دانشگاه اصفهان پژوهش های چینه نگاری و رسوب شناسی 2008-7888 39 3 2023 09 23 Organic facies and organic petrographic characteristics of the Pabdeh Formation in the Kilur-Karim Oilfield, SW Iran بررسی رخسارۀ آلی و ارزیابی خصوصیات پتروگرافی آلی سازند پابده در میدان نفتی کیلورکریم، جنوب ‎‍غرب ایران 1 14 28056 10.22108/jssr.2023.139363.1270 FA سارا امیری گروه زمین شناسی نفت و حوضه های رسوبی- دانشکدة‌ علوم زمین- دانشگاه شهید چمران اهواز- اهواز- ایران مجید علیپور گروه زمین شناسی نفت و حوضه های رسوبی- دانشکده علوم زمین- دانشگاه شهید چمران اهواژ- اهواز- ایران Journal Article 2023 10 05 Abstract In this research, the organic matter present in the Pabdeh Formation is investigated using Rock-Eval pyrolysis and organic petrographic techniques. For this purpose, a total of 22 cutting samples were collected at 50-meter intervals from 3 wells in the Kilur-Karim Oilfield. According to hydrogen index (HI) versus maximum temperature (Tmax) diagrams, the Pabdeh Formation in this oilfield contains type II/III kerogen, with thermal maturity corresponding to the early stages of hydrocarbon generation. In general, according to the Rock-Eval pyrolysis data, the middle parts of the Pabdeh Formation show a higher potential for hydrocarbon generation. On the other hand, organic petrography results show that samples from the middle parts of the Pabdeh Formation contain high amounts of amorphous organic matter along with solid bitumen. In addition, the upper and lower parts of this formation contain lower amounts of organic matter and mainly have abundant fossil content. A combination of results from two analytical techniques reveals that the Pabdeh Formation in the Kilur-Karim Oilfield contains a relatively organic-rich zone in the middle, which is surrounded by organic-poor facies above and below. These results can be helpful for a better understanding of the organic facies and palaeodepositional environments of the Pabdeh Formation in the study area. Keywords: Pabdeh Formation, Organic petrography, Organic facies, Kilur-Karim Oilfield Introduction In this study, the organic geochemical characteristics of the Pabdeh Formation in the Kilur-Karim Oilfield have been investigated using the Rock-Eval pyrolysis and organic petrography methods. The Kilur-Karim Oilfield is one of the hydrocarbon fields located in the southern Dezful Embayment, which is located in the vicinity of the Bibi Hakimeh Oilfield about 40 km north of the Persian Gulf. This oilfield generally has a northwest-southeast trend and is separated from the Bibi Hakimeh structure by a thrust from the north side. It is worth mentioning that in previous studies, the organic geochemistry of the Pabdeh Formation has been investigated using different geochemical techniques (such as Rock-Eval pyrolysis and gas chromatography-mass spectrometry analyses) and also modeling techniques (Alizadeh et al. 2012, 2020; Karimi et al. 2016; Vatandoust et al. 2020; Safaei-Farouji et al. 2021). However, microscopic studies have not been conducted on this formation so far. Therefore, the organic petrographic characteristics of the Pabdeh Formation remain largely undocumented in the Zagros Basin. The purpose of the present study is to investigate the organic matter contained in the Pabdeh Formation using organic petrographic techniques in order to better understand its Palaeodepositional environments. In addition, organic petrographic results are used in combination with the Rock-Eval pyrolysis data to throw light on the organic facies of the Pabdeh Formation. Materials & Methods In this study, a total of 22 cutting samples from the Pabdeh Formation were analyzed using a Rock-Eval 6 pyrolysis instrument. The Rock-Eval 6 device is one of the most cost-effective laboratory methods for the geochemical evaluation of hydrocarbon source rocks and for the evaluation of thermal maturity (Lafargue et al. 1998). Among the advantages of this device are raising the analysis temperature to about 850 degrees centigrade, measuring the amount of total organic carbon (TOC) with higher accuracy, as well as distinguishing between organic and inorganic carbon (Behar et al. 2001). In this study, the standard method (Espitalié et al. 1977; Peters 1986) was followed for Rock-Eval pyrolysis. The cutting samples were taken at regular intervals of 50 meters from the studied wells. Since the Pabdeh Formation in the studied wells was drilled with oil-based mud, to remove the effects of contamination, first the samples were washed using a solvent (diluted chloroform) or detergent and placed in the oven for 72 hours. In the next step, the allochthonous organic matter (such as mica pieces and iron shavings) was separated from the samples. The cleaned samples were pulverized into pieces smaller than 80 microns using a mortar and 50 to 70 mg of them were subjected to the Rock-Eval pyrolysis. This analytical method provides valuable data, including numbers of peaks, that enable geochemists to infer the amount, type and maturity of the organic matter. One of the applications of the data derived from the Rock-Eval pyrolysis is to plot HI data either in front of the oxygen index (OI) or Tmax data (Hunt 1996). These diagrams are used to determine the type of organic matter and thermal maturity respectively (Tissot and Welte 1984; Dembicki 2009). In addition, data from the Rock-Eval pyrolysis can be used for drawing geochemical logs, which provide an opportunity to investigate the vertical and lateral changes in geochemical characteristics (Peters and Cassa 1994; Peters 1986). A number of geochemical parameters are calculated based on the raw data provided by the Rock-Eval device. The HI is calculated from the ratio of S2 to the TOC and the OI is calculated from the ratio of S3 to the TOC. Finally, the hydrocarbon generation potential is calculated using the ratio of the S1/(S1+S2) (Behar et al. 2001). In this research, after carefully investigating the results from the Rock-Eval pyrolysis (22 samples of Pabdeh Formation), a total of 10 samples were selected for organic petrographic studies. For this purpose, polished pellets were prepared according to standard methods (Taylor et al. 1998). At first, the cuttings were placed inside the epoxy resin in a way that the resin penetrated into the space between all the particles. Then, an activator was added to the prepared mixture and after hardening, the surface of the samples was polished (Bustin et al. 1983). For organic petrographic inspection, the polished pellets were examined using a Zeiss-AxioPlan-II reflective microscope under 100x magnification and in oil immersion. Discussion of Results & Conclusion We report the results of Rock-Eval pyrolysis and organic petrography of samples related to the Pabdeh Formation in three wells of the Kilur-Karim Oilfield. According to the HI versus OI diagram, this formation contains type II/III kerogen, which is at the beginning of the hydrocarbon generation window. According to diagrams of S1+S2 versus TOC, the hydrocarbon generation potential of the Pabdeh Formation in this oilfield is fair to good. The relatively high values of S2 and TOC in the middle parts of the Pabdeh Formation indicate that these parts are richer than the lower and upper parts. Based on organic petrographic observations, the middle parts of the Pabdeh Formation were deposited under anoxic conditions and contain significant amounts of amorphous organic matter. The upper and lower parts of this formation were deposited under oxic conditions and contain lower concentrations of organic matter. These results support the conclusion that the middle parts of the Pabdeh Formation contain a distinct type of organic facies, which has higher hydrocarbon generation potential. On the other hand, the lower and upper parts of the Pabdeh Formation are characterized by poor organic facies with limited generative potential. Our results suggest that this pattern of organic facies variation within the Pabdeh Formation is mainly controlled by the palaeo-depositional conditions. As a result, it can be concluded that only the middle parts of the Pabdeh Formation in the studied oilfield have hydrocarbon generation potential. These results can be helpful not only for modeling hydrocarbon systems, but they are also very important in reconstructing the palaeodepositional conditions during the deposition of the Pabdeh Formation. در این تحقیق، ‌‌مواد آلی موجود در سازند پابده، با استفاده از روش‎‍های پیرولیز راک ـ ایول[1] و پتروگرافی آلی[2] بررسی شده است. برای این منظور، تعداد ۲۲ نمونه خردۀ حفاری با فواصل منظم ۵۰ متری از ۳ چاه در میدان کیلورکریم گردآوری‌ و‌ آنالیز شد. براساس نمودار HI در مقابل Tmax، سازند پابده در این میدان، کروژن نوع II/III و در مراحل آغازین هیدروکربن‎‍زایی قرار دارد. به‎‍طور کلی‌ بر‌اساس داده‎‍های راک ـ ایول، بخش‎‍های میانی سازند پابده، پتانسیل بهتری را برای زایش هیدروکربن نشان می‎‍دهند. از سوی دیگر، نتایج پتروگرافی آلی نشان می‎‍دهد‌ نمونه‎‍های مربوط به بخش‎‍های میانی سازند پابده، مقدار درخور توجهی مادۀ آلی آمورف به‌همراه بیتومن جامد[3] دارد که در یک زمینۀ تیره‌رنگ قرار گرفته‎‍ است. در مقابل، بخش‎‍های بالا و پایین این سازند، در بر دارندة مقادیر ناچیزی از مادۀ آلی‌اند و عمدتاً‌ حجرات فسیلی متعدد و زمینۀ روشن دارند. تلفیق نتایج حاصل از این دو روش، مبین آن است که سازند پابده در میدان کیلورکریم، یک بخش نسبتاً غنی از مادۀ آلی دارد که با بخش‎‍های فقیرتر در بالا و پایین احاطه شده است. این نتایج برای شناخت بهتر رخسارۀ آلی و شرایط محیط رسوبی دیرینۀ سازند پابده در منطقة مطالعه‌شده راهگشاست. [1] Rock-Eval pyrolysis [2] Organic petrographic [3] Solid bitumen

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