در مطالعۀ حاضر، سازند ایلام از گروه بنگستان با سن کرتاسۀ پسین در شمالغرب آبدانان واقع در تاقدیس کبیرکوه ازنظر ویژگیهای رسوبی، فرایندهای دیاژنزی و ژئوشیمیایی بررسی شده است. مطالعههای صحرایی نشان میدهند این سازند در برش مطالعهشده با ضخامت 281 متر از سنگآهکهای تودهای، متوسط تا نازکلایه با میانلایههای شیل و آهک آرژیلی تشکیل شده است. مرز بالایی و پایینی سازند ایلام در برش مطالعهشده بهترتیب با سازند گورپی و سازند سروک بهطور ناپیوسته است. بر اساس نتایج پتروگرافی، نه ریزرخساره و یک پتروفاسیس برای سازند ایلام شناسایی شده است که عمدتاً در زیرمحیط رمپ خارجی- حوضه نهشته شدهاند. فرایندهای مهم دیاژنزی که سازند ایلام را در برش مطالعهشده تحتتأثیر قرار دادهاند، عبارتند از: میکرایتیشدن، سیمانیشدن، تراکم مکانیکی و شیمیایی، انحلال، پیریتیشدن، فسفاتیشدن و گلاکونیتیشدن. ویژگیهای ژئوشیمیایی (عناصر اصلی و فرعی) توالیهای کربناتۀ ایلام، بستهبودن محیط دیاژنتیکی سازند را نشان میدهند. تغییرات V/Cr، V/(V+Ni) و Ni/Co نشان میدهند نمونههای کربناتۀ سازند ایلام در شرایط نیمهاکسیدی و عمدتاً احیایی قرار گرفتهاند. تغییر دادههای ژئوشیمیایی عناصر Sr/Ca، Sr/Na،Sr/Mn، Ag، V، Ni، Zn، Sr، Fe، Co و P به شناسایی مرز دو سازند سروک و ایلام در تاقدیس کبیرکوه در برش آبدانان کمک کرده است.
عنوان مقاله [English]
Depositional-diagenetic and geochemical characteristics of the Ilam Formation in northwest of Abdanan, Kabir Kuh
In this study, the Late Cretaceous Ilam Formation of the Bangestan group in the northwest of Abdanan in the Kabir-Kuh anticline has been investigated for sedimentological and geochemical properties, and diagenetic processes. Field studies indicate that this formation in studied section reach a thickness of 281 m and composed of medium to thin-bedded limestones with shales and argillaceous limestones intercalations. The upper and lower boundaries of the Ilam Formation in the studied section are disconfirmable with the Gurpi and Sarvak formations, respectively. Based on the petrographic studies, nine microfacies and one petrofacies have been identified for the Ilam Formation which mainly deposited in the outer ramp-basin setting. The main diagenetic features affecting the Ilam Formation in the studied section include micritization, cementation, mechanical and chemical compactions, dissolution, pyritization, phosphatization, and glauconitization. Geochemical characteristic (major and trace elements) of the Ilam carbonate deposits indicate closed diagenetic system. Variations of V/Cr, V/(V+Ni) and Ni/Co indicate that the carbonate samples of the Ilam Formation are subjected to suboxic and mostly anoxic conditions. Separating the Sarvak and Ilam boundary can be aided by some geochemical data variations such as Sr/Ca, Sr/Na, Sr/Mn, Ag, V, Ni, Zn, Sr, Fe, Co and P at the Kabir-Kuh anticline in the Abdanan section.
Key words: Oxic-Anoxic conditions, Sedimentary environment, Zagros, Upper Cretaceous, Diagenesis
During the Cretaceous, the Zagros basin had different environmental conditions so that this variation caused lateral and vertical sedimentary facies changes and numerous source rocks and oil reservoirs were created in this basin. Therefore, the Cretaceous sequences have long been of interest to many geologists (e.g. Kashfi 1976; Bahroudi and Talbot 2003; Alavi 2004; Sepehr and Cosgrove 2004; Insalaco et al. 2006; Fakhari et al. 2008; Van Buchem et al. 2010; Agard et al. 2011; Esrafili-Dizaji and RahimpourBonab 2013; Asadi Mehmandosti et al. 2013; Spina et al. 2018).
From Albian to Campanian, a large sedimentary cycle of the Kazhdumi, Sarvak, Surgah and Ilam formations can be identified in the Zagros region, called the Bangestan Group. The Sarvak and Ilam formations of this group, after the Asmari Formation, are the most important reservoir rocks in the Zagros region that are exposed in Iran and the countries around the Persian Gulf basin.
In this study, an outcrop of Ilam Formation in the Kabir Kuh anticline (Abdanan section) is selected to study. The aims include: to identify lithological features, study of microfacies, diagenesis processes, identification of pale-sedimentary environment and determination of redox conditions based on Ni/Co, V/Cr and V/(V+Ni) changes. In addition, due to the lithological similarities between the Ilam and Sarvak formations and the absence of the Surgah Formation in between in the study area, precise determination of the boundaries between these two formations through geochemical data is considered. These studies can provide a detailed picture of the sedimentological status of the studied basin at the time of the Ilam Formation carbonate deposition.
Material & Methods:
To determine sedimentary characteristic of the Ilam Formation, 90 thin sections have been studied. Thin sections were stained with potassium ferricyanide and alizarin-red S solution (Dickson 1965). The petrographic classification for carbonates is based on the Dunham limestone classification (1962) and Embry and Klovan (1971). Flügel (2010) facies belts and sedimentary models were also used. After petrographic studies, twenty powdered micrite samples (18 samples from Ilam Formation and 2 samples from Sarvak Formation) were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) by 4-Acid Digestion method for trace, major and rare earth element contents at the Met-Solve Analytical Services Inc., BC, Canada.
Discussion of Results & Conclusions
Field work studies in the Kabir Kuh anticline (Abdanan section) indicate that the Ilam Formation has 281 m thickness and composed of medium to thin-bedded limestones with shales and argillaceous limestones intercalations.
Based on the petrographic studies, nine microfacies and one petrofacies have been identified for the Ilam Formation as follows: MF T1: Bioclast Ooid Packstone, MF T2: Algal Bioclast Packstone, MF T3: Peloid/Benthic Foraminifera Packstone, MF T4: Bioclast Packstone, MF T5: Echinoderm Fragments Oligosteginid Packstone, MF T6: Peliodal/Planktonic Foraminifera/Oligosteginid Wackestone to Packstone, MF T7: Oligosteginid Packstone, MF T8: Planktonic Foraminifera/Oligosteginid Wackestone to Packstone, MF T9: Planktonic Foraminifera Mudstone, PF: Shale/Calcareous Shale. The Ilam Formation mainly deposited in the outer ramp- basin setting at studied section.
The main diagenetic features affecting the Ilam Formation in the studied section include micritization, cementation, mechanical and chemical compactions, dissolution, pyritization, phosphatization, and glauconitization. The diagenesis sequence were effected the Ilam Formation carbonates during the three stages of eogenesis, mesogenesis and telogenesis.
Investigation of geochemical data shows high amounts of Fe, V/Cr, V/(V+Ni) and Ni/Co ratios in studied carbonates and indicate that the carbonate samples of the Ilam Formation are subjected to suboxic and mostly anoxic conditions.
Early pyrite observed in the Ilam Formation also indicates the abundance of iron ions in seawater and the dominance of the reductive conditions at their formation.
In addition, Sr/Mn and Sr/Ca changes indicate that the Ilam Formation diagenetic system is closed in the studied section, which is in agreement with the low porosity observed in these carbonates.
Therefore, with respect to the sedimentary environment, the existence of anoxic conditions at the time of formation and close diagenetic system that prevented the formation of suitable porosities in the Ilam sediments in Abdanan section, it can be concluded that this formation do not have a good reservoir quality in studied section.
Separating the Sarvak and Ilam boundary can be aided by some geochemical data variations such as Sr/Ca, Sr/Na, Sr/Mn, Ag, V, Ni, Zn, Sr, Fe, Co and P at the Kabir-Kuh anticline in the Abdanan section.