Organic petrography and geochemistry of the Pabdeh Formation in Khaviz and Mansourabad anticlines, SW Iran
Abstract This paper presents the results from organic geochemical evaluation of the Paleocene–Eocene Pabdeh Formation in the Khaviz Anticline and Mansourabad Oilfield by a combination of Rock-Eval pyrolysis method and organic petrographic techniques. A total of 24 cutting samples, collected from the...
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| Format: | Article |
| Language: | English |
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University of Isfahan
2025-03-01
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| Series: | Journal of Stratigraphy and Sedimentology Researches |
| Subjects: | |
| Online Access: | https://jssr.ui.ac.ir/article_29310_c34b143f8b006bbc7a00fad62a776ecd.pdf |
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| Summary: | Abstract
This paper presents the results from organic geochemical evaluation of the Paleocene–Eocene Pabdeh Formation in the Khaviz Anticline and Mansourabad Oilfield by a combination of Rock-Eval pyrolysis method and organic petrographic techniques. A total of 24 cutting samples, collected from these anticlines, were subjected to Rock-Eval pyrolysis, and 12 samples were subsequently selected for complementary petrographic inspections. According to the Rock-Eval pyrolysis results, the middle parts of the Pabdeh Formation contain predominantly type II organic matter, while the lower and upper parts are mainly characterized by a type II/III kerogen. In addition, Rock-Eval pyrolysis results indicated that the thermal maturity of the studied samples corresponds to the early stages of the oil generation window. Similarly, our organic petrographic observations supported the presence of organic-rich facies in the middle parts of the Pabdeh Formation. The organic-rich facies is characterized by large concentrations of amorphous organic matter (AOM), framboidal pyrite, and solid bitumen (SB). In contrast, the organic-poor zones contain abundant fossils and euhedral pyrite in a calcareous-marly matrix. These results suggest that the middle parts of the Pabdeh Formation were deposited under predominantly anoxic conditions with enhanced preservation of sedimentary organic matter. In contrast, the organic-poor zones were formed under oxidizing conditions which led to the poor preservation of organic matter.
Keywords: Pabdeh Formation, Khaviz Anticline, Mansourabad Oilfield, Organic facies, Paleo-depositional environments.
Introduction
The petroleum industry has an important role in the worldwide supply of energy and the improvement of global economic/technological growth. The Zagros fold and thrust belt of Iran is one of the most prolific petroleum provinces in the world with several supergiant oil and gas accumulations (Alipour 2024). Most of the oil in the Zagros Basin has accumulated in the Dezful Embayment (Bordenave and Huc 1995). The Aptian–Albian Kazhdumi Formation is one of the most important petroleum source rocks that charged the Bangestan Group (Ilam/Sarvak) reservoirs in the Dezful Embayment (Alipour 2022, 2024). The Paleocene–Oligocene Pabdeh Formation is a young source rock with minor petroleum contribution due to limited thermal maturity across most of the Zagros Basin (Alipour 2024).
In recent years the Pabdeh Formation has been evaluated using bulk Rock-Eval pyrolysis techniques (Alizadeh et al. 2012; Safaei-Farouji et al. 2021), organic petrographic methods (Amiri and Alipour 2023a, b; Khazami and Alipour 2024), and advanced biomarker analyses (Alizadeh et al. 2020). Nevertheless, little information is available about the organic petrographic characteristics and organic facies properties of the Pabdeh source rock in the Dezful Embayment. Therefore, the purpose of this study is to investigate the organic petrographic characteristics of sedimentary organic matter contained in the Pabdeh Formation to understand its paleo-depositional environments. Results from this study can provide new insights into the lateral-vertical organofacies variation within the Pabdeh Formation of the Dezful Embayment and surrounding regions.
Material & Methods
In this study, a total of 24 cutting samples (i.e., 10 samples from well#11 of the Mansourabad Field and 14 samples from well#2 of the Khaviz Anticline) were collected from the Pabdeh Formation. These samples were subjected to Rock-Eval pyrolysis analysis using a Vinci Rock-Eval 6 instrument. After preliminary screening, a total of 12 samples (i.e., 6 samples from Mansourabad and 6 samples from Khaviz) were selected for detailed organic petrographic inspections under reflected light.
Rock-Eval pyrolysis: In the present study, in order to represent the entire thickness of the Pabdeh Formation, cutting samples were collected every 20 meters. The cutting samples were cleaned under a binocular to remove non-indigenous particles like mica fragments from drilling mud additives and iron filings from the erosion of the drill bit. In addition, the cutting samples were treated with detergent and n-hexane to remove non-indigenous hydrocarbons arising from drilling mud additives. The cleaned samples were then pulverized in a porcelain mortar. The powdered samples were placed in the oven for 24 hours at a temperature of about 50 ֯C to remove any possible moisture. To perform the analysis, approximately 50–70 mg of the powdered samples were placed in the crucible and then subjected to Rock-Eval pyrolysis following the standard procedure published elsewhere (Behar et al. 2001). The Rock-Eval 6 device is used to evaluate the source rocks in terms of the amount, type, and maturity of their contained organic matter. In addition, analytical parameters derived from Rock-Eval pyrolysis can be used to infer the paleo-depositional environments of source rocks and the prevalence of oxic/anoxic conditions during their sedimentation (Kotarba et al. 2003). The amount of organic matter in a source rock includes the total organic carbon in the sample, which is expressed as a weight percentage and includes both kerogen and bitumen (Hunt 1996).
Organic petrography: In the present study, organic petrographic examinations were conducted by a Zeiss Axioplane II microscope under reflected light in oil immersion with 100x objective magnification. The organic matter contained in fine-grained shales has different petrography with respect to the organic matter preserved in coals (Gonçalves et al. 2024). Kerogen in coal seams may contain 90% detectable maceral and only 10% amorphous organic matter, but kerogen in shale rocks usually contains more than 50% amorphous organic matter (Mastalerz et al. 2018).
Discussion of Results & Conclusions
The combined application of Rock-Eval pyrolysis and organic petrography confirms the existence of an organic-rich interval sandwiched between organic-poor facies. According to our organic petrographic observations, the middle parts of the Pabdeh Formation contain distinct organic facies characterized by abundant AOM, SB, and framboidal pyrite in a dark-colored marl matrix. In contrast, the organic-poor intervals are characterized by calcareous lithology with abundant fossils and negligible AOM. These observations are consistent with the enhanced organic preservation during deposition of the middle parts of the Pabdeh Formation, which was most likely related to the prevalence of anoxia within the paleo-depositional environments. However, the organic-poor sections were deposited under suboxic regimes with minor preservation of the organic matter. These findings have practical implications for paleo-environmental reconstructions during deposition of the Pabdeh Formation. In addition, they provide new insights into variations of organic facies in the Pabdeh Formation as a potential source rock in the Cenozoic succession of the Zagros basin. |
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| ISSN: | 2008-7888 2423-8007 |