Evaluation of B-keybed of the Asmari reservoir Caprock, Pazanan Oil field, Iran
Amir Sasan Zarvani1*, Bahman Soleimani1 , Hassan Amiribakhtiyar2
1- Department of Petroleum Geology, Ahvaz University, Iran
2- Head of Geology Department, National Iranian South Oil Company
Tell and Fax: +986113331059
Correspondence E-mail: sasangeo_p@yahoo.com
Abstract:
Recently, petroleum geologists focused their attention on the study of source rock, migration and accumulation
with the help of different geochemical methods. Of these, carbon isotope and biomarkers or chemical fossils are
new tools in petroleum geology especially in source-oil correlation. Gachsaran Formation (Miocene) is the cap
rock of the Asmari reservoir throughout the Zagros. The member 1 of this formation is divided into 6 keybeds
that of these B keybed is very important. It is considered as a lithological indicator in drilling of Pazanan oil
wells. The aim of the present work is to evaluate the cap rock in this oil field in view of lithology, isotopic
(carbon and sulfur) and chemical variations. The geochemical analytical data of bituminous shale (B key bed)
applied to interpret their sedimentary basin and thermal maturity. It seems that they are probable source rocks to
produce petroleum derivations detected in the caprock. Biomarkers such as tricyclic terpanes, bisnorhopane and
low Gammacerane index found in bitumen extracted from the shale are attributed to low salinity. Individual
organic geochemical indicators are also indicating marine-terrestrial environment and low thermal maturity.
δ13C in extracted saturate and aromatic parts of bitumen is measured -25.1 and -22.8‰, respectively.
Carbon isotope value (δ13C) of carbonate sample (C-key bed) is -20.7‰. The value δ34S of Anhydrite in
mudstone (C-Key bed) is 16.6-16.8‰, but Anhydrite samples belong to D-key bed are showing higher values
(20.8-21.1‰). These values reflects the presence of two fluids, diagenetic (brackish) and primary sea water.
These measurements and lithological variations are consistent to biochemical data and verify a Sabkha-lagoonal
environment.
Key word: Pazanan, Caprock, Bitumen shale, key-bed, Biomarker
Corresponding author: Amir Sasan Zarvani, Petroleum Geology Group, Chamran University, Ahvaz, Iran.
Tell: +989171113578 and Fax: +986113331059
E-Mail: zarvani@scu.ac.ir and sasangeo_p@yahoo.com
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Introduction
The study of each part of a petroleum system is necessary. However, recently, petroleum
geologists have focused their attention on the study of the source rock, migration and
accumulation using a variety of different geochemical methods. Of these, carbon isotopes and
biomarkers or chemical fossils are relatively new tools in petroleum geology, especially in
correlation (Miller, 1995).
Caprock analysis is an important part of any exploration project. Unsuitable caprock lithologies
and under pressured conditions can lead to destructive effects, such as well collapse, during the
drilling program. The caprock thus seems to us to be the most important part of the petroleum
system. However, compared to the enormous literature devoted to reservoir rocks and the
massive effort devoted to understanding the geochemistry of source rocks, virtually nothing has
been written on the nature of caprocks. The following articles may be cited: Schowalters (1976),
Mial (2000), Weber et al. (1978), Downey (1984), Grunau (1987), Leythaeuser et al. (1982), and
Grunau (1987).
In all oil fields, the caprock is considered a seal, preventing loss, and a guide. In the southern oil
fields of Iran the caprock also plays an important role in separating two differently-pressured
units (Soleimani et al. 2007).
The aim of this study is the evaluation of the caprock in the Pazanan oil field in terms of its
lithological, isotopic and chemical variations. analysed bituminous shale, carbonate and
anhydrite samples to identify biomarkers and carbon/sulfur isotopic values.
Geology
The Pazanan oil field (Fig.1) is one of the Zagros oil fields and is located 150km SE of Ahvaz
city in the SW of Iran. It is 60km long and 4-6 km wide (Fig. 2). The Pazanan reservoir is a one
of the largest condensate gas reservoirs in Iran, with a thin, but economically producible, oil
column. The Asmari reservoir (of Oligocene age) was capped by the lower part of the Gachsaran
Formation (Member 1). Member 1 was deposited in the early Miocene and is up to 50m thick.
Caprock depth varies from 2580 to 2717m in the northern part of the field, from 1704 to 2444m
in the central part, and from 2050 to 2490m in southern part (Soleimani et al, 2007).
Methodology
Sample Collection- Samples were collected from different depths and locations to provide a
better understanding of the chemical variation, distribution and well defined the key beds of the
caprock.
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GC-MS analysis: Selected samples were prepared by cleaning and pulverizing, followed by
dissolution in suitable solvents and analysis by Liquid Chromatography (GC-MS) By this
procedure, biomarkers of saturated and aromatic bitumen fractions were determined. The results
were reported by Humble Geochemical Services, U.S.A.
Carbon and Sulfur Isotope Analyses: Generally, carbon and sulfur isotopic variation is a
function of environment, and is usually considered as a good paleoindicator. The reported data
expressed as δ13C ‰ and δ34S‰. To carry out carbon and sulfur isotope analyses, selected
samples of caprock, including carbonate, anhydrite and bituminous shales, were sent to Humble
Geochemical Services Laboratory, U.S.A.
It should be noted that continental samples have very low δ13C. When the amount of aromatic
carbon isotope (δ13CAro) is plotted against that of saturates (δ13CSat), marine and terrestrial
environments of deposition may be separated into two domains (Sofer, 1984) given by the lines:
δ13CAro = 1.14 δ13CSat + 5.46
δ13CAro (‰) = 1.12 δ13CSat + 5.45
Discussion and Results:
Sonic and Gamma-Ray log data indicated that the caprock consists mainly of marl and mudstone
(Fig. 3).
Table-1 provides a summary of the general lithological characteristics of the caprock in the
Pazanan oil field. Lithologically, it can be divided into 6 keybeds. Fig.3 showing changes in
thickness of caprock. This variation is also reflected that the surface structure of Pazanan
anticline is similar to a saddle form (normal to axis) (Alavi, 2006). Therefore, it can be expected
that the elevation of central part is less than other parts. In spite of this matter, there is no
evidence of petroleum leakage at present (Fig.4). However, the presence of hydrocarbon traces in
caprock is possible to be related to ancient leakage, but it needs isotope study. Lithological
variations from the bottom to top of caprock markedly show that dominated sedimentary
conditions transitionally changed upward to a quiet environment (lagoon). It should be kept in
our mind an important fact that the bituminous shale keybed (i.e. B) is traceable in all positions
of the field.
These factors as well as lithological variation cause the well-collapse during drilling project.
Bituminous shale sample was analysed geochemically. The analytical data of Bitumen`s
biomarkers belong to member 1 of Gachsaran Formation applied to evaluate sedimentary basin
and maturity evaluation of probable source rocks in Pazanan oil field. GC-MS data (Appendix
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I,II,III) revealed the presence of bisnorhopane C2817a18a21b(h)or BNH C28 which is approved
that bituminous shale deposited in non perfectly reduction conditions (Table 2).
The detection of Gammacerane Index indicates also low salinity (Fig. 5). It can be verified that
these shales are fluculated and deposited during income fresh water or raining season.
Gammaceran eIndex 
gammacerane
17 ( H ), 21 ( H )  hopane
 100
geochemical indicators such as C29/C28+C27 ratio and Pentacyclic terpanes (Table2) applied to
interpret depositional conditions. When the former is low and the later is high refers to non
marine environment (Fig. 6).
The ratio of TS/ hopane and C29TS/ (C29hopane+C29TS) as maturity indicators (Alimi, 2005) were
used. The results showed low maturity (Fig. 7). The samples with high Oleanane Index (more
than 30%) indicating terrestrial sources and the presence of high plants. But low Oleanane Index
(10%) referring a marine source (Hunt, 1996). High ratio of Oleanane/(Oleanane+Hopane) (>0.2)
related to contribution of Tertiary source rocks (Peters et al., 2002). High C29/ C30 ratio in
bituminous shale (0.46) is the reason of non marine deposition.
The presence of tricyclic terpanes, bisnorhopane and low quantity of Gammacrane index is
subsidiary of low salinity. These data showed that bitumen shale of keybed B is immature (Fig.
7). δ13C in extracted saturate and aromatic parts of bitumen is -25.1 and -22.8‰ respectively.
These values verify a marine-terrestrial environment. If the Sofer eq. is used:
1)δ13CAro=-22.8‰> 1.14 × (-25.1) +5.96 = -24.64;
2)δ13CAro = -22.8‰> 1.1 × (-25.1) +3.75= -23.86
3)δ13CAro= -22.8‰ = 1.12 × (-25.1) +5.75= -22.66
The calculated of Eq.3 is the nearest value to measured data. We find that the oil is paraffinic and
terrestrial-marine source.
δ13C‰ (per mil whole rock) of selected carbonate samples of caprock (Table 3) indicates 20.7‰. Bituminous fractions contain -25.1 (in saturate) and -22.8‰ (in aromatic). If we plotted
These data in Sofer diagram (Sofer, 1984) (Fig.8) introduces the terrestrial environment of the
caprock sediments.
Carbon isotope value (δ13C) of carbonate samples is -20.7. These data and lithological variations
verify a sabkha-lagoonal environment. The hot-wet and hot-dry climate can be concluded.
The value δ34S of anhydrite in mudstone is (Table 4) 16.6 to 16.8‰, but it increases in anhydrite
samples (20.8-21.1‰). Therefore, these measurements are indicating marine to nonmarine
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source. The variation reflects the presence of two fluids. One is diagenetic (brackish) with the
salinity of lesser than the primary sea water.
CONCLUSIONS
Studied key beds of Asmari caprock in the Pazanan oil field presents the following results:
1-Mudstone is a dominant lithology.
2- The caprock can be divided into six keybeds: A, B, C, D, E, and F.
3- CV=1.25 that display organic matter with terrestrial source.
4- The presence of pentacyclic terpanes with high ratio (about 61.2%) indicating a terrestrial
environment.
5- High C29/ C30 ratio in bituminous shale (0.46) is the reason of non marine deposition.
6- The presence of tricyclic terpanes, bisnorhopane and low quantity of Gammacrane index are
indicating low salinity. δ13C in saturate and aromatic parts is -25.1 and -22.8‰ respectively.
Bituminous shale of keybed B is immature.
7- The value δ34S of anhydrite in mudstone is 16.6-16.8‰, but it increased in anhydrite samples
(20.8-21.1‰). These values reflects the presence of two fluids, diagenetic (brackish) and
primary sea water. This measurements indicating marine to nonmarine source.
8- Geochemical, isotopic data and sedimentary sequence offer a sabkha-lagoonal environment.
9- Chemical composition variations of key beds demonstrated the changes weathering from wethot to dry-hot during sedimentation.
Acknowledgements
We express our thanks to Shahid Chamran University authorities. We specially thank to Dr.
Amiri Bakhtiar, the Head of Geology Department of National Iranian South Oil Company, and
Prof. H. Alimi, Vice president of Humble Company-Texas. The authors would like to thank
J.L.Berry and Tiller of AIPG (American Institute of Professional Geologist) for reading the
manuscript and making helpful comments.
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References
[1] Alavi, M., 2006, Regional stratigraphy of the Zagros Fold-Thrust belt of Iran and its
proforeland evolution, American Journal of Science, Vol. 304, P. 1–20.
[2] Alimi, H., (2005). Application of petroleum geochemistry to hydrocarbon exploration and
exploitation, Petroleum Geochemistry Short Course at RIRI, 5-7 November, Tehran, Iran.
[3] Downey, M.W. 1984 Evaluating seals for hydrocarbon accumulations. Bulletin American Association of
Petroleum Geology. 68. 1752-1763.
[4] Hunt, J.M. (1996). Petroleum Geochemistry and Geology. 2nd Edition. W.H. Freeman and
Company, New York. 743 p.
[5] Grunau, H.R. 1987 A world wide look at the cap rock problem. Petroleum Geology.,10-245.266.
[6] Leythaeuser, D., Schaefer, R.G and Yukler, A. 1982 Role of diffusion in primary migration of
Hydrocarbons. Bulletin American Association Petroleum Geology, 66.408-429.
[7] Miall, A.D., 2000, Principles of sedimentary basin analysis, 3rd Springer-Verlag, 616p
[8] Prasad, U. 2000, Economic geology, C.D.S., Publications, Daryaganj, 318P.
[9] Peters, K.E., Fowler, M.G. (2002). Application of petroleum geochemistry to exploration and
reservoir management. Organic Geochemistry, v. 33, pp. 5-36.
[10] Schowalter, T. 1976 The mechanics are secondary hydrocarbon migration and entrapment Wyoming
Geology Association Earth science bulletin. ,9,1-43.
[11] Soleimani, B., and Zarvani, A.S., 2007, Lithological and petrophysical evaluation of the
caprock keybeds (Miocene), Asmari reservoir of Pazanan oil field, International Conference
Frontiers and Technologies. Petroleum Engineering, ICFT, Phuket, Thailand, P. 37-41.
6
[12] Sofer, Z.(1984). Stable carbon isotope compositions of crude oils: application to source
depositional environments and petroleum alteration. American Association of Petroleum
Geologists Bulletin, v. 68, pp. 31-49.
[13] Weber, K. J., Mandle, G., Pilaar. W.F., Lehner, F. and Precious, R.G, (1978) The role of faults in
hydrocarbon migration and trapping in Nigerian growth fault structure. 2643-2651. offshore Technology
Conference Houston, paper OTC 3356.
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Figures:
Figure 1: Geological and Geographical location of Pazanan oil field in south west of
Iran.
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Fig.2- Isopach UTM Pazanan oil field map, and location of oil wells.
9
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Fig.3– Thickness variation of caprock in Pazanan oil field.
Fig. 4 – UTM 3D schematic plot of the Pazanan oil field, diagram of the thickness of the
caprock.
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12
Fig. 5. GC-MS Hopanogram related to Bitumen shale of B-key-bed. Small pick
display Gammacerane index that indicator low salinity.
Gammacerane
Fig.6- The C29/C28+C27 ratio indicator
non marine environment.
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Fig.7. The ratio of TS/ hopane and C29TS/ (C29hopane+C29TS) as maturity indicators
(Volkman et al., 1983) were used. The results showed low maturity in Bitumen
shale.
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Fig. 8– The plot of δ13Csat and δ13Carom (Sofer diagram), indicating depositional
environment of the Pazanan caprock bituminous shale.
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Tables:
Table.1-General lithological characteristics of keybeds of caprock, Pazanan oil field
Key
Bed
A
B
C
D
E
F
Description
Anhydrite sometimes forms coarse crystals (0.5mm long), with
interlayers of limestone (what is proportion anhydrite vs lime?)
Bituminous shale with mudstone, with sparse anhydrite.
Mudstone to wackestone with traces of bitumen and dolomite.
Interspace of allochem was filled by anhydrite.
Mudstone dominant. Anhydrite is a minor phase as pore filling,
nodules or veins. The crystal size (of what?) is up to 0.5mm.
Mudstone with interlayers of anhydrite (acicular shape up to
0.4mm long)
Wackestone, mudstone or occasionally packstone, with
anhydrite as filling cast, and sparse quartz (0.08 mm in long
dimension)
Table 2. Bitumen shale biomarkers geochemical analysis results
Peak Label
R.Time
(min.)
40.71
Peak
Area
17172
Ppm
(A.)
21.3
Peak
Height
6653
Ppm(ht.)
C19t
Compound
Name
C19 tricyclic diterpane
C20t
C20 tricyclic diterpane
42.97
24249
30.1
8363
32.4
C21t
C21 tricyclic diterpane
45.29
50484
62.7
19561
75.8
C22t
C22 tricyclic terpane
47.42
72003
89.4
18833
72.9
C23t
C23 tricyclic terpane
49.82
27818
345.2
84202
326.1
C28BNH
C28 17a18a21b(H)bisnorhopane
64.7
25643
31.8
11897
46.1
Gam
Gammacerane
69.53
26964
8
61.2
334.6
87210
337.7
186.6
C29Ts/(C29Ts+C29H)
15198
7
0.15
Ts/(Ts+Tm)
0.18
0.21
0.46
0.40
% Pentacyclic Terpanes
Ts
Ts 18a(H)-trisnorhopane
C29H/C30H
62.13
16
25.8
61.1
50189
0.16
194.4
Table 3- Carbon isotopic data of different bitumen parts and carbonate
Well No.
δ13C per mil
Saturate
δ13C per mil
Aromatic
Pz-1
-25.1
-22.8
Pz-2
δ13C per mil
Whole rock
-20.7
Table 4- Sulfur isotopic data of different parts of anhydrite-mudstone
Well No.
δ34S per mil
whole rock
δ34S per mil
whole rock
δ34S per mil
whole rock
Pz-1
Pz-2
Pz-3
16.6
-------
16.8
-------
---20.8
21.1
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Appendix I:
Appendix II
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19