelitian dan Pengembangan Geologi Bandung
Nasution, J., Indragiri, N.M., 2007. Peta Anomali Bouger Lembar Tapaktuan, Sumatra. Skala 1 : 250.000.
Pusat Survei Geologi, Bandung.
Rose Robert, 1983. Miocene carbonate rocks of Sibolga basin northwest Sumatra, Proceeding Indonesian
Petroleum Association Annual Convention. Twelfth Annual Convention, June 1983, p. 107-125
Setiadi, I., Mirnanda, E., 2007. Peta Anomali Bouger Lembar Langsa, Sumatra. Skala 1: 250.000 Pusat Survei
Geologi, Bandung
Setiadi, I., Nasution, J., 2007. Peta Anomali Bouger Lembar Sinabang, Sumatra. Skala 1:250.000. Pusat
Survei Geologi, Bandung
Setyanta, B., Setiadi, I., 2007. Anomali Gaya Berat dan Tataan Tektonik Sekitar Perairan Laut Banda dan Pulau
Seram, Jurnal Sumber Daya Geologi Vol. XVII No.6, Pusat Survei Geologi, Bandung
Sjarif, N., Tasno, D.P., Manurung, A., Widijono, B.S., Mirnanda, E., 2000. Peta Anomali Bouger Lembar Medan,
Sumatra. Skala 1:250.000. Pusat Penelitian dan Pengembangan Geologi, Bandung
Telford, W.M., Geldart, L.P., Sheriff, R.E., 1990, Applied Geophysics (2nd Edition), Cambridge University Press,
London
167
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STRATIGRAFI DAN SEDIMENTOLOGI ENDAPAN DATARAN PASANG-SURUT DI KALI
TULIS, BANJARNEGARA
S. Bachri, E. Slameto dan I. Nurdiana
Jl. Diponegoro No.57 Bandung 40122
Sari
Endapan dataran pasang-surut Formasi Merawu di Kali Tulis termasuk ke dalam Formasi Merawu, dan dapat dibagai
menjadi dua bagian atau anggota, yaitu bagian bawah atau anggota batulumpur, dan bagian atas atau anggota batupasir,
yang batas antara keduanya bersifat gradasional. Bagian bawah dikuasai oleh batulumpur dan sebagian besar
diendapkan pada lingkungan dataran lumpur (mud flat), sementara bagian atas dikuasai oleh batupasir yang sebagian
besar diendapkan pada dataran pasir (sand flat). Beberapa lapisan batupasir dan batulempung di bagian bawah
mengandung foraminifera yang menunjukkan umur Miosen Awal – Miosen Tengah, sedangkan pada bagian atasnya
sejauh ini belum dijumpai fosil. Pada kedua anggota dijumpai struktur sedimen tulang ikan, nendatan berskala kecil,
perarian sejajar dan silang-siur, bioturbasi, flute cast, bed load, serta penghalusan butir ke atas. Batupasir pada kedua
satuan tersebut banyak mengandung fragmen batuan gunung api yang mengindikasikan bahwa batuan asalnya adalah
batuan gunung api dalam busur gunung api.
Kata kunci : dataran pasir (sand flat), dataran lumpur (mud flat), struktur tulang ikan, Formasi Merawu
Abstract
The tidal flat deposits of the Merawu Formation in Kali Tulis belongs to the Merawu Formation, and can be divided into
two members, i.e. the lower part or the mudstone member, and the upper part or the sandstone member, with
gradational contact between the two members. The lower parts is dominated by mudstone which is mostly deposited in
mud flat environment, while the upper part is dominated by sandstone which is dominantly deposited in sand flat
environment. Some sandstone layers in the lower part contains foraminifers of Early – Middle Miocene age, whereas the
occurrence of fossils in the upper part has not been known so far. Sedimentary structures found in the two members
include herring-bone structure, small scaled slump, parallel and cross laminations, bioturbation, flute cast, bed load,
and fining upwards grain size. The sandstone of the two members contains volcanic rock fragments abundantly,
suggesting that the provenance is volcanic rock in volcanic arc.
Keywords: sand flat, mud flat, herring bone structure, Merawu Formation
Pendahuluan
Pengukuran stratigrafi pada runtunan batuan
sedimen Neogen di sepanjang Kali Tulis di Desa
Sokaraja, Banjarnegara, telah menyingkap bahwa
batuan yang dikenal sebagai Formasi Merawu (Van
Bemmelen, 1949; Condon drr., 1975) atau Formasi
Rambatan (Condon drr.,1996) di lintasan tersebut
ternyata sebagian besar mengindikasikan sebagai
endapan dataran pasang-surut, bukan sebagai
anggota turbidit seperti yang ditafsirkan oleh
beberapa penulis sebelumnya. Di samping endapan
pasang-surut, juga dijumpai endapan laut dangkal,
khususnya di bagian bawah runtunan, yang dicirikan
oleh adanya batulempung gampingan yang
mengandung fosil foraminifera.
Naskah diterima :
Revisi terakhir :
12 Maret 2010
25 Juni 2010
Berdasarkan analisis laboratorium (paleontologi)
dapat ditentukan umur anggota tersebut, sedangkan
dari analisis petrografi dapat dikenali adanya
kandungan material vulkanik yang cukup melimpah.
Pengamatan struktur sedimen beserta fosil jejak di
lapangan sangat penting untuk mengenali jenis
lingkungan pengendapannya.
Pengukuran stratigrafi endapan dataran pasang surut
di Kali Tulis dimulai dari sebelah bawah bendungan
Kali Tulis sampai di dekat muara Kali Sidawangi
(Gambar 1). Kondisi singkapan cukup bagus,
terutama di bagian hulu.
Ciri Litologi Dan Struktur Sedimen
Berdasarkan pengukuran stratigrafi terperinci skala
1:100 di lintasan Kali Tulis, Formasi Merawu dapat
169
Geo-Sciences
dibagi menjadi dua anggota, yaitu anggota
batulumpur di bagian bawah, dan anggota batupasir
di bagian atas. Bagian bawah runtunan dikuasai oleh
batulumpur (batulanau dan terutama batulempung),
dan bagian atas dikuasai oleh lapisan-lapisan
batupasir berbutir sedang (Gambar 2, 3 dan 4).
Batas antara anggota batulumpur dan anggota
batupasir bersifat gradasional, yang ditunjukkan
oleh semakin banyak dan semakin tebalnya lapisan
batupasir ke arah atas. Batas yang sifatnya
gradasional ini mengindikasikan hubungan selaras
tanpa adanya perbedaan atau perubahan umur
maupun lingkungan pengendapan secara mencolok
atau tiba-tiba.
Anggota Batulumpur
Anggota batulumpur terdiri atas batulempung dan
batulanau yang diselingi batupasir dan setempat
batupasir konglomeratan, dengan perbandingan
batupasir/batulumpur sekitar 70:30 (Gambar 5).
Batupasir umumnya tipis-tipis kurang dari 30 cm,
hanya di beberapa tempat bersifat konglomeratan
dan mencapai tebal 50 -90 cm (Gambar 6).
Setempat dijumpai konglomerat terpilah buruk,
dengan perlapisan bersusun, dengan komponen
batuan andesitan, mencapai tebal 90 cm. Di
beberapa tempat, batupasir dan batulumpur bersifat
gampingan dan mengandung fosil foraminifera kecil,
khususnya pada bagian bawah anggota batulumpur.
Berdasarkan pengamatan petrografi, batupasir pada
anggota ini banyak mengandung fragmen batuan
gunung api, hampir mencapai 50%, serta
mengandung material silisiklastik lebih banyak lagi,
dengan jenis batupasir umumnya litharenit atau
volcanic arenite yang kaya akan gelas vulkanik,
dengan bentuk butir umumnya runcing – bersudut
tanggung (Gambar 7). Komposisi dan tekstur
batupasir pada anggota ini mengindikasikan adanya
sumber batuan gunung api atau lajur magmatik yang
relatif dekat.
Pada angota batulempung Formasi Merawu juga
sering dijumpai fosil jejak pada bagian bawah
lapisan batupasir, termasuk di antaranya adalah
Cruziana (Gambar 8) di bagian tengah anggota
batulumpur yang menunjukkan lingkungan laut
dangkal. Adapun struktur sedimen yang sangat
penting adalah struktur tulang ikan (herringbone
170
Gambar 1. Peta lokasi penelitian.
structure) yang dijumpai pada beberapa lapisan
batupasir di bagian atas (Gambar 9). Di samping itu,
pada beberapa bagian, baik di bagian bawah maupun
atas anggota ini, sering dijumpai struktur nendatan
beskala kecil, berukuran beberapa puluh sentimeter
(Gambar 10).
Dalam anggota batulumpur dijumpai beberapa
terobosan diorit yang diduga setara dengan diorit
Pliosen yang dijumpai di tempat lain yang juga
menerobos Formasi Merawu (Condon drr., 1975).
Anggota Batupasir
Anggota batupasir Merawu dicirikan oleh dominasi
batupasir berbutir sedang, sebagian kecil berbutir
kasar dan konglomeratan yang menunjukkan
penghalusan ke atas. Perbandingan batupasir dan
batulumpur rata-rata sekitar 75 : 25.
Batupasir dijumpai berlapis tebal-tebal, rata-rata
sekitar 20 cm sampai sekitar 1 meter. Anggota ini
tersingkap baik di bawah bendungan Kali Tulis
(Gambar 11).
Di samping perlapisan sejajar juga banyak dijumpai
perlapisan bergelombang, nendatan berskala kecil,
serta struktur tulang ikan. Anggota ini tidak
menunjukkan sifat gampingan.
No.
Lokasi
Skala tebal
(M)
Geo-Sciences
Litologi dan struktur sedimen
Pemerian
Gambar 2. Bagian bawah penampang anggota batulumpur Formasi Merawu di Kali Tulis.
171
No. Lokasi
Litologi
Skala tebal
(M)
No. Lokasi
Skala tebal
(M)
Geo-Sciences
Litologi
Gambar 3. Penampang bagian tengah (kiri) dan atas (kanan) anggota batulumpur Formasi Merawu di Kali Tulis. Simbol mengikuti Gambar 2.
172
Geo-Sciences
Gambar 4a. Penampang terukur bagian atas dan tengah anggota
batupasir Formasi Merawu di bawah bendungan Kali Tulis.
Simbol mengikuti Gambar 2.
Gambar 4b. Bagian paling atas penampang stratigrafi terukur anggota
batupasir di Kali Tulis. Simbol litologi dan struktur
sedimen mengikuti Gambar 2.
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Geo-Sciences
Gambar 5. Anggota batulumpur Formasi Merawu di Kali Tulis, terdiri
atas lapisan-lapisan batulempung (10 – 100 cm) diselingi
batupasir (5-25 cm), pada batupasir dijumpai perarian
sejajar dan silang-siur, load cast dan flute cast.
Gambar 8. Fosil jejak Cruziana pada bagian bawah lapisan batupasir
anggota batulumpur Formasi Merawu di Kali Tulis sisi barat,
S.07o19' 46”, E.109o47' 43.5”.
Gambar 6. Batupasir sangat kasar, kelabu terang, bagian bawah
konglomeratan dengan komponen batupasir, di Kali Tulis,
S.07o19'49.2”, E. 109o47'43.4”.
Gambar 9. Struktur herringbone pada sisipan batupasir dalam anggota
batulumpur.
_!
!
!
!
!
!
!
!
!
!
!
!
!
1
2
3
4
5
6
7
8
9
Gambar 7. Mikrofoto batupasir litharenite (volcanic arenite) pada
anggota batulumpur dari Kali Tulis, pandangan nikol
menyilang. Tampak pemilahan buruk, dan komponen
butirannya sangat dikuasai oleh kepingan batuan vulkanik
kaya gelas. Tampak retakan batuan yang telah terisi oleh
kalsit anhedral (Maryanto, 2006).
174
Gambar 10. Struktur nendatan pada sisipan batupasir gampingan pada
anggota batulumpur.
Geo-Sciences
Penelitian sebelumnya (Raharjanti, 1996) pernah
melaporkan adanya fosil bentonik yang menunjukkan
lingkungan batial atas. Di samping itu, analisis
petrografi yang dilakukannya menunjukkan bahwa
jenis batupasirnya adalah wacke yang mencirikan
pengendapan cepat. Atas dasar hal tersebut maka
yang bersangkutan menafsirkan bahwa Formasi
Merawu telah diendapkan melalui mekanisme
pengendapan arus turbid. Namun, dalam makalah ini
dimunculkan penafsiran lain sesuai dengan data baru
yang diperoleh dari pengukuran stratigrafi terperinci
skala 1:100, terutama bukti-bukti struktur sedimen,
fosil jejak, dan jenis litologinya.
Gambar 11. Singkapan anggota batuasir Formasi Merawu di bawah
bendungan Kali Tulis.
Umur Dan Lingkungan Pengendapan
Pada penelitian ini dapat dikenali adanya kandungan
fosil foraminifera pada bagian bawah anggota
batulumpur. Dari analisis paleontologi pada
beberapa percontoh batulempung gampingan di Kali
Tulis, hanya dua percontoh yang dapat ditentukan
umurnya, yaitu percontoh pada lokasi 08RY18
(lokasi 109 o 47'07,1”BT 07 o 20'35,2”LS dan
percontoh 06RY13B (lokasi 109o47'31,2”BT, 07o20'
12,5”LS) (Sudijono, 2006). Percontoh 08SB18
mengandung fosil foraminifera planktonik
Globigerinoides subquadratus Bronnimann,
Globorotaloides suteri Bolli dan Praeorbulina
transitoria (Blow). Adapun foraminera bentoniknya
berupa Anomalinoides sp., Ammodiscus sp.,
Bathysiphon sp., Nodosaria sp., Nonion sp., dan
beberapa genus bercangkang agglutinated yang
tidak terdeterminasi. Berdasarkan table umur dari
Blow (1969), maka kumpulan fosil planktonik
tersebut menunjukkan umur N8-N9 (Miosen Awal –
Miosen Tengah). Sementara itu percontoh 06RY13B
hanya mengandung satu spesies foraminifera
plankton berupa Globigerinoides subquadratus
Bronnimann yang mengindikasikan kisaran umur
N4-N13 (Miosen Awal–Miosen Tengah).
Berdasarkan analisis paleontologi tersebut maka
umur anggota batulumpur dapat ditentukan pada
kisaran Miosen Awal – Miosen Tengah. Adapun pada
anggota batupasir sejauh ini belum ditemukan fosil
penunjuk umur. Namun berdasarkan hubungannya
dengan anggota batulumpur yang bersifat
gradasional, maka diduga kuat umurnya masih
dalam kisaran Miosen Awal–Miosen Tengah.
Keberadaan batulempung gampingan dengan fosil
foraminifera di bagian bawah anggota batulumpur,
kemudian keberadaan struktur herringbone di bagian
atas anggota batulumpur dan bagian bawah anggota
batupasir, menunjukkan bahwa lingkungan
pengendapan Formasi Merawu di Kali Tulis adalah
laut dangkal (bagian bawah anggota batulumpur)
yang berangsur ke atas berubah menjadi dataran
lumpur (mud flat) dan dataran pasir (sand flat). Di
dalam satuan anggota mud-flat sendiri sering
dijumpai interval relatif pendek yang dikuasai oleh
batupasir yang menunjukkan adanya perubahan
lingkungan jangka pendek menjadi lingkungan sandflat (Gambar 2,3 dan 4). Sebaliknya, dalam satuan
atau anggota batupasir juga setempat diselingi
interval relatif pendek yang dikuasai oleh batulumpur.
Hal ini menunjukkan pula adanya perubahanperubahan pendek dari lingkungan sand-flat ke
lingkungan mud-flat. Di beberapa bagian pada
anggota batupasir maupun batulumpur juga dijumpai
interval pendek dengan perbandingan lapisan
batupasir dan batulumpur hampir seimbang, yang
mengindikasikan adanya lingkungan mixed flat yang
merupakan zone peralihan dari dataran pasir ke
dataran lumpur, atau sebaliknya.
Adanya beberapa lapisan batupasir konglomeratan
dengan komponen batulempung dan batupasir di
bagian bawahnya mengindikasikan adanya alur-alur
yang berasosiasi dengan daerah dataran pasangsurut. Oleh karenanya, endapan batupasir
konglomeratan dan konglomerat tersebut ditafsirkan
merupakan endapan tidal channel. Keberadaan fosil
bentonik laut dalam seperti bathysiphon di lokasi
08SB18 yang berasosiasi dengan tidal channel di
dekatnya tidak sesuai dengan lingkungan batuan di
sekitarnya. Oleh karena itu, fosil-fosil tersebut diduga
merupakan hasil rombakan dari batuan yang lebih
tua.
175
Geo-Sciences
Kesimpulan
Formasi Merawu pada lintasan Kali Tulis dapat dibagi
menjadi dua satuan atau anggota, yaitu anggota
batulumpur di bagian bawah, dan anggota batupasir
di bagian atas. Secara umum, kedua anggota
tersebut merupakan endapan dataran pasang surut,
yaitu pada lingkungan mud flat di bagian bawah dan
lingkungan sand flat pada bagian atas. Meskipun
demikian, secara terperinci terdapat keragaman atau
asosiasi lingkungan pengendapan, yang ditunjukkan
adanya endapan laut dangkal di bagian bawah
anggota batulumpur, adanya interval pendek yang
dikuasai oleh batupasir pada satuan anggota
batulumpur, serta adanya interval pendek yang
dikuasai oleh batulumpur dalam satuan atau anggota
batupasir. Demikian pula dengan keterdapatan
batupasir konglomeratan dan konglomerat
mengindikasikan adanya intervensi endapan alur
pada dataran pasang surut.
Secara umum terjadi peningkatan kelimpahan
lapisan batupasir dari bagian bawah ke bagian atas
Formasi Merawu, atau dari anggota batulumpur ke
anggota batupasir.
Ucapan Terima Kasih
Ucapan terima kasih kami ucapkan kepada rekanrekan yang telah ikut bekerja sama dalam penelitian
di lapangan, serta yang telah memberikan masukan
melalui diskusi-diskusi yang sangat berharga. Dr.
Hermes Panggabean, Dr. Surono, Ir. D.A.
Agustiyanto, M.Phil, Ir. Rachmansyah, R.
Fachruddin, R.Y. Saragih, Suprijono dan Sudijono
telah banyak membantu selama pelaksanaan
penelitian ini.
Acuan
Bemmelen, R.W.van, 1949. The Geology of Indonesia. vol. 1A, Martinus Nijhoff, The Hague.
Blow, W.H., 1969. Late Middle Eocene to Recent Planktonic Foraminiferal Biostratigraphy. 1st International
Conference Planktonic Microfossils, Geneva, Proc. Leiden, E.J. Bull. Vol.1.
Condon, W.H., Pardiyanto, L. & Ketner, K.B., 1975. Peta Geologi Lembar Banjarnegara dan Pekalongan. Skala
1 : 100.000, Direktorat Geologi, Bandung.
Condon, W.H., Pardiyanto, L., Ketner, K.B., Amin, T.C., Gafoer, S. dan Samodra, H., 1996. Peta Geologi
Lembar Banjarnegara dan Pekalongan, Jawa. skala 1 : 100.000, Edisi ke 2, Puslitbang Geologi,
Bandung.
Raharjanti, P.Y., 1996. Geologi Daerah Pagarpelah dan Sekitarnya, Kecamatan Banjarmangu, Kabupaten
Banjarnegara, Propinsi Jawa Tengah. Skripsi S1 Jurusan Teknik Geologi, Fakultas Teknologi Mineral,
Universitas Pembangunan Nasional “Veteran” Yogyakarta, tidak terbit.
Maryanto, S.,2006. Hasil Uji Petrografi Batuan dari Banjarnegara. Laporan Internal, Pusat Survei Geologi,
tidak diterbitkan.
Sudijono, 2006. Hasil Uji Mikropaleontologi Foraminifera. Laporan internal Laboratorium Pusat Survei Geologi,
tidak diterbitkan.
176
PANDUAN
PENULISAN MAKALAH ILMIAH
JURNAL SUMBER DAYA GEOLOGI
UMUM
1. Naskah merupakan karya asli yang belum pernah diterbitkan di manapun sebelumnya.
2. Naskah dalam Bahasa Inggris ataupun Indonesia yang baik dan benar, dilengkapi dengan Sari
dalam Bahasa Indonesia dan Abstract dalam Bahasa Inggris.
3. Teks harus tercetak jelas; gambar dan foto harus asli dengan ukuran maksimum 19,5x15 cm.
4. Naskah harus ditelaah dan disunting paling tidak oleh dua orang dari Dewan Redaksi
dan/ataupun Editor Ilmiah (Scientific Editor) sesuai dengan mekanisme yang berlaku.
5. Naskah yang masuk ke Dewan Redaksi, harus disertai Surat Pengantar dari Kelompok
Program/Pimpinan Unit (khusus dalam lingkungan DESDM).
6. Dewan Redaksi berhak menolak naskah/makalah yang kurang memenuhi syarat sebagai tulisan
ilmiah.
7. Soft copy yang berisi teks, gambar, dan potret yang telah diperbaiki sesuai dengan telaahan dan
suntingan, dan dinyatakan dapat diterbitkan oleh Dewan Redaksi, diserahkan kepada Ketua
Dewan Penerbit/Kepala Bidang Informasi.
NASKAH
1.
Halaman pertama naskah berisi judul makalah, sari dan abstract, serta kata kunci dan keywords.
Nama penulis, nama instansi, alamat dan nomor telepon/hp dituliskan pada lembar tersendiri.
2.
Naskah diketik dengan komputer dalam MS-Word dengan huruf Times New Roman, Font-12, dua
spasi.
3.
Beri dua spasi antara heading dan teks di bawahnya, tiga spasi antaralinea tanpa menggunakan
indentasi.
4.
Naskah berisi :
a. Judul (Title)
b. Sari/Abstract; harus ringkas dan jelas mewakili isi makalah (concise summary), paling banyak 200
kata (words) diketik satu spasi (single space).
c. Kata kunci (keywords); 4 sampai 6 kata ditulis di bawah sari/abstract.
d. Pendahuluan (Introduction) : Latar belakang, Permasalahan, Tujuan Penelitian, Lokasi Daerah.
(Scientific Background, Scientific Problem, Aim(s), Studied Area).
e. Metodologi (Methods)
f. Analisis dan Hasil (Analyses and Results)
g. Diskusi (Discussion)
h. Kesimpulan dan Saran (Conclusions/Recommendations)
I. Ucapan Terima Kasih (Acknowledgment)
5.
Acuan (References); harus diacu (cited/referred) dalam tulisan, mendukung isi tulisan dan ditulis
dalam daftar serta disusun menurut abjad. Hindari penulisan nama penulis/pengarang maupun
Call for paper:
editornya dengan huruf besar. Semua nama penulis harus ditulis, tidak boleh hanya nama penulis
pertama dengan tambahan drr.
Contoh :
Prosiding (Proceeding):
- Koning, T. and Darmono, F.X., 1984. The Geology of the Beruk Northeast Field, Central
th
Sumatra. Oil production from pre-Tertiary basement rocks. Proc. 13 Ann. Conv.
IPA, Jakarta, Indonesia.
Jurnal/Buletin:
- Reich, M., Parada, M.A., Palacos, C., Dietrich, A., Schultz, F. and Lehman, B., 2003. Adakitelike signature of Late Miocene intrusions at the Los Pelambers giant porphyry
copper deposit in the Andes of central Chile: metallogenic implications.
Mineralium deposita, 38: 876-885.
Peta:
- Simandjuntak, T.O., Surono, Gafoer, S., dan Amin, T.C., 1991. Geologi Lembar Muarabungo,
Sumatera, skala 1:250.000. Pusat Penelitian dan Pengembangan Geologi, Bandung
Laporan tidak diterbitkan:
- Siagian, H.P. dan Mubroto, B., 1995. Penelitian Magnet Purba di daerah Baturaja dan
Sekitarnya, Sumatera Selatan. Laporan intern Pusat Penelitian dan Pengembangan
Geologi, Bandung (Tidak diterbitkan).
Tesis (Skripsi, Disertasi):
- Stone, I.G., 1963. A morphogenetic study of study stages in the life-cycle of some Vitorian
cryptograms. Ph.D. Thesis, Univ. of Melbourne.
Buku :
- George, S., 1967. Language and Silence. Faber and Faber, London: 96pp.
Dalam Buku :
- Carter, J.G., 1980. Environmental and biological controls of bivalve shell mineralogy and
microstructure. In: Rhoads, D.C. and Lutz, R.A. (Eds.), Skeletal growth of aquatic
organisms. Plenum Press, New York and London: 93-134.
Publikasi Khusus (Special Publication):
- Kay, E. Alison, 1979. Hawaiian Marine Shells.B.P. Bishop Museum Special Publication 64(4):
653pp. Major Treatment.
Informasi di internet:
- Lunt, P., 2003. Biogeography of some Eocene larger foraminifera, and their application in
distinguishing geological plates. Paleontologica Electronica 6(1):22pp, 1.3MB;
http://paleo-electronica.org/paleo/2003-2/geo/issue 2-03.htm
6.
Dalam draft, gambar/peta/potret diletakkan pada halaman akhir makalah.
7.
Keterangan gambar ditulis terpisah dari gambar “caption figures”, jangan ditulis di bawah gambar
8.
Keterangan tabel juga diketik dalam satu spasi, diletakkan di atas tabel, tidak diakhiri dengan titik.
Setiap awal kata, ditulis dengan huruf besar, kecuali kata depan dan kata sambung.
9.
Naskah dikirim ke Dewan Redaksi sebanyak 2 copy

00-cover_Jurnal-juni 2010.cdr

Transcription

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