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Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 41-49
Shale Gas: Geological Perspective of Baong Formation
for Future Chances of North Sumatra Basin;
Compared to Fort Worth Basin in USA
Gas Serpih: Perspektif Geologis Formasi Baong
untuk Kemgungkinan Masa Depan Cekungan Sumatra Utara;
dibandingkan dengan Cekungan Fort Worth, Amerika Serikat
Nur Amalia Murtrijito, Fetty Maria Naibaho, and Wijayanti Ashuri
Department of Geological Engineering, Bandung Institute of Technology, Indonesia
ABSTRACT
Baong Formation, situated in North Sumatra Basin, has other potential resources besides the ability of
formation to be an oil-prone sediments. Gas is generated in an interbedded black shale and limestone
of this formation. The black shale derived from the deposition of organic-rich material and influenced
by the presence of sandy shale is possible to produce natural fractures within this formation leading to
the increasing porosity and permeability significantly. The presence of interfingering limestone also
contribute to the natural fracture amount. The Baong Formation has particular similarities with Barnett
Shale of the Fort Worth Basin in petroleum system and lithology. Therefore, it is not impossible if the
Baong Formation become a commercial gas resource as Barnett Shale. On the other hand, Peutu-, Arun,
and Belumai Formations, comprising limestone with minor dolomite and sand, which lithologically
are similar with Ellenburger Formation in Fort Worth Basin, may become good quality reservoirs for
the underlying Parapat-Bampo source rock. Moreover, Parapat – Bampo Formations as fluvial deposits
become a good source rock as Riley Formation in Fort Worth Basin consisting of sandy fluvial deltaic
sediments. Besides the similarity in lithology, the Baong Formation, tectonostratigraphically is also
similar to Barnett Formation in Fort Worth Basin. In syn-rift depositional period, the Riley Formation
in Fort Worth Basin and Parapat - Bampo Formations in North Sumatra Basin were deposited first. It
was continued by the deposition of Ellenburger Formation in Fort Worth Basin, and Peutu -, Arun -,
and Belumai Formations in North Sumatera Basin, then followed by the conformably deposition of
the potential gas-prone sediments, those are Baong Formation in North Sumatra Basin and Barnett
Shale in Fort Worth Basin. During the gas-prone sediment formation, active faults formed fault belt
system impacting thermal subsidence and uplift could occurred.
Keywords: Baong Formation, North Sumatra Basin, Barnett Shale, Fort Worth Basin, Indonesia, USA
SARI
Formasi Baong, yang terletak di Cekungan Sumatra Utara, selain sebagai batuan sedimen bersifat
oil-prone, juga berpotensi sebagai sumber gas serpih yang terbentuk di dalam perselingan serpih hitam
dan batugamping. Serpih hitam yang terendapkan sebagai sedimen yang kaya akan bahan organik
dan mengandung sisipan serpih pasiran sangat memungkinkan untuk mengalami rekahan secara
alamiah, sehingga akan meningkatkan porositas dan permeabilitasnya secara signifikan. Keberadaan
batugamping yang menjemari dengan serpih juga menambah peringkat rekahan alamiah tersebut.
Formasi Baong memperlihatkan kesamaan sistem petroleum dan litologi dengan Serpih Barnett dari
Cekungan Fort Worth. Hal ini tidak menutup kemungkinan jika Formasi Baong menjadi sesumber
gas seperti Serpih Barnett. Di lain fihak, Formasi Peutu, Arun, dan Belumai yang tersusun terutama
oleh batugamping, dengan sedikit dolomit dan batupasir, dan secara litologi sama dengan Formasi
Ellenburger di Cekungan Fort Worth, akan berfungsi sebagai reservoir berkualitas bagus bagi batuan
sumber yang mengalasinya, yaitu Formasi Parapat-Bampo. Lebih jauh lagi, Formasi Parapat-Bampo
yang merupakan endapan fluviatil akan menjadi batuan sumber yang baik seperti Formasi Riley di
Naskah diterima: 24 September 2012, revisi terakhir: 05 April 2013, disetujui: 08 April 2013
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Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 41-49
Cekungan Fort Worth, yang terdiri atas batuan sedimen delta-fluviatil pasiran. Selain kesamaan
dalam litologi, Formasi Baong dengan Formasi Barnett juga memperlihatkan kesamaam secara
tektonostratigrafis. Dalam periode pengendapan syn-rift, Formasi Riley di Cekungan Fort Worth dan
Formasi Parapat-Bampo di Cekungan Sumatra adalah paling awal terendapkan. Selanjutnya diikuti
oleh pengendapan Formasi Ellenburger di Cekungan Fort Wort dan Formasi Peutu, Arun, dan Belumai
di Cekungan Sumatra Utara. Kemudian, secara selaras di atasnya terendapkan Formasi Baong di
Cekungan Sumatra Utara dan Serpih Barnett di Cekungan Fort Worth, yang keduanya bersifat oilprone. Selama pengendapan kedua formasi ini, terjadi sesar aktif yang membentuk sistem jalur sesar
dan berpengaruh kuat terhadap penurunan dan pengangkatan secara termal di kawasan tersebut.
Kata kunci: Formasi Baong, Cekungan Sumatra Utara, Serpih Barnett, Cekungan Fort Worth,
Indonesia, Amerika Serikat
INTRODUCTION
Shale gas is an unconventional hydrocarbon
or natural gas produced from shale formations, which has been developed in several
countries. Indonesia has shale gas potential
in some basins, one of which is North Sumatra Basin, where Baong Formation was
deposited as a regional seal for the three main
reservoirs below the Baong Formation, those
are the Belumai, Arun, and Peutu Formations.
The purpose of this study is to examine the
potential of unconventional hydrocarbons
from geological point of view, so the authors
have tried to compare with the success of
shale gas that has been first produced in the
Barnett Shale in the Fort Worth Basin, Texas,
USA, based on the petroleum system, geochemical analysis, and the forming of North
Sumatra Basin related to basin infill.
GEOLOGICAL BACKGROUND
North Sumatra Basin is a back-arc basin
(Figure 1). Almost the entire basin fill is
marine deposits, much of them, especially
in the north, comprise basinal deeper marine
claystone, shale, and shallow water reefal
limestone. The later developed on structural
highs. Regressive shallow water deltaic
facies are found in the southeast area. The
sequence predominantly composed of argil42
laceous material and divided into four-basin
stages is somewhat arbitrary (Doust and
Noble, 2008).
Late post-rift (Middle Miocene to Pliocene)
stage is a regressive sequence comprises
argillaceous Baong Formation, in which
turbidite sands occur, and the overlying
paralic shale, silt, and sand of the Keutapang
and Seurula Formations. In the north, deeper
marine facies continued, while towards the
southeast, these formations became shallower with the deposition of regressive
deltaic sand of moderate to good reservoir
quality (Doust, and Noble, 2008).
TECTONIC SETTING
NNorth Sumatra Basin, a back-arc basin,
is located in the northeast area of Bukit
Barisan (Figure 1). This basin extends in
the northwest - southeast direction and is
bounded by Bukit Barisan to the west and
Malacca Platform in the east.
The Burman Plate being slowly dragged
northward by the Indo-Australian Plate.
A clockwise rotation of the Burman and
Shan Plates led to the opening (rifting) of
the Mergui, North Sumatra, Martaban, and
Central Burman Basins. Early Miocene plate
reconstruction and basin evolution show a
continuing northward drift of the Burman
Plate and rotation of the Burman and Shan
Shale Gas: Geological Perspective of Baong Formation for Future Chances of North Sumatra Basin;
Compared to Fort Worth Basin in USA (N. A. Murtrijito et al.)
6 N
o
Key
North
Sumatra
Basin
Active volcano
Base of trench slope
re
Asa
h
Arc an
h
Fo
Regional fault system
Malacca
Platform
rc
A
Fore arc ridge
dg
Ri
e
y st
M
ub
eS
S)
tiv
(SF
em
ge
sin
Ba
ed
W
Central
Sumatra
Basin
tS
y
lga
ar
o
Ac
0
l
Fau
tra
ma
Su risan
Ba
n
tio
o
Sib
re
cc
A
Nias
ult
Mentawai Islands
ne
Zo
Bangka
Fa
on
ai
cti
taw
du
en
Tigapuluh
High
Sy
South
Sumatra
Basin
m
ste
)
N oE
FS
(M
e
To
of
ge
lan
me
e
dg
Jakarta
e
dg
we
400 km
we
200
ge
0
of
lan
ch
ren
aT
nd
6o S
Bengkulu
Basin
p
To
me
N
Su
Relative
Plate
Motion
Figure 1. Regional tectonic setting of Sumatra.
Plates. Middle Miocene plate reconstruction
and basin evolution display an accelerated
northward drift of the Burman Plate causes
the opening of the modern Andaman Basin
west of the Mergui Ridge (Figure 2). Compression intensifying plate rotation causes a
major left-lateral movement along the Ranong Fault. Early Pliocene plate reconstruction and basin evolution indicate sediments
pouring off the rising Barisan Mountains
on Sumatra, and then causing a significant
downwarping of the southern North Sumatra
Basin margin (Anderson et al., 1997).
There are two main structure trends in North
Sumatra Basin, those are the N-S trend
representing pre-Miocene structure, and the
NW-SE that is post Miocene structure. Both
trends are a product of three tectonic events,
comprising initial extension phase in Late
Eocene-Early Oligocene, wrench tectonism
in Middle Miocene, and compressional tectonism in Pliocene-Pleistocene.
METHODOLOGY AND CHARACTERIZATION OF SHALE GAS
Shale gas consists of 70 - 90% dry methane
gas (some in the form of wet gas), 20% is a
mixture of ethane, butane, and propane, and
the rest comprises CO2, O2, N, and H2S.
Shale gas was once considered to be in a formation serving as a source rock and seal that
accumulates near the reservoir sandstones
and carbonates (from onshore gas). Shale
is usually deposited in a quiet low-energy
environments such as lacustrine or seawater
environment in which clay sediment was deposited in a suspension mechanism through
43
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 41-49
A
16 Ma
30 Ma
Burman
Plate
Indian
Plate
Shan
Plate
Burman
Plate
Shan
Plate
Indian
Plate
Indian
Plate
Indian
Plate
Burman
Plate
Burman
Plate
Shan
Plate
Shan
Plate
Figure 2. The tectonic evolution of North Sumatera Basin (Anderson et al.,1997).
a quiet water environment. During deposition of this clay grains, shale can also accumulate with organic material (algae, plants,
animal shell debris). This layer is very thin
(sheet-laminae) and make arrangement of
horizontal permeability of the shale which
is very small, and also limited permeability
in the vertical succession.
As a comparison, Shale Gas Potential of
the most vibrant play is in the Barnett Shale
located in Fort Worth Basin, where horizon44
tal drilling and stimulation techniques have
resulted in multiple zones of growth of 0.2
bcf/d in 1999-2004 to more than 1.3 bcf/d in
2005. Shale gas exploration is the complex
interplay and balance of the controlling factors of organic richness, thermal maturity,
lithology, mineralogy, depth, and fracturing characteristics that lead to this type of
gas accumulation. Factors like production
technique and infrastructure determine the
economic value of shale gas resources.
Shale Gas: Geological Perspective of Baong Formation for Future Chances of North Sumatra Basin;
Compared to Fort Worth Basin in USA (N. A. Murtrijito et al.)
RESULTS
Screening Analysis of Baong and Barnett
Formations
Rock-Eval analysis and petrographic data
obtained from the Baong Formation show
that TOC varies between 0.8 - 2% with
type II kerogen, vitrinite reflectance (Rv)
of 0.55 - 0.6%, HI varies from 50 - 150,
and maximum temperature (T max) from
420 - 455o C. In addition, shales and mudstones of the Bampo and Peutu Formations
are dominantly type III kerogen with TOC
ranging from 0.5% to 3% (Buck and McCulloh, 1994).
Organic richness and composition of the
Barnett Shale lead to an excellent to mixed
oil-gas-prone source rock potential at low
to moderate thermal maturity (Jarvie et al.,
2005; Kinley et al, 2008). Geochemical
analysis shows that the Barnett Shale tends
to indicate a little unique aspects, shown by
its type II kerogen, TOC 3 - 5%, vitrinite
reflectance (Rv) of 1.0 - 1.4%, HI from 13
- 59, and the maximum temperature (Tmax)
> 455o C (Jarvie et al., 2005).
Gas originated from the shale in Fort Worth
Basin produces in two types, dry gas and wet
gas. Therefore, when the organic material
have a secondary cracking would be wet
and when biodegradation occurs, dry gas
formed. Gas produced in Barnett Shale is
of thermogenic origin produced from greater
depth (Montgomery et al., 2005), in the
form of dry gas with a little bit of sulphur.
It indicates mature hydrocarbons, shown
by Tmax > 455o C . Gas yields are very high
in the Barnett Shale in accordance with
the abundant organic content and original
hydrocarbon generation potential. Type of
gas produced in the reservoir is strongly
influenced by the composition of gas and
isotopic carbon and the content of gas itself
is directly proportional to the TOC (in the
gas window, TOC can be reduced by 30 50%), kerogen type, and level of maturity.
Depositional Environment
Based on stratigraphic column which has
been generalized (Figure 3), in the Early
Oligocene, Parapat Formation was deposited in a fluvial-lacustrine environment,
then Bampo Formation deposited in a
deep marine environment with dominant
lithology comprises marine shale. Upwards,
Peutu and Arum Limestone formed during
the marine transgression is composed of
reef and carbonate build-up. At the same
time, phase of basinal slope carried sandsize material from Asahan Highs forming
Belumai Formation that is overlain by
deep marine Baong shale interbedded with
sandy Baong itself. Above the regional seal
of Middle Miocene unit, that is the Baong
Formation, the Keutapang Formation was
formed widespread within a shallow marine
environment, which then was continued by
a deposition of Seurula Formation due to
rapidly shallowing basin through quantity
of clastic sediment influx from the uplifted
Barisan. Then from the shallow marine, the
environment changed gradually to be deltaic, then to fluvial environment, where Julu
Rayeu Formation was deposited. Thereby,
when viewed from the bottom to Julu Rayeu
Formation, it shows a coarsening upward
pattern consisting of sandstones and shales
in vertical succession, in general.
Figure 4 display that at the Fort Worth Basin,
shelf carbonate system was deposited in
the Ordovician Ellenburger Formation on
a passive continental margin (Montgomery
et al., 2005: Pollastro, 2007), while the
Barnett Shale of Late Mississpian is characterized by basin edge carbonate system.
Both these carbonate systems are variation
of the carbonate platform. Then, during the
main phase of subsidence in Pennsylvanian
time, a fluctuation of sea level progressively
45
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 41-49
Holocene
Late
Pliocene
Early
Miocene
Oligocene
East
Hiatus
Late
Julu Rayeu Formation
Early
Late
Eocene
West
Alluvium
Seurula Formation
Keutapang Formation
Middle
Baong Formation
Early
Peutu
Formation
Middle
Bampo
Formation
Arun
Formation
Early
Late
Middle
Belumai
Formation
Hiatus
Parapat
Formation
Early
Figure 3. Generalized stratigraphic section of North Sumatra Basin (Pertamina BPPKA, 1996).
occurred represented by the deposition
of Marble Falls Formation (Montgomery
et al., 2005; Pollastro, 2007). Interbedding dark limestone and grey-black shale
at lower part and limestone at upper part
constitute the formation. Loucks and Ruppel (2007) found that phosphate indicates
marine upwelling. The marine upwelling is
the chance of explosion of development of
organisms below the sea water which is the
hemipelagic plume condition by deposition
of fine-sized sediment (shaly) in the sea.
Exposure of carbonate sediments dominated
by crinoidal carbonates, pinnacle reef, and
shell material and all derived from the slope
toward the hemipelagic basin (carbonate grains are coarser), can be originated
46
through the mechanism from debris flow.
Some of the Ellenburger carbonates crop
out to the surface.
LITHOLOGY
North Sumatra Basin located in Sumatra was
filled in with sediments deposited during
Oligocene through Pliocene epoch. The first
formation is Basement of the basin consists
of low grade metamorphics and metagreywacke. During Early Oligocene, the Parapat
Formation was deposited, consisting of
sandstone that is characterized by porosity
of <5%. After that, the Bampo Formation
was formed in Late Oligocene, composed of
Shale Gas: Geological Perspective of Baong Formation for Future Chances of North Sumatra Basin;
Compared to Fort Worth Basin in USA (N. A. Murtrijito et al.)
claystone. In the Early Miocene, the Peutu
Limestone was conformably deposited
on top of Bampo Formation. Overlying
the Bampo Formation are the Arun and
Belumai Formations, comprising reef and
sandstone, respectively. Furthermore, the
Peutu Formation consisting of limestone
and Arun Formation are interbedded. Upwards, Keutapang and Seurula Formations
are present as shallow marine sandstone and
shale. Finally, the Julu Rayeu Formation
composed of sandstone began to be deposited in Pliocene (Figure 4).
On the other hand, Fort Worth Basin in Texas
was infilled by sediments during Cambrian
through Cretaceous (Figure 4). Basement
of the basin is composed of granite, diorite,
and metasediments of Precambrian age.
The first sediments occurring in Fort Worth
Basin is Upper Cambrian Wilberns-RileyHickory Formations composed of granitic
conglomerate, sandstone, and shale as marine deposits. There are no Silurian and
Devonian sedimentary strata recognized,
probably they might be eroded. Ordovician
until Lower Pennsylvanian was dominated
by carbonates. Deposition of Ellenburger
Group began in Lower Ordovician, followed by Simpson Group, and then Viola
Limestone on Upper Ordovician. In Missis-
Period
Stratigraphy Unit
Cretaceous
Undifferentiated
Permian
Cisco Group
Pennsylvanian
Mississippian
Oil
Gas
Canyon Group
Strawn Ss
Caddo Ls
Pregnant Sh
Atoka Ss and Smithwick Sh
Bend
Marble Falls Ls
Comyn
Upper Barnett
Barnet Shale
Forestburg Ls
Lower Barnett
Ordovician
Chappel
Ls
Simpson Gp
Ellenbuger Da/Ls
Cambrian
Riley
Hickory
Figure 4. Stratigraphy of Fort Worth Basin (Pollastro, 2007).
Precambrian
Granite & Diorite
47
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 41-49
sippian, Barnett Shale comprising siliceous
shale, limestone, and minor dolomite, occurs as marine sediments, unconformably
deposited on Viola Limestone (Montgomery
et al., 2005). A significant proportion of
limestone within the Barnett Shale called
Forestburg Limestone was deposited in a
series of debris flows into the deeper part
of the basin (Bowker, 2002, in Montgomery
et al., 2005), accompanying sea level drop,
in Early Pennsylvanian. This unit interfingering with the middle part of the Barnett
Shale tends to divide the Barnett Shale into
two members, those are Lower Barnett
Shale and Upper Barnett Shale. The Lower
Member consists of five shale unit which is
separated by the interbedded of limestone
with 10 - 30 ft thick. Meanwhile the Upper
Barnett Shale is thinner than Lower Barnett
Shale (Figure 3).
The Parapat Formation is fluvio-lacustrine
sediments whilst the Bampo one occurs
as marine sediments. Hydrocarbon began
expelled through Peutu, Arun, and Belumai Formations, so that the Peutu -, Arun
-, and Belumai Formations are present as a
good quality reservoirs, when it begin to be
fractured, and then hydrocarbon moved to
the Baong Shale. Baong Formation become
source rock from its shale and became seal
of the accumulated hydrocarbon below it.
So that Keutapang Formation has become
both good quality source rock and regional
seal rock.
DISCUSSION AND CONCLUSION
Determination of initial maturity of organic
material is at a temperature of 435o C and
pyrolysis 0.6% Ro (Peters and Cassa, 1994).
From this reference, it was found that the
organic content in the Baong Formation is
good, but not yet mature, whereas in the Barnett Shale organic matter is at the end of the
maturity level and both tend to produce gas
based on kerogen type and its HI content.
Fort Worth Basin has an unconventional petroleum system when it begins from Barnett
Shale as source rock, and it regionally seals
Marble Falls Limestone overlying above
and Viola Limestone and Simpson Group
underlying below, so that the Marble Falls,
Viola Limestones, and Simpson Group could
become barriers stabilizing the Barnett Shale
when it began to be fractured. Moreover,
the Barnett Shale itself becomes a reservoir.
Barnett shale itself as source rock from it
shale and from accumulation fractured Ellenburger Formation, so that Barnett Shale
become a seal for Ellenburger Formation.
When the system is conventional, Pennsylvanian rock become good quality reservoir,
and the important source rock for this basin
is Barnett Shale.
North Sumatra Basin has a particular way to
its petroleum system. The first source rock
formed is Parapat and Bampo Formations.
48
Based on Peters and Cassa (1994), TOC
in Baong Formation has an enough good
content of organic material, whereas the
TOC of Barnett Shale has very good content
of organic material. Maturity of the rock
sample is determined based on the pyrolysis
temperature (Tmax) and vitrinite reflectance
(Ro).
Both Forth Worth Basin and North Sumatra
Basin are almost similar in their lithology
and petroleum system, but the difference between them is in their age, hydrogen index,
and maturity. Therefore, North Sumatra Basin has a chance to become unconventional
energy for the future.
ACKNOWLEDGEMENT
The paper has been presented in The 41st Annual
Convention and Exhibition IAGI Yogyakarta 2012.
The authors acknowledge committee of the convention for permission to publish the paper in Majalah
Geologi Indonesia (MGI) IAGI. Thank is due to
Shale Gas: Geological Perspective of Baong Formation for Future Chances of North Sumatra Basin;
Compared to Fort Worth Basin in USA (N. A. Murtrijito et al.)
Kang Dadan who always give support to authors in
writing and discussing the paper.
Texas and Southeastern New Mexico. American
Association of Petroleum Geologists, Bulletin, 92
(8), p.967-991.
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49