Geochemical Evaluation of Nayband and Shemshak Formations as

The 1 st International Applied Geological Congress, Department of Geology, Islamic Azad University - Mashad Branch, Iran, 26-28 April 2010
Geochemical Evaluation of Nayband and Shemshak Formations as
Possible Source Rocks in Tabas Basin, Central Iran
Alipour, Majid; Alizadeh, Bahram and Hosseini, Seyed Hossein
Department of Geology, Faculty of Earth Sciences and Remote Sensing,
S. Chamran University of Ahvaz, Iran
Corresponding Author: [email protected]
Abstract
The Tabas Basin, a small part of the Central East Iranian Micro-continent (CEIM), is an intracontinental depression with a complex structural and stratigraphic history. Occurrence of
numerous coal seams intercalated with Jurassic shaly units, emphasizes their importance as
possible source units in this basin. Due to the lack of any exploratory wells for finding
hydrocarbon accumulations, geochemical characterization of the studied source rocks are mainly
based on data gathered by analyzing outcrop samples. In order to determine the TOC content,
level of thermal maturation and hydrocarbon potential of these formations a total of 54 samples
were collected from outcrops of both Nayband and Shemshak formations in the Shotori Range
(The Eastern border of the Tabas Basin). Rock-Eval 6 pyrolysis was carried out on samples.
Screening the data (mainly on the basis of their TOC content and Rock-Eval Tmax values), some
samples were selected for supplementary petrographic studies to acquire reliable data on thermal
maturity of the studied samples. According to the TOC values from Rock-Eval pyrolysis, samples
from Nayband and Shemshak formations show a fair source rock potential with an average TOC
of 0.62 and 0.53 wt% respectively. Average HI values for all of the studied samples are below 50
(mg HC/gr TOC) and are consistent with type IV kerogen. The observed pyrolysis S2 values,
generally lower than 0.2 (mg HC/gr rock), has left the Tmax values ineffective for a precise
maturity assessment (i.e. thermal maturity of the samples is needed to be further supplemented by
vitrinite reflectance measurements). According to average Tmax values, all the studied samples
are grouped in post-mature category. However, some of the observed Tmax values are not
consistent with Ro values. This is assumed to be a result of low TOC content and weathering of
some samples in the outcrop.
Keywords: Rock-Eval 6 pyrolysis, Vitrinite Reflectance, Nayband and Shemshak Formations,
Source Rock Potential, Tabas Basin, Shotori Range.
1. Introduction
The Tabas Basin is an intra-continental depression and part of the Central East Iranian
Micro-continent, (CEIM; Takin, 1972) that has experienced a complex structural history
(Fig. 1). From Precambrian to Permian, central Iran was part of the northwestern
Gondwana (Fursich et al., 2009a). During Permian the CEIM was detached from
Gondwana and moved towards the Eurasia, thereby closing the Paleo-Tethys Ocean. Final
collision between CEIM and Eurasia in the Middle-Late Triassic time (Early Cimmerian
tectonic phase) resulted in the formation of highlands which served as a source for thick
molassic type sediments of the Nayband and Shemshak Formations (Seyed-Emami et al.,
2004; Wilmsen et al., 2009b; Fursich et al., 2009a). Due to the occurrence of coal seems
in these formations they have been the subject of study by numerous authors. In this study
1546
The 1 st International Applied Geological Congress, Department of Geology, Islamic Azad University - Mashad Branch, Iran, 26-28 April 2010
outcrop samples were collected from Southern most section of the Shotori Mountains
(Kamarmacheh Kuh Section). In this section, marine sediments of Nayband Formation
were deposited with no sharp boundary with the overlying Shemshak Formation. The
section has a thickness of up to 1560 m and is composed mainly of shale and sandstone
with some rare intercalations of limestone.
2. Materials and Methods:
Selected samples (54 outcrop samples) were analyzed using Rock-Eval 6 pyrolysis
instrument. A subset of 4 samples was selected for further petrographic analysis in order
to obtain a reliable estimate of the level of thermal maturity. Zeiss Axioplane II
microscope equipped with a J&M photo-multiplayer was used in petrographic studies.
3. Results and Discussion:
Rock-Eval pyrolysis data were used to determine the type, level of thermal maturity and
generative potential of the organic matter. The Rock-Eval pyrolysis maturity parameter
(Tmax) may give erroneous estimate of maturity in poor/lean source rocks (Peters, 1986).
Hence, additional petrogrphic analysis was performed on selected samples for reliable
assessment of thermal maturity.
Results from Rock-Eval pyrolysis, clearly demonstrate the quantity, quality (type) and
thermal maturity of the contained Organic Matter (Table 1).
Amount of the OM contained in a source rock determines the source rock capability to
generate and expel hydrocarbons (Hunt, 1996; Alizadeh et al., 2006). This vital parameter
is achieved by determining Total Organic Carbon (TOC). The TOC content of the studied
samples, are determined to be lean.
Plotting the HI versus OI values, from Rock-Eval pyrolysis data, is the most common
method to determine the type of OM. The OM in the studied samples is mixed Type III
and IV kerogen (Fig. 2).
A precise estimate of the level of thermal maturation in a basin can be extremely useful in
hydrocarbon exploration strategy. Thermal maturity of the OM can be determined by
Rock-Eval data (Tmax and PI) and/or by visual analysis of samples under reflected light.
As for the outcrop samples, the Tmax data have limited usability due to the effects of
processes as sub-aerial weathering.
Tmax values for samples from Nayband and Shemshak Formations range from 454-609
C and 228-612 C respectively. The average Tmax values for Nayband and Shemshak
Formations (518 C and 498 C respectively) put them in the post-mature stage.
Another maturity indicator from Rock-Eval pyrolysis is PI, which is defined as S1/(S1+S2)
ratio. The average PI values for Nayband and Shemshak Formations are 0.21 and 0.18
respectively, indicating a maturity corresponding to end of oil window.
Among the data provided by Rock-Eval pyrolysis, HI and the ratio of S2/S3 are used to
determine the type of hydrocarbons a source rock may generate upon maturation (Peters,
1986). Average HI values for Nayband and Shemshak Formations clearly indicate that the
samples are capable of generating gas (Table 1). Moreover the average S2/S3 values for
Nayband and Shemshak Formations are in accordance with the previous data and indicate
that the source rocks are gas-prone (Peters, 1986).
1547
The 1 st International Applied Geological Congress, Department of Geology, Islamic Azad University - Mashad Branch, Iran, 26-28 April 2010
To achieve the most reliable maturity assessment organically rich samples were selected
for organic petrographic studies. Polished pellets of selected samples were prepared from
bulk rock samples according to the method described by Stach et al., 1982 (Table 2).
Average Ro values of 0.89 and 0.86 % for Nayband and Shemshak Formations are
consistent with peak stage of maturity for the OM contained (Peters et al. 2005) in these
formations (Table 2).
4. Conclusion
This study shows that the OM contained in the Nayband and Shemshak Formations in the
studied section, is mainly mixed Type III and IV kerogen, with some potential to generate
gaseous hydrocarbons. According to the observed Tmax and Ro values, the studied
samples are in the peak to late stage of thermal maturation. The high level of maturity
along with low average HI and S2/S3 values demonstrate that these formations are
considered to be poor source rocks with some potential to generate gas. Hence, the
possibility of generation of some gaseous hydrocarbons in geologic past by these
formations cannot be ignored.
References
[1] Alizadeh B., Tezhe, F., Adabi, M. (2006). “Hydrocarbon Potential of probable source rock in
Marun Oilfield using Rock-Eval 6 Pyrolysis”, Journal of Science University of Tehran, Vol.
32, No. 3. pp. 267-274, autumn 2006.
[2] Akinlua, A. Akinlua A., Jarvie D. M., Ajayi T. R., 2005. A pre-appraisal of the application of
rock-eval pyrolysis to source rock studies in the Niger Delta, Journal of Petroleum Geology,
vol. 28, pp. 39-48.
[3] Hunt, J.M., 1996. Petroleum Geochemistry and Geology. Freeman, New York, p. 743.
[4] Peters, K. E., 1986, Guidelines for evaluating petroleum source rock using programmed
pyrolysis: AAPG Bulletin, v. 70, p. 329.
[5] Peters, K. E., Clifford, C. E. and Moldowan, J. M., 2005. The Biomarker Guide. 2nd ed.
Prentice Hall, New Jersey.
[6] Seyed- Emami, K., Fursich, F.T. & Wilmsen, M., 2004 - Documentation and significance of
tectonic events in the northern Tabas Block (east – central Iran) during the Middle and late
Jurassic. Riv. Ital. Paleont. Strat. 110(1), 163-171.
[7] Takin, M. 1972. Iranian geology and continental drift in the Middle East. Nature, 235, 147150.
[8] Stach, E., M.-Th. Mackowsky, W. Teichmuller, G. H. Taylor, D. Chandra, and R.
Teichmuller, 1982, Coal petrology: Gebruder Borntraeger, berlin, 535 p.
[9] Tissot, B., Welte, D.H., 1984. Petroleum Formation and Occurrence. Second Ed. Springer
Verlag, Berlin.
[10] Wilmsen, M., Fursich, F. T., Seyed-Emami, K. & Majidifard, M. R. 2009b. An overview of
the stratigraphy and facies development of the Jurassic System on the Tabas Block, eastcentral Iran. In: Brunet, M.-F., Wilmsen, M. & Granath, J. W. (eds) South Caspian to Central
Iran Basins. Geological Society, London, Special Publications, 312, 323–343.
1548
The 1 st International Applied Geological Congress, Department of Geology, Islamic Azad University - Mashad Branch, Iran, 26-28 April 2010
[11] Fursich, F. T., Wilmsen, M., Seyed-Emami, K. & Majidifard, M. R. 2009a. The MidCimmerian tectonic event (Bajocian) in the Alborz Mountains, Northern Iran: evidence of the
break-up unconformity of the South Caspian Basin. In: BRUNET, M.-F., WILMSEN, M. &
GRANATH, J. W. (eds) South Caspian to Central Iran Basins. Geological Society, London,
Special Publications, 312, 189–203.
Table 1. Selected Samples Showing Rock-Eval Parameters From Nayband and
Shemshak Formations.
Formation
Nayband
Nayband
Nayband
Nayband
Nayband
Nayband
Nayband
Nayband
Nayband
Nayband
Nayband
Nayband
Nayband
Nayband
Nayband
Nayband
Shemshak
Shemshak
Shemshak
Shemshak
Shemshak
Shemshak
Shemshak
Shemshak
Shemshak
Shemshak
Shemshak
Shemshak
Sample
MZY-869
MZY-870
MZY-871
MZY-872
MZY-873
MZY-874
MZY-875
MZY-876
MZY-877
MZY-878
MZY-879
MZY-880
MZY-881
MZY-882
MZY-883
MZY-884
MZY-885
MZY-886
MZY-887
MZY-888
MZY-889
MZY-890
MZY-891
MZY-895
MZY-896
MZY-908
MZY-909
MZY-911
TOC
0.54
0.47
0.51
0.57
0.62
0.57
0.57
0.58
0.52
0.59
0.56
0.56
0.42
1.27
0.91
0.67
0.68
0.91
1.92
0.52
0.61
0.94
0.58
0.57
0.26
0.51
0.55
0.46
S1
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.02
0.03
0.02
0.02
0.04
0.14
0.03
0.04
0.02
0.03
0.01
0.01
0.04
0.03
0.05
S2
0.02
0.02
0.03
0.04
0.08
0.03
0.02
0.4
0.02
0.05
0.03
0.18
0.01
0.32
0.29
0.13
0.26
0.25
0.61
0.13
0.08
0.23
0.18
0.03
0.02
0.2
0.13
0.21
S3
0.42
0.31
0.39
0.31
0.34
0.47
0.49
0.3
0.4
0.4
0.34
0.46
0.18
0.35
0.15
0.31
0.12
0.25
0.45
0.22
11
0.27
0.13
0.31
0.24
0.23
0.25
0.13
Tmax
521
522
516
520
609
525
525
523
515
518
525
496
454
526
489
514
508
506
461
490
605
517
513
531
604
610
484
380
HI
3.70
4.26
5.88
7.02
12.90
5.26
3.51
68.97
3.85
8.47
5.36
32.14
2.38
25.20
31.87
19.40
38.24
27.47
31.77
25.00
13.11
24.47
31.03
5.26
7.69
39.22
23.64
45.65
OI
77.78
65.96
76.47
54.39
54.84
82.46
85.96
51.72
76.92
67.80
60.71
82.14
42.86
27.56
16.48
46.27
17.65
27.47
23.44
42.31
1803.28
28.72
22.41
54.39
92.31
45.10
45.45
28.26
S2/S3
0.05
0.06
0.08
0.13
0.24
0.06
0.04
1.33
0.05
0.13
0.09
0.39
0.06
0.91
1.93
0.42
2.17
1.00
1.36
0.59
0.01
0.85
1.38
0.10
0.08
0.87
0.52
1.62
PI
0.33
0.33
0.25
0.20
0.11
0.25
0.33
0.02
0.33
0.17
0.25
0.05
0.50
0.06
0.09
0.13
0.07
0.14
0.19
0.19
0.33
0.08
0.14
0.25
0.33
0.17
0.19
0.19
TABLE 2. Average Ro Values for Selected Samples From Nayband and Shemshak Formations.
Formation
Nayband Fm.
Shemshak Fm.
Sample
MZY-872
MZY-876
MZY-885
MZY-887
MZY-909
Ro (%)
0.89
0.88
0.8
0.75
0.82
Figure 1. Generalized geological map of the Tabas Area with location of the
studied section shown by asterisk (modified after Seyed-Emami et al., 2004).
1549
The 1 st International Applied Geological Congress, Department of Geology, Islamic Azad University - Mashad Branch, Iran, 26-28 April 2010
A)(
B)(
Figure 2. Plots of HI versus OI (A), and HI versus Tmax (B), to determine the type and thermal
maturity of the studied formations (modified after Hunt, 1996 and Akinlua et al., 2005).
1550