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The Geochemical Characterization of
Sediments from Early Cretaceous SembarFormationA. Nazir
a, T. Fazeelat
a& M. Asif
a
a
Chemistry Department, University of Engineering & Technology,Lahore, Pakistan
Version of record first published: 04 Oct 2012.
To cite this article: A. Nazir, T. Fazeelat & M. Asif (2012): The Geochemical Characterization ofSediments from Early Cretaceous Sembar Formation, Petroleum Science and Technology, 30:23,
2460-2470
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Petroleum Science and Technology, 30:24602470, 2012
Copyright Taylor & Francis Group, LLC
ISSN: 1091-6466 print/1532-2459 online
DOI: 10.1080/10916466.2010.519756
The Geochemical Characterization of Sedimentsfrom Early Cretaceous Sembar Formation
A. NAZIR,1 T. FAZEELAT,1 AND M. ASIF1
1Chemistry Department, University of Engineering & Technology, Lahore,
Pakistan
Abstract The soluble organic matter (SOM) and kerogen of sediments contain
organic molecules, and can be interpreted in terms of source, generative poten-
tial, thermal maturity, and depositional environment of the organic matter. EarlyCretaceous sedimentary sequence of Sembar Formation comprising five sedimentssamples was analyzed. Both SOM and hydrocarbons bound to the kerogen terms as
pyrolyzed organic matter (POM) were characterized geochemically. Hydrous pyrolysiswas carried out to release hydrocarbons from extracted sediments and fractionated
by liquid chromatography. Saturated fractions from both SOM and POM were furtheranalyzed by gas chromatography-flame ionization detector. The study suggested that
Cretaceous sequence of Sembar Formation has fair to good hydrocarbon source poten-tial. Thermal maturity parameters indicate onset of oil genesis zone. The presence of
even carbon numbered n-alkenes, Pr/Ph and Pr/n-C17 versus Ph/n-C18 plots revealmarine algal source and anoxic depositional environment of organic matter from
Sembar Formation.
Keywords early Cretaceous, geochemical characterization, hydrous pyrolysis, kero-gen, Sembar formation
Introduction
Sediments provide a dynamic and long-term reservoir for organic species that include
lipids (solvent-soluble organic matter, including hydrocarbons, fatty acids, and alcohols)
and macromolecular organic matter, all derived from natural biogenic and geologic
sources (Simoneit, 1978). The quantity and quality of organic matter preserved during
diagenesis of sediments ultimately determine the petroleum-generative potential of the
rock. The deposition of organic-rich sediments is favored by a high rate of production oforganic matter and a high preservation potential. Sediments are defined as oxic, dysoxic,
suboxic, and anoxic depending on the oxygen content of the overlying waters. Most
petroleum is generated from source rocks deposited under anoxic to dysoxic environments
because they contain more hydrogen-rich organic matter than do oxic sediments. Anoxic
depositional environments are created from the lack of water circulation below the
photic zone in marine or lacustrine sediments. Anaerobic degradation of organic matter
is thermodynamically less efficient than aerobic degradation (Claypool and Kaplan,
1974). This observation supports the prevailing belief that anoxia is the main cause
for enhanced preservation of hydrogen- and lipid-rich organic matter in petroleum source
Address correspondence to A. Nazir, Chemistry Department, University of Engineering &Technology, Lahore 54890, Pakistan. E-mail: [email protected]
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Geochemical Characterization of Sediments 2461
rocks (Demaison and Moore, 1980). Where oxic conditions exist, the organic matter is
largely destroyed during sedimentation and diagenesis, even when organic productivity
is high. The distribution of alkanes and alkenes in sediments is characterized by distinct
carbon number ranges and has been used for the diagnosis of paleo environments (Pearson
and Obaje, 1999) and organic source input (Cranwell, 1982).
Potential petroleum source rocks are described in terms of the quantity, quality, and
level of thermal maturity of the organic matter. A potential source rock contains adequate
amounts of the proper type of dispersed kerogen to generate significant amounts of
petroleum but is not yet thermally mature. A potential source rock becomes an effective
source rock only at the appropriate levels of thermal maturity (i.e., with the oil-generative
window). The main objective of this work was to evaluate depositional environment,
hydrocarbon-generating potential and thermal maturity of Sembar Formation from the
Lower Indus Basin (Khaskheli-1). This geochemical evaluation will provide information
about the source potential of Sembar Formation that subsequently helps to locate oil and
gas in the area.
Experimental
Geological Description of Sembar Formation
The sediment samples analyzed in this study belong to the Sembar Formation of Creta-
ceous age (well Khaskheli-1). The Khaskheli oilfield is situated in the Badin Block, the
southern central portion of the Lower Indus Basin and is about 31 km in the northwest of
Badin town. The samples were provided by British Petroleum Exploration and Production
Inc. (Islamabad, Pakistan). The Lower Cretaceous Sembar Formation consists mainly of
shale with subordinate amount of siltstone and sandstone. Shale of Lower Cretaceous
Sembar Formation is the main source rock in the Lower Indus Basin and the major
component of oil discovered in Badin Platform is believed to have been sourced from
Sembar shales (Kadri, 1993). Most of the Cretaceous shale contains abundant organic
matter and is deposited over most of the Indus Basin in marine depositional environment
and thickness varies from a few meters to 260 m (Iqbal and Shah, 1980; Kadri, 1993).
Total organic carbon (TOC) values of Sembar in Badin area wells range from 0.5% to
3.5% and average about 1.4% (Wandrey et al., 2004). The Sembar Formation is thermally
mature in the western deeply buried part of the shelf and becomes shallower and less
mature toward the eastern edge of the Indus Basin. Geochemical analyses of rock samples
and produced oil and gas in the Indus Basin have shown that the bulk of the hydrocarbons
produced in the Indus Basin are derived from the Lower Cretaceous Sembar Formation(Wandrey et al., 2004).
Determination of TOC
The TOC was determined by wet combustion titration method (Fazeelat and Yousaf,
2004). Briefly, finally grounded sediment (100 mg) was taken in a dry conical flask and
solution of chromic acid (0.4 N, 10 mL) was added to it. It was placed on a sand bath
for digestion at approximately 175C for 3 min. The contents of the flask were allowed
to cool. The volume was then made up to 100 mL by adding distilled water and five
drops of diphenylamine indicator (0.1 g diphenylamine in 50 mL concentration H2SO4)
were added. The contents of the flask were titrated against ferrous ammonium sulfate
(0.2 N) until endpoint green color was obtained. A blank titration was run parallel to
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2462 A. Nazir et al.
each analysis.
Total Organic Carbon (wt%) D2.16(Blank titer Sample titer)
Blank titer Sample weight
Soxhlet Extraction of Organic Matter
The Soxhlet extractor containing antibumping beads, thimble, and glass wool was pre-
extracted with a mixture of solvents, CH3OH, and CH2Cl2 (1:1, 150 mL) for 24 hr. After
pre-extraction, a known weight of grounded sediment (5 g) was taken in the thimble then
covered with glass wool and placed in the extractor. Solvent mixture (CH3OH:CH2Cl2,
1:1; 150 mL) was taken in the round-bottom flask containing some pre-extracted boiling
chips to ensure smooth boiling and extraction was performed for 72 hr. The soluble
organic matter (SOM) was obtained by careful evaporation of the solvent using rotary
evaporator. The extracted sediments were further used for hydrous pyrolysis.
Hydrous Pyrolysis of Extracted Sediments
Hydrous pyrolysis was performed on extracted sediments (Lewan et al., 1979). The
stainless steel tube reactor was washed with water, alcohol, acetone, and dichloromethane
successively. The extracted sediment sample (500 mg) was taken in the tube reactor
(capacity 25 mL) containing 10 mL triply distilled water. It was purged with nitrogen
(for removal of air and to provide inert atmosphere) and sealed. The tube reactor was
heated at 330C for 72 hr in a furnace. The expelled organic matter from extracted
sediments was obtained after evaporation of water and is called pyrolysate (pyrolyzed
organic matter [POM]). Organic matter thus generated (POM) was subjected to column
chromatography.
Fractionation of SOM and POM by Column Chromatography
Both SOM and POM were fractionated into saturates; aromatics; nitrogen, sulfur, and
oxygen (NSO); and asphaltene and resins fractions by column chromatography on silica
gel. A glass column (40 cm 1.2 cm) was packed with slurry of activated silica gel
(105C, 24 hr, 5 g) in n-hexane (25 mL). The bitumen dissolved in n-hexane (50 l) was
introduced onto the column. The saturated fraction was eluted with three bed volumes
of n-hexane, the aromatics with three bed volumes of 95:5 mixture of n-hexane: diethyl
ether, and NSO with three bed volumes of methanol and asphaltene and resins with threebed volumes of chloroform. The fractions were recovered by careful evaporation of the
solvent on a sand bath followed by the removal of residual solvent under nitrogen. The
fractions were collected in pre-weighed sample vials (Fazeelat and Saleem, 2007).
Gas Chromatography-Flame Ionization Detector Analysis
Sample dilution for gas chromatography was made as follows: 1 mg saturated fraction D
50 L n-hexane, 10 mg saturated fraction D 500 L n-hexane (where 1 mL D 1000 L).
Analysis of saturated fraction was carried using a Shimadzu GC-14B Series (Tokyo,
Japan) gas chromatograph, equipped with a flame ionization detector (FID) and fused
silica capillary column, coated with methyl silicone (OV-1), and 30 m 0.25 mm i.d.
film thickness was 0.25 m. The sample 1 L was injected in splitless mode at 60C.
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Geochemical Characterization of Sediments 2463
Detector (FID) and injector temperatures were 300C and 280C, respectively. The oven
temperature was programmed from 60C to 300C a t 4C/min. Nitrogen at a linear
velocity was used as the carrier gas. The data were collected from retention time 066
min (Fazeelat and Saleem, 2007).
Results and Discussion
A total of five sediments were analyzed and geochemically characterized from the Lower
Indus Basin, Pakistan. Column chromatography and gas chromatography-flame ionization
detector (GC-FID) results from both SOM and POM were used to evaluate depositional
environment, generative potential, and thermal maturity of organic matter of Sembar
Formation.
Depositional Environment
The Pr/Ph ratios, presence of n-alkenes, and saturates/aromatics ratios were used toclassify depositional environment of the sediments.
The Pr/Ph ratio can be used to determine the redox conditions of the sediments
during deposition under the assumption that both pristane and phytane originate from
the same source (e.g., phytol side chain of chlorophyll a). Typically Pr/Ph ratios
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Table1
n-A
lk
anesparametersformaturity,S
OM
,anddepositionalenvironmentofthesedimentsandrelativeabundance(%)ofevencarbonnumbered
n-a
lkenesinthePOM
ofthesediments
SOM
POM
Pr/Ph
Pr/n-C17
Ph/n-C18
Relativeabundance(%)
ofn-alk-1
-enes
Sample
Depth,
m
TOC,
wt%
Bit/
TOC
SOM
,
ppm
POM,
ppm
CPI
OEP
CPI
OE
P
POM
SOM
POM
SOMP
OM
SOM
16
18
20
22
24
A
2,55760
0.7
0.101
900
800
0.92
0.88
0.80
0.8
2
0.68
0.89
0.24
0.17
0.31
0.45
9.68
8.57
9.19
9.41
9.57
B
2,56066
0.8
0.095
1,100
1,200
0.96
0.97
0.89
0.8
8
0.52
0.74
0.23
0.16
0.40
0.36
26.33
30.48
28.23
29.53
29.01
C
2,57985
0.9
0.090
1,300
1,600
0.91
0.80
0.76
0.8
8
0.63
0.89
0.18
0.16
0.37
0.37
25.21
26.78
30.29
26.03
27.48
D
2,58591
0.95
0.092
1,800
1,900
0.95
0.94
0.81
0.8
9
0.51
0.60
0.18
0.15
0.27
0.30
23.51
22.3
22.25
21.96
21.7
E
2,59197
1.00
0.091
2,100
2,700
0.99
0.99
1.06
0.9
1
BDL
0.12
BDL
0.10
BDL
0.28
15.27
11.87
10.04
13.07
12.24
Not
e.CPID
(C21
C
C23
C
C25
C
C27
C
C29)C
(C23
C
C25
C
C27
C
C29
C
C31)/2(C22
C
C24
C
C26
C
C28
C
C30);OEPD
(C21
C
6C23
CC25)/4C22
C
4C24;
relativ
eabundanceofn-alk-1-enesD
peakareaofC16
n-alk-1-ene/peakareaofC16-C24
n-alk-1-enes*100;SOM
D
weightofextract/weightofsedim
entstaken*100;
POM
D
weightofextract/weightofsedim
entstaken*100;16D
C16,n-alk-1-ene;18D
C18,n-alk-1-ene;20D
C20,n-alk-1-ene;22D
C22,n-alk-1
-ene;24D
C24,
n-alk-1-ene.
2464
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Geochemical Characterization of Sediments 2465
Figure 1. Pristane/n-C17 versus phytane/n-C18 (SOM and POM) to infer organic matter and
depositional environment of Sembar Formation sediments (adapted from Connan and Cassou
[1980]). *Isoprenoid/n-alkanes ratios of POM 2591-97 m were below detection limits.
and acids in organic matter, which had been reduced to respective olefins with even
number carbon chain under reducing conditions. The presence of even carbon n-alk-1-
enes with maxima at n-C16 and n-C18 (Figure 3) shows contribution of algal, bacterial,
and fungal organic matter (Alboro, 1976).
Table 2
Column chromatography results of SOM and POM
Relative percentages
Saturates Aromatics NSOs Asphaltenes
Saturates/
Aromatics
Sample POM SOM POM SOM POM SOM POM SOM SOM POM
A 14.00 18.20 34.00 37.45 39.00 29.10 13.00 15.25 0.49 0.41
B 18.64 19.42 31.82 36.00 38.54 31.86 11.00 12.72 0.54 0.59
C 19.60 17.86 29.40 32.14 34.15 32.71 16.85 17.29 0.55 0.67
D 22.50 20.71 27.50 30.71 35.25 32.00 14.75 16.58 0.67 0.82E 21.43 21.66 24.29 28.33 35.43 35.66 18.85 14.35 0.76 0.87
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2466 A. Nazir et al.
Figure 2. Gas chromatograms showing distribution of n-alkanes and n-alk-1-enes (*) in saturated
fraction of POM (Samples B and E). Number on peaks refers carbon number of n-alkanes. Pr D
pristane; Ph D phytane; IS D internal standard.
Figure 3. Plot of relative abundance (%) versus carbon number of n-alkenes showing decrease in
concentration ofn-alkenes with both depth and carbon number in POM of sediments. (color figure
available online)
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Geochemical Characterization of Sediments 2467
Figure 4. Classification of sediments (SOM and POM) on ternary diagram (after Tissot and Welte,
1984). Different regions of ternary plot are the following: AA D aromatic asphaltic; AI D aromatic
intermediate; AN D aromatic naphthenic; N D naphthenic; PN D paraffinic naphthenic; P D
paraffinic.
The presence of n-alk-1-enes in sediments is also an indicator of anoxicity of organic
matter (Alboro, 1976) that supported earlier findings about the depositional conditions
(see previous). Table 2 suggests organic matter type from the relative amount of weight
percent of compound classes supported by the ternary plot. The position of the samples of
the well Khaskheli-1, on ternary plot (Figure 4) for bitumen and POM show that on going
from 2,557 to 2,597 m the bitumen composition changes from aromatic naphthenic to
paraffinic naphthenic. It shows a gradual increase in paraffinic and naphthenic character
with an increase in depth. The plot also suggests that the organic matter type is marine.
Generative Potential of Sediments from Sembar Formation
Generative potential is the total amount of organic matter present as solvent soluble
hydrocarbons and kerogen (i.e., SOM and POM). The amount of SOM illustrates thefraction of generative potential that has been effectively transformed into hydrocarbons,
while the amount of POM indicates remaining hydrocarbon potential yet to generate
hydrocarbons in sediments. Generative potential of the Sembar Formation sediments is
determined by measuring TOC, SOM, and POM.
The primary prerequisites for a potential source rock are the quantity, quality, and
thermal maturity of organic matter it contained. TOC values of the analyzed sediments
lie in the range of 0.71.0 wt% (Table 1). Peters and Cassa (1980) suggested TOC values
as poor (TOC 2 wt%). The TOC results from Sembar Formation sediments indicate that most
of them have good source rock potential.
The quantity of organic matter in sediments is interpreted in terms of extracted
(SOM) and bound hydrocarbons (POM). Detailed compositional analysis of SOM in
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2468 A. Nazir et al.
conjunction with kerogen yields the necessary information to make at least semiquanti-
tative predictions about the amount of petroleum, which have been or will be generated
by a given amount of source rock. Generally, the samples containing value of SOM
less than 300 ppm are not considered for source rock potential evaluation (Wasim et al.,
2004). The values of SOM for the analyzed samples range from 900 to 2,100 ppm, which
shows good potential. The values of POM for the analyzed samples range from 800 to
2,700 ppm (Table 1), which is also consistent with SOM and indicates good source rock
potential (Peters and Cassa, 1994).
Thermal Maturity of Sediments From Sembar Formation
Thermal maturity of the samples was determined by bitumen transformation ratio (Bit/
TOC), carbon preference index (CPI), odd-even predominance (OEP), saturates/aromatics
ratio, and isoprenoids to n-alkane ratios. The ratio of extractable bitumen to TOC, called
the transformation ratio, ranges from near zero in shallow sediments to 0.25 (i.e., upto 250 mg/g of TOC) at peak oil generation (Peters and Cassa, 1994). Bit/TOC ratio
is shown in Table 1, where the values (0.090.10) indicate thermal maturity of Sembar
Formation sediments lay within the oil window (cf. Peters and Cassa, 1994). CPI and
OEP are influenced by degree of maturation and type of organic matter. CPI and OEP
values are calculated by using the formula given by Bray and Evan (1961) and Scalan
and Smith (1970).
The CPI are calculated from n-alkanes in the range of C21-C31 carbon numbers and
OEP in the range of C21-C25 carbon numbers (Scalan and Smith, 1970); however, the
range can be adjusted to include any specified range of carbon numbers. The OEP values
increase from 0.80 to 0.99 for SOM and 0.82 to 0.91 for POM as they move from top to
the bottom of the sample sequence. Similarly, CPI values changes from 0.91 to 0.99 for
SOM and 0.76 to 1.06 for POM as the move down to depth of the sediments samples.
This increase in CPI and OEP values reveals thermal maturity of Sembar Formation
sediments increases with increase in depth. The OEP and CPI significantly above or
below 1 indicate thermally immature oil or extract, whereas value of 1 indicates higher
thermal maturity (Peters et al., 2005).
The hydrocarbon composition of sediments was determined using capillary gas
chromatography and Figure 4 shows the GC-FID chromatograms of the representative
sample. Similarly each sediment sample saturated fraction exhibits unimodel n-alkanes
distribution with slight even predominance in the carbon range of C20-C30. SOM was
fractionated into saturates, aromatics, NSOs, and asphaltenes components using col-umn chromatography on silica gel (Table 2). The column chromatographic analysis
of SOM and POM reveals high proportion of aromatic hydrocarbons and polars than
saturated hydrocarbons. The saturate/aromatic ratio observed for the samples increases
with depth from 0.49 to 0.76 for SOM and 0.41 to 0.87 for POM. It is suggested that
saturates/aromatics ratio increases with maturity that indicate maturity of the samples is
increasing with depth (Peters et al., 2005).
The Pr/n-C17 and Ph/n-C18 ratios decrease with increasing thermal maturity as more
n-paraffins are generated from kerogen by thermal cracking (Tissot et al., 1971). The
ratios can be used to evaluate the thermal maturity of nonbiodegraded oils and bitumens
(Peters et al., 2005). The Pr/n-C17 and Ph/n-C18 ratios for the samples are shown in
Table 1 and values are in the range of 0.180.24 and 0.270.40 for POM and 0.10
0.17 and 0.280.45 for SOM, respectively (Table 2). The isoprenoids to n-alkanes ratios
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Geochemical Characterization of Sediments 2469
decrease with depth, which indicates a rise in thermal maturity with increasing depth of
Sembar Formation (Table 1).
Conclusions
Pr/Ph ratio indicates anoxic depositional environment of sediments. Position of SOM and
POM on ternary diagram shows reducing depositional conditions and marine source of
organic matter. Presence of even carbon n-alkenes indicates anoxicity and low maturity
of sediments, particularly in the upper part of the sedimentary column as compared
with deeper samples and also probably show marine anoxic depositional environment.
Pr/Ph and Pr/n-C17 versus Ph/n-C18 plots indicate source of organic matter is marine
algal, deposited under reducing sedimentary environment and increasing maturity with
depth. SOM, POM, and TOC reveal good source rock potential for the Sembar Formation
sediments. Thermal maturity indicated by Bit/TOC ratio reveals that the organic matter
of sediments is within the oil window. CPI and OEP values show increasing thermal
maturity with increasing depth.
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