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Journal of South American Earth Sciences 47 (2013) 166e178
Contents lists available at ScienceDirect
Journal of South American Earth Sciences
journal homepage: www.elsevier.com/locate/jsames
Molecular organic geochemistry of the Apiay field in the Llanos basin,
Colombia
J.E. Cortes a, b, *, J.E. Niño a, J.A. Polo a, A.G. Tobo a, C. Gonzalez a, S.C. Siachoque a
a
b
Antek S.A. e Environmental & Petroleum Geochemistry Laboratory, Calle 25B #85B-54, Bogotá, Colombia
PetroMarkers, Inc. e Petroleum Geochemistry Laboratory, 830 Bay Star Blvd., Webster, TX 77598, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 May 2013
Accepted 13 July 2013
The bulk properties and molecular organic geochemical composition for crude oils from the Apiay, Suria
and Reforma/Libertad producer areas, which integrate the Apiay field located in the southwest area of
the Llanos Basin in Colombia were analyzed by gas chromatography (GC/FID), isotopic analysis and gas
chromatography/mass spectrometry (GC/MS) analysis.
The main producing intervals in the Apiay field are known as the K2 and K1 units of the Guadalupe
Formation, a thick siliciclastic sequence deposited during the Upper Cretaceous to Upper Eocene in a
fluvial and transitional marine system. The crude oils analyzed are paraffinic, with saturate fraction
>60%, the d13C isotopic composition ranging from 26.19 to 25.62 for the saturated fraction, 25.84 to
24.02 for the aromatic fraction, and canonical variable (C.V.) <0.47, which characterized them as nonwaxy marine oils. The saturated fraction analyzed by GC/FID presents a unimodal distribution between
n-C10 to n-C33 with n-C15 to n-C17 as the major peaks. CPI is close or slightly greater than 1.0, Pr/Ph ratio
>1.5, low z high molecular weigh hydrocarbons indicating an input of algal/microbial organic matter
with a significant input of terrigenous matter (higher plants).
Branched/Cyclic biomarkers, previously separated from n-alkanes by silicalite/ZSM-5 (S-115), were
analyzed using SIM-GC/MS. Samples from the Apiay area showed higher concentration of tricyclic terpanes than samples from Suria and Reforma-Libertad, respectively, which suggests an early diagenetic
influence of marine saline water, consistent with early generation from marine organic matter. However,
the presence of a great suit of sesquiterpanes and diterpanes in all samples confirming an angiosperm
input. Ts/Ts þ Tm falls in the range of 0.25e0.66, all samples present gammacerane, C31-Hopane
isomerization index ranged between 0.50 and 0.71. A predominance of C29 over C27 and C28-steranes in
the Apiay area indicates terrigenous source rock for most of the samples, however samples from the
Reforma-Libertad and Apiay areas show mixing characteristics of crude oils originated from marine and
terrigenous sources.
Diasteranes are higher than regular steranes, which predicts a siliciclastic lithology for these
Upper Cretaceous sourced oils. Thermal maturity, according to 20S/20S þ 20R-C29 and bb/aa þ bbC29 steranes and aromatic parameters, suggests that some of the oils were generated in the peak oil
window. Biomarker results suggest a transitional fluvio-deltaic depositional environment with a
predominance of continental fluvial type facies with marine episodes, which agrees with the marine
input (algal/microbial) and with a moderate input of highland organic matter. The norhopane index
indicates a greater biodegradation process in the Apiay area that in Suria and Reforma/Libertad
areas.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Sesquiterpanes
Hopanes
Steranes
Biodegraded mixed oils
Apiay field
Colombia
1. Introduction
* Corresponding author. Petromarkers, Inc. e Antek S.A., 830 Bay Star Blvd.,
Webster, TX 77598, USA. Tel.: þ1 713 261 4828.
E-mail address: [email protected] (J.E. Cortes).
0895-9811/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jsames.2013.07.007
The Apiay field is one of the most important petroleum producing field under administration of the Colombian Petroleum
Company (Ecopetrol S.A.). The field has an area of 15,000 km2 and is
located in the Apiay-Ariari sub-basin of the Llanos basin in Meta,
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J.E. Cortes et al. / Journal of South American Earth Sciences 47 (2013) 166e178
Colombia. Discovered in 1981, at the present time, its original reserves have been estimated in 50,000 barrels of crude oil and
15 million-ft3 of natural gas (Carta Petrolera, 1999; El Tiempo,
2011).
Various studies of the Apiay field have been reported in the
literature. The National University of Colombia has done various
studies related with the stratigraphic and the petroliferous potential of the basin, in addition to several petrographic and paleontological studies in oils and source rocks (Mayorga and Vargas, 1995;
Varela, 1997; Cabrera, 1999). The Colombian Petroleum Institute
(ICP) has done many multidisciplinary studies of source rocks and
crude oils samples in the Llanos basin (Tegelaar et al., 1995; Rangel
et al., 1996; Rangel et al., 1999). Bonilla (1996) evaluated 82 samples
from this basin using SARA, GC/FID, and biomarkers analysis. Luna
et al. (1996) studied samples of crude oils and seeps from the Apiay
and Castilla fields to characterize its origin, maturity, and biodegradation. Dzou et al. (1999) have reported the application of new
diterpane biomarkers to evaluate source, biodegradation and
mixing effects on samples from the central Llanos basin. Palmer and
Russell (1988) have defined 5 oil families in Llanos basin based on
the analyses of 53 crude oils. Ramon et al. (2001) evaluated the
evolution of the Cretaceous organic facies in Colombia studying
samples from the Llanos basin without including samples from the
Apiay field. More recently, Cortes et al. (2010) have used CSIA and
GC/MS to differentiate Cretaceous and Tertiary crude oils from the
Llanos basin.
167
This paper presents a detailed analysis of the bulk properties
and biomarkers composition crude oils from the Apiay field in order to predict the geochemical characteristics of its source rock,
depositional environment, maturity, organic matter origin, and age.
The analyses were performed by gas chromatography (GC/FID),
bulk isotopic analysis, and gas chromatographyemass spectrometry (GC/MS).
2. Geological setting
Fig. 1 shows the localization of the area study. As it is observed,
the Apiay oil field has three producing areas named Apiay, Suria
and Reforma-Libertad. Fig. 2 presents a generalized stratigraphic
column of the Llanos basin (NHA, 2010; NHA, 2012). The sedimentary column is represented by rocks from the Lower Paleozoic
(Cambro-Ordovician), Upper Mesozoic (Upper Cretaceous) and
Cenozoic (Tertiary and Quaternary). The units of petroliferous interest are found in the Upper Cretaceous and Upper Eocene rocks
(Bohorquez and Valderrama, 1991).
The Cretaceous sequence presents ages ranging from Cenomanian to Maestrichtian, thinning to 1650 feet. This sequence
has been divided from base to top in the K2 and K1 operational
units. Perez and Bolivar (1985) argue that the K2 unit lies unconformably over Paleozoic rocks with a thickness of 700 feet
and is comprised of thick grained of quartz sand; which overlies
in transitional contact the K1 unit with a thickness of aprox. 450
Fig. 1. Regional map and well localization of the Apiay, Suria and Reforma-Libertad areas in the Apiay Oilfield.
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J.E. Cortes et al. / Journal of South American Earth Sciences 47 (2013) 166e178
Fig. 2. Generalized Lithostratigraphic column of Llanos basin (NHA, 2010, 2012).
feet, comprised of greasy dark shale, sand sandstone, and occasionally carbon mantles. These authors have postulated a
littoral to continental environment for K2 unit and coastal
transitional environment for K1 unit. Castro (1989) defines the
K2 unit as a fluvial system with light transgressive pulses and
the K1 unit as a transitional environment with continental influence in the lower part of the sequence grading up to marine.
This author correlated the K2 unit with the Une formation and
the K1 unit with the Chipaque formation and Guadalupe group
(Castro, 1989).
During the Cenozoic era a thick sedimentary sequence was
deposited, which had a great variety of depositional environments,
according to the existent paleogeographic characteristics and the
marine transgressive process. The Tertiary is represented by rocks
deposited from the Upper Eocene to the Middle-Upper Miocene, to
which belongs the Mirador formation. In the Apiay-Ariary subbasin, Kendall et al., (1982), describe the Mirador formation as
marine-influenced, sand-rich valley fill deposits that passed
upward into muddier coastal plain sediments, and is considered
one of the major reservoir in the basin (Dzou et al., 1999).
The Middle Oligocene-Lower Miocene sequence is represented
by the Carbonera formation, which consists of a marine-influenced
lower coastal plain. The Leon formation has been described as a
series of green shales with occasional presence of limolite, whose
thickness varies from 130 ft. to 1900 ft.
The younger section in the Middle Miocene-Plio-Pleistocene
sequence has not been studied in detail and its nomenclature and
correlation are different throughout the basin. In general, they are
known as Guayabo and Necesidad formations, respectively. Perez
and Bolivar (1985) describe this formation in the Apiay-Ariari
sub-basin, as a sequence of red layers with brown sandy mudstones interbedded with gray, white, and reddish sandy mudstone.
It is understood that the sequence was deposited in a continental
environment, with few deposits of marine environment.
It has been proposed that the Apiay-Ariari sub-basin was
deposited under a deltaic transitional marine environment with a
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J.E. Cortes et al. / Journal of South American Earth Sciences 47 (2013) 166e178
169
Table 1
Bulk parameters of selected crude oils from the Apiay Oilfield, Llanos basin.
Well
Area
SAT
ARO
RES
ASPH
Sat/Arom.
Resin/Asph
d13Ca SAT
d13Ca AROM
CV
APIAY-3/K1
APAIY-5
APIAY-11 APIAY
SURIA-1/K1
SURIA-3/K1
GUAYURIBA-2
POMPEYA-1/K1
POMPEYA-1/K2
TANANE-1/K1
REFORMA-2/K1
LIB NORTE-3/K2
LIBERTAD-4/K1
LIB. NORTE/K1
APIAY
APIAY
64.00
SURIA
SURIA
SURIA
SURIA
SURIA
SURIA
REF/LIB
REF/LIB
REF/LIB
REF/LIB
62.46
63.39
17.64
60.13
65.36
75.54
77.48
78.96
78.16
58.06
69.39
80.60
63.77
19.34
22.11
9.08
18.45
15.57
6.74
16.99
17.68
16.78
21.66
18.07
16.09
17.72
9.11
5.53
9.28
19.78
17.26
16.58
4.01
1.80
3.36
16.87
11.26
2.45
17.59
9.09
8.97
3.63
1.64
1.81
1.14
1.52
1.56
1.70
3.41
1.28
0.86
0.92
3.23
2.87
0.98
3.26
4.20
11.21
4.56
4.47
4.66
2.68
3.84
5.01
3.60
1.00
0.62
2.55
12.06
9.54
14.54
2.64
1.15
1.98
4.95
8.80
2.85
19.12
26.19
26.24
26.45
25.75
25.87
25.94
25.62
25.71
25.67
25.83
25.67
25.77
25.74
25.84
25.86
25.87
23.90
25.15
25.45
24.04
24.03
24.06
24.35
24.18
24.02
24.12
2.8
2.7
2.2
0.4
2.0
2.5
0.2
1.3
0.1
0.4
0.4
0.2
0.1
d13C relative to PDB. CV: canonical variable ¼ 2.53*d13CSAT þ 2.22d13CARO 11.65.
3. Experimental
3.1. Samples
Crude oil samples were collected in the head of producing wells, so
they are considered “fresh” samples. Samples were collected, after
purging the line, in 1-liter capacity amber glass bottles with screw cap
and Teflon septa. Each sample was carefully labeled, stored in polystyrene coolers and moved to the laboratory (Antek S.A., 2011).
3.2. Methods
3.2.1. SARA analysis
After precipitation of asphaltene with 20 mL of n-pentane, and
filtration, the maltene fraction was separated by conventional
liquid chromatography on activated alumina. The saturated fraction
was eluted with n-hexane, the aromatic fraction was eluted with nhexane:dichloromethane (70:30), and the polar (NSO) fraction was
eluted with dichloromethane:methanol (98:2). The saturated
fraction was analyzed by GC/FID. The branched and cyclic fraction
(biomarkers) were separated from n-alkanes by silicalite powder S115 (Union Carbide, Des Planes, IL) and then analyzed by GC/MS.
The aromatic fractions were analyzed directly by GC/FID and some
selected samples were analyzed for aromatic biomarkers by gas
chromatographyemass spectrometry (GC/MS).
3.2.2. Stable carbon isotope analysis
The saturate, aromatic, resin and asphaltene fractions of
selected samples were analyzed using a static combustion method
described by Engel and Maynard (1989) and the carbon isotope
ratios were measured in a Finningan Delta-E Mass Spectrometer.
The reference material employed was a NBS-22 working standard
(d13C ¼ 29.81&) relative to PDB carbonate (d13C ¼ 0&).
3.2.3. Chromatographic conditions
GC/FID analysis for both saturate and aromatic fractions was
made on a Hewlett Packard 5890 Series II-Plus gas chromatograph
equipped with FID and coupled with a 30 m 0.25 mm 0.25 mm
HP-5 fused silica capillary column. The GC oven temperature was
programmed from 40 C to 300 C at 5 C/min. Helium was used as
carrier gas at a linear velocity of 50 cm/s (12 psig). Data handling
was collected with Agilent Chemstation chromatographic software.
GC/MS of branched and cyclic fractions (biomarkers) were
analyzed using a Varian 3400 CX gas chromatograph coupled to a
Finningan-MAT TSQ 70 Mass Spectrometer. The separation was
carried out using a 60 m 0.25 mm 0.25 mm DB-5-MS fused silica
capillary column heated from 40 C to 140 at 15 C/min and then to
300 C at 2 C/min. The SPI injector temperature was programmed
from 40 C to 300 C at 180 C/min and then kept isothermal until
the analysis was finished. The column was coupled directly to the
ion source through a transfer line operated at 300 C, the ion source
was operated in electronic impact (EI) mode at 70 eV. The MS was
operated in multiple ion detection (MID) monitoring the following
ions: internal standard, C24-d50 (m/z ¼ 98 and 114); n-alkanes (m/
z ¼ 99); sesquiterpanes and diterpanes (m/z ¼ 123), terpanes (m/
z ¼ 191), demethylated hopanes (m/z ¼ 177), and steranes (m/z 217
and 218). The Interactive Software Chemical Information System
(ICIS) version 7.0 (Finningan Corp.) was utilized as data acquisition
system. Biomarker quantitation was done using the high peaks
from the GC/MS traces.
4. Results and discussion
4.1. Bulk properties
The bulk properties studied in selected samples of the Apiay
field are listed in Table 1. In general, samples from the Apiay field
have an API gravity varying from 17 to 57 API, with 0.1e2.0% sulfur
-28,0
-27,5
-27,0
-26,5
-26,0
-25,5
-23,5
-24
AROMATICS
predominance of fluvial continental facies with marina episodes to
the top. Bohorquez and Valderrama (1991) concluded from their
organic and inorganic sedimentary structure studies, facies analysis, body geometries, and the stratigraphic relations, a fluvialdeltaic environment for this sub-basin.
-24,5
-25
13C
a
-25,5
-26
-26,5
-27
13C
APIAY
SATURATES
SURIA
REF/LIB
Fig. 3. Cross-plot of bulk carbon isotope values of aromatic vs. saturate fractions for
selected samples from the Apiay Oilfield.
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content (results not shown). En general, most of the crude oils are
considerer heavy oils, except for Tanane-1/K1 (57 API). High to
medium API gravities and low sulfur contents are generally associated with non-biodegraded oils (Tissot and Welte, 1984; Pan et al.,
2003). SARA analysis was done using “stabilized” (non-topped)
crude oil ranging between 15.57% and 22.11%, low values of resins
(NSO compounds), except for Suria-1/K1, Suria-3/K1, Guayuriba-2,
Reforma-2/K1, and Libertad Norte/K1 (>16%). Crude oils are relatively low in asphaltenes, with the notable exception of The Apiay3, The Apiay-5 and The Apiay-11 wells.
Saturate d13C values for all crude oils span in a relatively narrow
range from 26.45 to 25.62&, whereas aromatic fractions range
between 25.87& and 23.90&, which permits to associate these
samples with a marine origin and suggests that these crude oils
were derived from a source rock composed of a similar type of
organic matter (Ramon and Dzou, 1999). A plot of aromatic vs.
saturate stable carbon isotope values is given in Fig. 3. As it can be
seen, all samples fall in the range of marine oils (non-waxy). For all
samples, the aromatic fractions are more enriched in d13C than
saturate fractions. Samples from the Suria and Reforma/Libertad
areas are more enriched in d13C than Apiay samples due to its
greater marine contribution. Canonical variables (C.V.) show results
<0.47, which confirms a non-waxy marine oil origin (Sofer, 1984).
More negative C.V. indicates more marine input (Peter et al., 2005).
Crude oils from the Apiay-Ariari sub-basin are richer in d13C than
other crudes oils of the Llanos basin, due to the generation of gas in
the sub-basin, which is very reduced in 13C such that the remaining
hydrocarbons in the fluid become d13C enriched (Wang, 1993).
Additionally, the d13C values in the samples present a low dispersion, except for the Apiay-3 well, indicating that there are not
significant variations of organic facies in the source rock. In general,
samples with marine characteristic show d13C values in the range
of 25 to 27&, while hydrocarbons with terrigenous (land plants)
signature are characterized by d13C values lighter than or less
than 30& (Langdon and Abrajano, 1999).
4.2. GC analysis
4.2.1. n-Alkanes and isoprenoids
Gas chromatographic profiles of selected crude oils are shown
in Fig. 4. The geochemical ratios based on n-alkanes distribution
are displayed in Table 2. In general, the n-alkanes are the most
abundant components in the hydrocarbon fraction for all samples
(>60%). Crude oils have an n-alkanes distribution between n-C10
and n-C33 with unimodal distribution and maximum between nC15 and n-C17, indicating an algal/microbial input. However, some
samples (e.g. Apiay-15, Libertad Norte-3/K2, and Reforma-2)
display a bimodal distribution with maximum at n-C15 and n-C25
to n-C27, indicating an algal/microbial input with a significant
input of terrigenous organic matter. A dominance of n-alkanes
Fig. 4. GC/FID chromatograms of saturate fractions of selected crude oils from the Apiay Oilfield.
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J.E. Cortes et al. / Journal of South American Earth Sciences 47 (2013) 166e178
171
Table 2
Biomarker ratios based on n-alkane and isoprenoid distribution in the Apiay Oilfield, Llanos basin.
Area
No
Well
CPI
Pr/Ph
Pr/n-C17
Ph/n-C18
LMWH/HMWH
n-Alkane dominant
APIAY
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
APIAY-1
APIAY-3
APIAY-4
APIAY-5
APIAY-7
APIAY-8
APIAY-9
APIAY-10
APIAY-12
APIAY-15
APIAY-16
APIAY ESTE
SURIA-1/K1
GUATIQUIA-2
GUAYURIBA-2/K1
SURIA-1/K2
SURIA-2/K2
SURIA-3/K1
SURIA-4/K2
SURIA-5/K2
SURIA SUR-1/K2
SURIA SUR-2/K2
SURIA SUR-3/K1
SURIA SUR-4/K1
SAURIO-1/K1
POMPEYA-1/K1
POMPEYA-1/K2
TANANE-1/K1
REFORMA-2/K1
LIB. NORTE/K1
LIBERTAD-1/K1
LIBERTAD-4/K1
LIB. NORTE-3
LIB. NORTE-2
1.03
1.02
1.03
1.03
1.02
1.02
0.99
1.02
1.02
1.01
1.02
1.07
1.02
1.04
1.02
1.02
1.01
1.02
1.01
1.01
1.02
1.01
1.00
1.01
1.02
1.01
1.01
1.01
1.02
1.01
1.01
1.01
1.02
1.01
3.36
1.52
3.31
3.63
3.36
3.60
3.55
3.48
3.47
3.06
3.54
1.92
2.99
2.72
3.03
3.10
3.02
3.29
3.10
3.12
3.12
3.00
3.11
2.99
3.14
3.08
2.99
3.01
2.62
2.84
2.73
2.88
3.02
2.93
0.47
0.47
0.47
0.47
0.46
0.46
0.48
0.46
0.47
0.52
0.47
0.56
0.50
0.52
0.50
0.50
0.51
0.59
0.50
0.51
0.51
0.50
0.51
0.46
0.54
0.53
0.51
0.52
0.56
0.47
0.49
0.51
0.47
0.46
0.15
0.14
0.15
0.14
0.14
0.14
0.14
0.14
0.14
0.19
0.14
0.34
0.18
0.20
0.18
0.17
0.18
0.19
0.17
0.18
0.18
0.18
0.18
0.18
0.18
0.18
0.18
0.18
0.24
0.18
0.18
0.19
0.17
0.17
1.01
0.96
0.98
1.41
1.18
1.36
1.24
1.09
0.90
1.26
1.06
1.03
1.54
0.92
1.77
1.04
0.99
1.15
0.82
1.06
1.09
1.19
1.25
1.10
1.16
1.18
1.03
1.05
0.86
1.27
0.73
0.90
0.91
0.75
n-C15-n-C17, n-C27
n-C15-n-C17, n-C21-n-C23
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17, n-C21-n-C23
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17
n-C15-n-C17, n-C27
n-C15-n-C17, n-C27
n-C15-n-C23, n-C27
n-C15-n-C19, n-C27
n-C15-n-C23, n-C27
n-C15-n-C23, n-C27
SURIA
REF./LIBERT
CPI: carbon preference index: ((n-C23 þ n-C25 þ n-C27) þ (n-C25 þ n-C27 þ n-C29)/2*(n-C24 þ n-C26 þ n-C28); Pr/Ph ¼ pristane/phytane ratio; LMWH/HMWH ¼ low/high
molecular weight hydrocarbons ratio.
around of n-C17 is thus interpreted as an indication of a marine
source (Jones, 1986).
There is not evidence of strong biodegradation of the oils based
on GC/FID profiles, although some crudes show a little unresolved
complex mixture (UCM) in the range between n-C12 to n-C33, which
decreases as the Apiay > Reforma/Libertad > Suria areas (Fig. 4).
However, as we will see later, the presence of high concentrations
of demethylated hopanes (25-norhopanes, m/z 177) would be taken
as evidence of severe biodegradation in some of these crude oils.
Thus, the presence of well preserved n-alkanes profiles plus high
concentrations of demethylated hopanes in the Apiay field samples
is suggested as a “mixed crude oil” evidence which is the result of
mixing of crude oils from old and fresh generation-migration
pulses.
Low molecular weight hydrocarbons (<n-C23) are quit similar than
high molecular weight hydrocarbons (>n-C25), establishing relations
of LMWH/HMWHz1.0 for most of the samples and with an odd-even
predominance around the unity (CPI ¼ 0.99e1.07), which confirms
homogenous maturity for all samples (Hunt, 1995; Peter et al., 2005).
However, some samples show values of LMWH/HMWH<1.0 (Apiay-3,
Apiay-4 and Apiay-12, Guatiquia-2, Suria-2, Suria-4, Reforma-2, Libertad-1, Reforma-4, Libertad Norte-2 and Libertad Norte-3) suggesting low thermal maturity for these crudes oils.
Pristane/Phytane (Pr/Ph) ratios show values in the range of
1.52e3.63 for Apiay area, 2.99 to 3.29 for Suria area, and 2.62 to 2.84
for Reforma-Libertad area, indicating light differences in the
depositional environment. The highest values correspond to crude
oils with oxic environment and terrigenous input, while the lowest
values are associated with sub-oxic environments with marine
influence. According to these results, Apiay crude oils seem to have
a more oxic environment with terrigenous input, while the Suria
area had a fluvio-deltaic marine-microbial environment more oxic
than the Reforma-Libertad depositional environment, which shows
a marine environment with minor terrigenous input.
The relationship between Pr/n-C17 and Ph/n-C18 has been proposed by Lijmbach (1975) to characterize the depositional environment of the crude oils. As shown in Table 2, samples present
homogenous values of Pr/n-C17 and Ph/n-C18 through the sub-
Fig. 5. Correlation between Isoprenoid hydrocarbons/n-alkanes showing the sources
of organic matter and the depositional environment in the Apiay Oilfield.
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Fig. 6. GC/FID of aromatic fractions of selected crude oils from the Apiay Oilfield.
4.2.2. Aromatic hydrocarbons
The GC/FID chromatograms of aromatic hydrocarbons of selected
crude oil are shown in Fig. 6 as reference. The major aromatic and its
alkyl compounds are methyl-, dimethyl- and trimethylnaphthalenes, phenanthrene, methyl- and dimethylphenanthrenes while
the major organosulfur components, analyzed by GC/MS, included
benzothiophene, dibenzothiophene and a series of methyl-,
dimethyl- and trimethyldibenzothiophenes. The distribution of the
aromatic compounds in Colombian basins will be analyzed and
discussed in detail in another paper ready for publication (Cortes et
al., 2012).
The dibenzothiophene/phenanthrene ratio (DBT/P) is thought to
be an indicator of source rock lithology (Radke et al., 1986; Hughes
et al., 1995). DBT/P values range from 0.09 to 0.22 showing the
highest values in the Apiay area and decreasing in Suria and
Reforma/Libertad areas, respectively. A plot of DBT/P vs. Pr/Ph ratio
depicted in Fig. 7 indicates that most samples from the Apiay area,
except for the Apiay Este well and Suria crude oils, fall in the border
between fluvio-deltaic shale and marine and lacustrine shale, while
samples from Reforma/Libertad are in the marine and lacustrine
shale classification. However, there is not evidence for a lacustrine
environment in Llanos basin, at least in Apiay sub-basin, which is
confirmed for steranes distribution (Fig. 13).
7
1A. Marine carbonates
1B. Marine marls
2. Hypersaline Lacustrine and marine anoxic
3. Marine and Lacustrine shale
4. Fluvio-deltaic shale and coal
6
DBT/PHENANTHRENE
basin, varying from 0.46 to 0.52 and 0.14 to 0.24, respectively,
except for The Apiay Este oil. Fig. 5 shows the Pr/n-C17 vs. Ph/n-C18
ratios, where all the samples fall in the region of terrigenous
organic matter (type III kerogen) in an oxidizing environment, with
the exception of Apiay Este, which falls in the border between
terrigenous and transitional environment (type II & III kerogen).
This last well seems to be the result of chemical transformation of
organic material from mixed marine/terrigenous sources (Barakat
et al., 1997). The total carbon preference index, CPI, calculated between n-C23 to n-C29 ranged from 0.99 and 1.04. Low maturity oils
often exhibit CPIs either greater than or less than 1 (Hunt, 1995).
1A
5
4
3
1B
2
1
2
3
4
0
0
1
2
3
4
5
6
7
Pr/Ph
APIAY
SURIA
REF/LIB
Fig. 7. Depositional environment for the Suria, Apiay and Reforma-Libertad areas in
the Apiay Oilfield.
Author's personal copy
J.E. Cortes et al. / Journal of South American Earth Sciences 47 (2013) 166e178
SURIA SUR 1/K2
8β(H)-HD
E+03
4.783
BIS
HD ISOM.
DMHD-2
EU
ND-1
50
RD-2
ND-2
RD-1
8β(H)-D
DMHD-1
NB
m/z:123
4β(H)-EU
100
173
600
700
800
900
1000
Fig. 8. Partial m/z 123 mass chromatogram for bicyclic sesquiterpanes distribution in Suria Sur 1/K2 crude oil. ND-1 & 2: Nordrimane 1 & 2 (M.W. ¼ 194), 4b-EU: C15 4b(H)Eudesmane, Nor-Bis: Norbisabolane, RD-1 & 2: Rearranged drimanes 1& 2 (M.W. ¼ 208), EU: Eudesmane, 8b(H)D: 8b(H)-Drimane, DMHD-1 & 2: Desmethylhomodrimane 1 & 2, Bis:
Bisabolane, HD-isom.: Homodrimane isomer (M.W. ¼ 222), 8b(H)HD: C16 8b(H)-Homodrimane.
4.3. Molecular biomarkers by GCMS
The biological markers were identified based on full scan GC/MS
analysis, comparing the mass fragmentograms of the specific biomarkers with data previously reported (Philp, 1985; Wang, 1993;
Peter et al., 2005) and with crude oils of known composition (e.g.
NGS NSO-1 oil sample, NPD-Nigoga; Boscan crude oil, Venezuela;
Cusiana & Cupiagua crude oils, Colombia) (NPD-Nigoga, 2000;
Antek S.A., 2011).
Bicyclic sesquiterpanes are polymethyl-sustituted decalins
present in crude oils and ancient sediments (Miceli and Philp, 2012;
Yang et al., 2013). Its origin has been associates to high plants/resins
(Van Aarssen and de Leeuw, 1999) but also to algae or bacteria
source (Yang et al., 2013). The 8b(H)-configuration seems to be
more stable than the 8b(H)- (biological configuration), the 8b(H)drimane and homodrimanes dominate in nature sediments and oils
(Noble et al., 1987; Nytoft et al., 2009). As biomarkers, the sesquiterpanes have been used as maturity (Weston et al., 1989), origin,
and depositional environment indicators (Wang, 1993).
Thirteen sesquiterpane compounds, including homodrimane,
drimane, eudesmane, bisabolane and two rearranged drimanes,
named RD1 and RD2, were found in all samples in variable concentrations, with the exception of the Suria-3/K1 and Reforma-2/K1
wells. The relatively high concentration of drimanes and homodrimanes found in the Apiay field samples may be related to prokaryotic organisms (Volkman, 1988; Wang, 1993), while the 4b(H)m/z:123
E+04
1.175
19NIP
100
eudesmane thought to be related to higher plants. Chromatographic profiles and results are shown in Fig. 8 and Table 3. The
Apiay area shows the highest concentrations of sesquiterpanes
followed by Suria and Reforma/Libertad. The results in Table 3
indicate that 8b(H)-homodrimane has higher concentration than
8b(H)-drimane in the Apiay area ranging between 42.8 and 85.6%,
indicating higher maturity over Suria and Reforma/Libertad areas;
while Suria area shows more homogeneous values, suggesting
similar maturity in this area, except for Suria Sur2/K2, Tanane-1/K1
y Guatiquia-2/K1. In the Reforma/Libertad area, the drimanes seem
to have a greater concentration. The wide distribution of the (HD/
30H) ratio ranging between 0.03 y 5.25% supports the concept of a
variable input of marine/terrigenous organic matter through the
sub-basin due to a great variety of depositional environments, according to the existent paleogeographic characteristics and the
marine transgressive process. In all the cases, 8b(H)-Homodrimane
is widely distributed and in higher concentration than eudesmane
compound. Thus, HD/HD þ EU vary between 41.3 and 99% through
the sub-basin. Previous studies have showed that the concentration
of C14 sesquiterpanes is higher at the immature stage, while those
of C15 drimanes and C16 homodrimanes are relatively lower (Yang
et al., 2013).
A set of bi-, tri- and tetracyclic diterpenoids were detected at m/z
123 in relatively high concentrations in all samples compared with
C15 and C16 sesquiterpanes (Fig. 9 and Table 3). The compounds
detected included 19-NIP, 18-NIP, b-labdane, phyllocladane, nor-
SURIA SUR 1/K2
6
2
9
βL
50
1
3
18NIP
7
4
αL
5
R IP
1100
1200
βF
αK
βK
αF
1300
1400
1500
Fig. 9. Partial m/z 123 mass chromatogram showing diterpane distribution in Suria Sur 1/K2 crude oil. 1, 2, 3, 4 & 5: unknown Diterpane (m.w: 274), 6: Bicyclane, 7: Nor-isopimarane
(tricyclane), bL: 8b(H)-Labdane, 9: unknown Diterpane, 19NIP: 19-Nor-isopimarane, aL: a-Landane, 18NIP: 18-Nor-isopimarane, R: Rimuane, IP: Isopimarane, bF: 16b(H)-Phylocladane, bK: ent-16b (H)-Kaurane, aF: 16a(H)-Phylocladane.
Author's personal copy
J.E. Cortes et al. / Journal of South American Earth Sciences 47 (2013) 166e178
23/3
m/z:191
A
E+03
6.771
APIAY EAST
29H
30H
C32
C33
C3
G1
2500
E+03
9.531
2500
E+03
1.224
D33
D31
D32
D30
1500
m/z:217
2000
25,28-BNH
D27/3
D28/3
D24/3
D25/3
25,28,30-TNH
D22/3
D23/3
50
100
1500
m/z:177
B
Tm
Ts
25-NH
22/3
19/3
100
25/3
50
24/4
26/3
20/3
24/3
21/3
100
25-NH
174
2000
C21
C27
C
Steranes
Pregnanes
Diasteranes
ααR
C28
50
C22
1400
ββR
ββS
ααR
C29
1600
1800
2000
Fig. 10. Mass fragmentograms showing the A) Terpane (m/z 191), B) Demethylated hopane (m/z 177), and C) Sterane (m/z 217) distribution in the Apiay East crude oil.
labdane, isopimarane, among others, which have been associated
with Podocarpaceae and Araucariaceae conifers contribution
(Macedo et al., 1999; Fabianska et al., 2003). Its origin has been
associated to conifer (gymnosperms) and resins (Peter et al., 2005).
However, some diterpanes have been found in Athabasca tar sand
and associated to microbial origin (Dimmler et al., 1984) and marine algae (Peter et al., 2005).
In general, the Apiay area shows higher concentrations of
diterpanes than the Suria and Reforma/Libertad areas. Isopimarane compounds show a greater concentration between the
diterpanes in which 19NIP shows the higher concentrations as
showed for IP/SDT ranging between 3.40 and 15.39% and 19NIP/
19NIPþ18NIP ratios varying between 45 and 94% throughout the
basin, except for Saurio-1/K1 well in the Suria area ratios. High
concentrations of diterpanes were found by Palmer and Russell
(1988) in Central Llanos oils. Samples from the Apiay field show
similar diterpanes distribution than samples reported by Dzou
et al. (1999). The abundance of sesquiterpanes over diterpanes
could be evaluated by HD/SDT ratios in Table 3, which vary
widely between 1.1 and 42.9%, except for Libertad-4/K1, confirming that sesquiterpanes are higher than diterpanes through
the sub-basin.
Fig. 10A shows a terpane mass fragmentogram (m/z 191) of the
Apiay Este well. The chromatographic profile contains significant
amounts of the tricyclic terpanes ranging from C19/3 to C30/3 whose
origin is associated with algal/microbial input. Tricyclic terpanes
are higher than pentacyclic terpanes in all samples with TT/
30H > 1.5, except for Libertad-4/K1 well (Table 4). C23/3 is the
dominant tricyclic terpane in the sub-basin, which can be associated with a marine source (Aquino Neto et al., 1983).
When Ts/Ts þ Tm is used as a maturity parameter, it is assumed
that the larger the value, the less mature a particular sample is in
relation to others of a similar source (Jones, 1986). According to the
results in Table 4, Ts/Ts þ Tm in the Apiay field vary from 0.25 to
0.66, indicating different maturity grades between samples. However, the Ts/Ts þ Tm ratio is affected by the thermal maturity
process and also by mineral composition of the siliciclastic source
rock, which catalyzes the biological configuration (Tm) into the
geological configuration (Ts). (Seifer and Moldowan, 1978; Peter
et al., 2005).
Oleanane, a biological marker diagnostic of higher plants has
been suggested as a marker for angiosperms (flowering plants)
(Ekwozor et al., 1979; Moldowan et al., 1994). The oleanane index
(O.I) provides information on the age of the source rock from oil
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J.E. Cortes et al. / Journal of South American Earth Sciences 47 (2013) 166e178
175
C-27
100
APIAY
90
SURIA
0
10
REF/LIB
20
80
PLANKTON
70
60
30
MARINE
50
50
40
40
60
ESTUARINE
70
30
80
20
10
TERRIGENOUS
LACUSTRINE
0
C-28
Fig. 11. Depositional environment for the Suria, Apiay and Reforma-Libertad areas in
the Apiay Oilfield based on Oleanane index vs. Pr/Ph ratios.
characteristics (Alberdi and Lopez, 2000). An O.I. over 0.2 is characteristic of Tertiary source rocks (Peter and Moldowan, 1993). The
results in Table 4 show that most of the samples display O.I. values
less than 0.2, which indicates that these samples were generated
from the Cretaceous or very early Tertiary source rocks. On the
other hand, samples with O.I. values >0.20, could indicate that
some samples in this sub-basin originated or received input from
source rocks deposited in the Tertiary (mixed crude oils). Fig. 11
correlates the depositional environment and geological time for
the Apiay field through the O.I. against Pr/Ph ratio indicating a
marine deltaic depositional environment for the Cretaceous source
rock.
Oleanane index data were used to consistently determine the
age of the source rock for the Apiay field. As it is shown in Fig. 12,
according to samples evaluated in this study, source rock range in
age from the Campanian-Maestrichtian (Late Cretaceous) to
Paleogene (Early Tertiary) which is consistent with the geological
data (see geological setting above). However, Fig. 12 permits us to
observe that samples from all three areas have an oleanane index in
HIGHER
PLANTS
90
100
C-29
Fig. 13. C27, C28, and C29-Steranes ternary diagram for selected samples from the Apiay
Oilfield.
both the Cretaceous and Tertiary, which could be interpreted as
crude oil being expelled from different source rock or organic
matter from the same source rock at different generation-migration
pulses (Lo-Monaco, 2013).
Demethylated hopanes (m/z 177) profiles are shown in Fig. 10B,
which shows the set of 25-norhopanes better preserved than terpanes (m/z 191) and steranes (m/z 217). Norhopane indices (NH
index) show high ratios for crude oils from the Apiay sector varying
between 0.70 and 3.80, while samples from Suria and Reforma/
Libertad show similar values ranging between 0.28 and 0.79, with
Guayuriba-1/K1, Suria Sur-3/K1, Suria-3/K1, and Suria Sur-2/K2 as
an exception, indicating a possible selective biodegradation process. Microbial conversion of hopanes to 25-norhopanes in oils is
the process currently invoked for the presence of these compounds
in crude oil mixtures (Peter and Moldowan, 1993). Samples containing both 25-norhopanes and n-alkanes suggest paleobiodegradation followed by filling of the reservoir by a second pulse of
non-biodegraded oil.
A typical partial steranes (m/z 217) mass fragmentogram of the
Apiay-Este sample is shown in Fig. 10C. The relative abundance of
Fig. 12. Age of Suria, Apiay and Reforma-Libertad crude oils in the Apiay Oilfield based on Oleanane index.
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J.E. Cortes et al. / Journal of South American Earth Sciences 47 (2013) 166e178
Table 3
Biomarker ratios based on Sesquiterpanes and Diterpanes in the Apiay Oilfield samples.
Area
No
Well
HD/HDþD (%)
HD/30H (%)
HD/HDþEU (%)
IP/SDT (%)
HD/SDT
19NIP/19NIPþ18NIP (%)
APIAY
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
APIAY-1
APIAY-3
APIAY-4
APIAY-5
APIAY-8
APIAY-10
APIAY-15
APIAY-16
APIAY ESTE
SURIA-1/K1
SURIA-3/K1
SURIA-4/K2
SURIA SUR-3/K1
SURIA SUR-4/K1
SURIA SUR-1/K2
SURIA SUR-2/K2
SAURIO-1/K1
TANANE-1/K1
GUATIQUIA-2/K1
GUAYURIBA-2/K1
POMPEYA-1/K2
REFORMA-2/K1
LIB. NORTE/K1
LIBERTAD-4/K1
LIB. NORTE-3
77.9
63.4
85.6
57.3
67.0
65.8
42.8
58.5
80.0
64.4
N.A.
64.8
67.7
75.4
52.7
35.1
56.7
46.0
47.4
54.4
63.9
N.A.
33.3
54.3
57.4
0.71
1.12
2.42
2.90
0.52
2.54
2.74
0.93
0.41
1.35
0.60
0.28
0.76
0.63
0.81
N.A.
0.91
5.25
1.65
2.80
0.64
0.16
0.08
0.03
0.87
92.1
87.4
92.0
67.9
77.8
85.4
77.6
77.7
74.3
91.2
83.9
91.4
54.7
84.8
86.4
65.9
69.4
74.2
46.9
79.8
70.6
86.7
99.0
54.7
41.3
6.48
5.81
3.40
14.18
7.25
14.96
7.38
7.38
15.39
7.05
5.86
5.93
7.35
5.64
6.32
13.07
5.72
8.01
12.70
5.70
8.32
5.19
7.69
8.27
5.50
11.4
22.1
29.0
38.0
5.5
36.2
42.9
19.7
7.5
23.4
12.7
1.1
14.1
9.3
14.1
3.8
15.2
15.4
40.2
33.9
7.8
3.5
1.6
0.6
16.7
83
81
81
51
82
45
79
80
79
57
81
80
64
82
84
80
15
89
75
64
76
82
81
86
94
SURIA
REF./LIBERT
N.A.: Not Available; HD: 8b(H)-Homodrimane; D: 8b(H)-Drimane; EU: 4b(H)-Eudesmane; IP: Isopimarane; 30H: 17a(H),21b(H) Hopane; SDT: Total Diterpanes.
C27, C28, and C29 steranes in the oils reflects the carbon number
distribution of the sterols present at the time of deposition and
reflects the organic matter type input to the source rocks of the oils
(Huang and Meinshein, 1979). Fig. 13 shows the ternary diagram of
C27, C28, and C29 steranes distribution based on results on Table 4.
The relative abundance of cholestane, methylcholestane and ethylcholestane from all samples range from 19 to 52%, 16 to 42%, and
24 to 63%, respectively. Based on data in Table 4, it can be seen that
the concentration of C29 regular steranes, especially the bb epimers,
is higher compared to the C27 and C28 regular steranes, which
suggests a terrigenous contribution in the Apiay area. The Suria
samples are comparatively rich in cholestane (C27), while the
Reforma/Libertad and Apiay samples are richer in ethylcholestane,
which permits to establish differences in the biological source input
Table 4
Biomarker ratios based on Terpanes (m/z 191), Demethylated hopanes (m/z177) and Steranes (m/z 217 & 218) in the Apiay Oilfield, Llanos basin.
Area
APIAY
No Well
1
2
3
4
5
6
7
8
9
SURIA
10
11
12
13
14
15
16
17
18
19
20
REFOR.
21
LIBERT 22
23
24
APIAY-1
`APIAY-2
APIAY-4
APIAY-5
APIAY-8
APIAY-10
APIAY-15
APIAY-16
APIAY ESTE
SURIA-1/K1
SURIA-3/K1
SURIA-4/K2
SURIA SUR-3/K1
SURIA SUR-4/K1
SURIA SUR-1/K2
SURIA SUR-2/K2
SAURIO-1/K1
TANANE-1/K1
GUAYURIBA-1/K1
POMPEYA-1/K2
REFORMA-2/K1
LIB. NORTE/K1
LIBERTAD-4/K1 3.67
LIB. NORTE-3
Ts/Ts TT/30H I.O
þ Tm
C31 I.I.,% C31 H.I.,% C27
C28
C29
D/S
steranes steranes steranes
C28/C29
C29
C29
H/S DBT/P NH
20S/20Sþ20R bb/aa þ bb bb-sterane
index
0.44
0.26
0.25
0.41
0.39
0.41
0.35
0.37
0.40
0.59
0.47
0.62
0.53
0.48
0.65
0.47
0.40
0.37
0.66
0.47
N.D.
0.52
0.50
0.47
0.64
0.52
0.58
0.49
0.54
0.62
0.63
0.45
0.65
0.64
0.66
0.54
0.61
0.59
0.51
0.71
0.52
0.56
0.64
0.68
0.49
0.59
0.51
0.59
0.43
0.33
0.40
0.53
0.18
0.44
0.24
0.42
0.51
0.60
0.52
0.75
0.50
0.36
0.39
0.60
0.48
0.53
0.60
0.38
0.29
0.29
0.24
0.44
4.61
5.25
5.94
5.69
3.93
4.57
6.46
3.71
9.41
2.47
1.93
2.86
2.90
2.65
2.32
2.19
3.75
6.62
2.49
1.58
1.52
2.62
0.50
3.80
0.23
0.16
0.07
0.08
0.14
0.29
0.12
0.12
0.09
0.13
0.16
0.29
0.24
0.08
0.28
0.14
0.23
0.09
0.27
0.08
0.19
0.08
0.36
0.16
36.8
35.7
31.1
30.3
34.0
35.2
35.0
35.4
29.8
31.2
38.1
33.7
29.3
38.6
36.6
24.3
33.3
34.7
29.9
39.9
36.5
39.2
52.2
33.2
20.2
21.9
32.6
19.6
29.8
34.3
21.2
31.1
35.6
52.4
20.2
41.7
40.6
20.6
26.8
39.3
26.5
38.1
38.1
28.5
22.3
33.8
18.9
30.8
29.3
35.6
21.4
24.6
16.0
16.0
35.2
27.3
22.2
19.3
37.0
34.5
34.0
17.1
24.2
32.5
27.3
33.2
23.8
31.0
30.0
19.5
17.6
28.3
50.5
42.5
46.0
55.6
54.2
49.6
43.7
41.6
42.1
28.4
42.9
23.7
24.4
62.3
49.0
28.2
46.2
37.7
38.1
40.5
47.7
46.6
63.5
42.0
4.34
3.47
5.08
5.64
3.86
4.32
4.18
3.52
3.87
12.6
5.51
13.7
9.09
3.94
3.63
11.2
3.53
5.94
6.79
9.66
7.12
5.48
3.05
6.08
0.48
0.54
0.54
0.65
0.68
0.48
0.64
0.53
0.33
0.62
0.60
0.66
0.54
0.52
0.61
0.53
0.57
0.62
0.63
0.60
0.64
0.73
0.65
0.58
0.58
0.83
0.46
0.44
0.29
0.32
0.80
0.65
0.52
2.71
0.86
1.45
1.39
0.27
0.49
1.15
0.59
0.80
0.62
0.76
0.62
0.41
0.27
0.67
4.67
3.34
4.73
4.80
5.37
4.06
6.33
4.56
2.92
5.90
4.06
4.88
6.47
2.85
5.31
N.A.
3.22
4.24
4.71
6.49
8.11
3.61
4.66
4.82
0.16
0.18
0.22
0.14
0.16
0.15
N.A.
N.A.
0.62
0.09
0.10
0.10
0.10
0.10
0.11
0.10
0.10
0.11
0.09
0.11
0.11
0.09
0.09
0.10
2.27
2.36
2.24
1.97
0.71
2.09
0.70
0.68
3.80
0.79
1.37
N.A.
2.22
0.44
0.28
0.06
0.68
0.64
2.91
0.48
0.43
0.51
0.54
0.53
N.A.: Not Available; Ts/(Ts þ Tm) ¼ 18a(H)-/[18a(H)- þ 17a(H)]-Trisnorhopane ratio; TT/30H ¼ C19eC28 Tricyclic Terpanes/C30-Hopane; O.I. ¼ Oleanane/C30-Hopane; C31e
I.I. ¼ C31-Isomerization Index ¼ C31 (S)/C31 (S) þ C31(R), the same calculus for C32 and C33; C31eH. I. ¼ C31-Homohopane Index ¼ 100*C31 (S þ R)/(C31eC35)-Homohopanes; C27, C28-, C29Sterane (%) ¼ relative percentages of C27, C28 and C29 steranes within the C27eC29 steranes; D/S: C27eC29 Diasteranes/Regular Steranes ratio; C29(20S/
20S þ 20R) ¼ C29aa20S/(20S þ 20R)(m/z 217); C29 (bb/aa þ bb) ¼ C29(bb 20S þ 20R Steranes/C29aa 20S þ 20R Steranes þ C29bb 20S þ 20R Steranes (m/z 217); C28/C29bbSterane ¼ C28bb-Sterane/C29 bb-Sterane; H/S: C30-Hopane (m/z 191)/C29aa 20S þ 20R Steranes (m/z 217); DBT/P: Dibenzotiophene/Phenanthere; NH Index¼ (BNH þ TNH)/
(C29 þ C30)-Hopanes.
Author's personal copy
J.E. Cortes et al. / Journal of South American Earth Sciences 47 (2013) 166e178
of these oils. Thus, Suria shows a greater input in marine organic
matter whereas Apiay and Reforma/Libertad show a greater
terrigenous organic matter input.
The Apiay field samples exhibit a medium to high (3.05e17.3)
abundance of diasteranes/sterane ratios, which suggests a siliciclastic source rock. Clay mineral catalysis, thermal maturation, and
oxic/anoxic depositional conditions have been proposed and discussed as the principal factors controlling diasteranes formation
(Peter and Moldowan, 1993). The C29 20S/20S þ 20R isomers range
from 0.18 to 0.60 (Table 4) decreasing systematically from Suria to
Reforma/Libertad (south to north). The most mature oils (Suria
area) fall in or near the equilibrium range of 0.50e0.55 (Mackenzie
et al., 1980). The ratio C29 bb/aa þ bb isomers range from 0.33 to
0.73 showing an increase from Reforma/Libertad to Apiay area
(westeeast). However, the Suria area shows the higher maturity
indices followed by the Apiay and Reforma/Libertad, respectively.
5. Conclusions
Crude oils from the Apiay field show the following characteristics:
high concentration of saturate hydrocarbons, LMWH/HMWH y1, nC15 and n-C17 are dominant n-alkanes in the Cþ
15 unimodal distribution
in the Apiay and Suria areas, while the Reforma/Libertad area shows,
in addition, a significant input of n-C27 and n-C29 alkanes. Pr >> Ph,
CPI y1.0, d13CSAT ¼ 25.62 to 26.45, d13CARO ¼ 23.90 to 25.87
with canonical variable ¼ 2.8 to 0.4, sesquiterpanes > diterpanes,
homodrimanes > drimanes, tricyclic terpanes > hopanes,
hopanes >> steranes; Ts < Tm, diasteranes/steranes>3.0; predominance of C29 steranes over C27 and C28 steranes, C29 20S/(20S þ 20R)
sterane ratio ranging from 0.18 to 0.60, while C29 bb/aa þ bb steranes
vary between 0.33 and 0.73 and presence of 25-norhopanes in high
concentration. These geochemical characteristics suggest a transitional fluvio-deltaic depositional environment with predominance of
continental facies of fluvial type and marine episodes, which agrees
with a marine input (algal/microbial) and with a moderate input of
highland organic matter.
A severe biodegradation process was identified in most of the
samples, which is supported by the presence of the complete 25norhopanes series. The presence of 25-norhopanes along with a
well-preserved profile of n-alkanes and isoprenoid hydrocarbons
characterized those samples as “mixed crude oils” as a result of
mixtures of highly biodegraded oil from Gacheta source rock with a
fresh non-biodegraded oil during accumulation in the reservoir.
Molecular organic geochemistry of the Apiay-Ariari sub-basin in
the analyzed samples indicates a complex generation history
(several generationeexpulsion processes), a severe biodegradation
process, and a mixing process from at least two (Cretaceous and
Tertiary) source rocks.
Acknowledgments
The authors wish to thank Ecopetrol, Gerencia Llanos for
permitting us monitoring samples. The experimental assistance of
J. Allen of the Organic Geochemistry Laboratory in the University of
Oklahoma and Dr. M.H. Engel and R. Maynard for the determination
of stable carbon isotope analysis is highly acknowledged. J. Cortes
wishes to thank to the University of Oklahoma and the Organic
Geochemistry Laboratory (Dr. R.P. Philp) in the Geology and
Geophysics School for permitting him to do the experimental work
for his Ph.D. Dissertation. The authors are grateful with the reviewers Dr. L. Lopez and Dr. S. Lo Monaco for their comments,
which contributed to improve the original manuscript. The original
paper has been beneficiated greatly from the advice and review of
Dr. H. Fuentes for their comments and suggestions.
177
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