بررسی رخساره‌های رسوبی و پارامترهای ژئوشیمیایی سازند آسماری (الیگوسن - میوسن) در میدان نفتی شادگان، فروافتادگی دزفول، جنوب غرب ایران

نوع مقاله : مقاله پژوهشی

نویسندگان

1 شرکت ملی مناطق نفت‌خیز جنوب، اهواز، ایران

2 گروه حوضه‌های رسوبی و نفت، دانشکده علوم زمین، دانشگاه شهید بهشتی، تهران، ایران

چکیده

در این پژوهش، محیط رسوبی و مطالعات ژئوشیمیایی سازند آسماری با ضخامت 9/363 متر در چاه شماره 11 از میدان نفتی شادگان در فروافتادگی دزفول، حوضه زاگرس بررسی شد. سازند آسماری در میدان نفتی شادگان عمدتا از سنگ آهک و دولوستون­های متخلخل در تناوب با ماسه سنگ­ها و شیل­ها تشکیل شده است. در چاه شماره 11، سازند آسماری با سن الیگوسن (شاتین) و میوسن زیرین (آکی­تانین - بوردیگالین) با ناپیوستگی هم شیب رسوبات مارلی و شیلی سازند پابده را می­پوشاند و توسط رسوبات تبخیری سازند گچساران پوشیده می­شود. در این توالی 26 ریزرخساره کربناته - تبخیری شناسایی گردید که در چهار محیط اصلی رمپ داخلی، رمپ میانی، رمپ بیرونی و حوضه نهشته شده­اند. محیط رسوبی سازند آسماری یک رمپ هموکلینال معرفی شده است. مطالعات ژئوشیمیایی و پتروگرافی کربنات­های سازند آسماری نشان می­دهد که آراگونیت، ترکیب کانی­شناسی اولیه این سازند بوده است. دامنه تغییرات ایزوتوپ اکسیژن در سنگ آهک­های آسماری بین ‰PDB 85/0- تا ‰PDB 96/8- (میانگین‰PDB 99/2-) و مقادیر ایزوتوپ کربن بین‰PDB 86/5- تا‰PDB 56/1 (میانگین‰PDB 74/0-) متغیر است. نتایج این مطالعه نشان می­دهد که ترکیب ایزوتوپی سنگ آهک­های سازند آسماری نسبتا اولیه بوده و عمدتا در تعادل ایزتوپی با آب دریای پالئوژن - نئوژن بوده است. اگرچه برخی از نمونه­ها توسط فرآیندهای دیاژنتیکی در طول تدفین در سیستم دیاژنتیکی نیمه بسته تا بسته بعدی تحت­تاثیر قرار گرفته­اند. مقادیر نسبتاً بالای Sr/Mn نیز حاکی از سیستم دیاژنتیکی بسته تا نیمه بسته (Closed to semi-closed diagenetic system) با نسبت پایین تبادل آب به سنگ (Water/rock interaction) برای کربنات‌های سازند آسماری است. سبک­ترین ایزوتوپ اکسیژن (‰96/8-) دمایی معادل 8/68 درجه سانتی گراد را نشان می­دهد که بدیهی است باید دمای محیط دیاژنز تدفینی باشد و سنگین ترین میزان ایزوتوپ اکسیژن (‰85/0-)، حداقل دمای رسوبگذاری معادل 23 درجه سانتی گراد را نشان می­دهد.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Investigation of sedimentary facies and geochemical parameters of the Asmari Formation (Oligocene-Miocene) in the Shadegan Oil Field, Dezful Embayment, SW Iran

نویسندگان [English]

  • Armin Omidpour 1
  • Roghayeh Fallah-Bagtash 2
1 National Iranian South Oil Company, Ahvaz, Iran
2 Department of Petroleum and Sedimentary Basins, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran
چکیده [English]

Introduction
Carbonates of the Asmari Formation form the youngest reservoir rock of the Zagros Basin (Ghazban, 2007). Various studies have shown that sedimentary geochemistry can be used to evaluate palaeotemperature and palaeoclimate by reconstructing the chemical and isotopic content of the ancient seawater and/or diagenetic fluids (Winefield et al, 1996; Adabi, 2004; Adabi and Mehmandosti 2008; Crowe et al, 2013; Swart, 2015; Fallah-Bagtash et al, 2020; Omidpour et al, 2021). The present study is based on a combination of different data such as core analysis, thin-section petrography, trace-element and stable-isotope analysis of Asmari carbonates to recognize the original carbonate mineralogy and diagenetic environment. 
Materials and methods
In this study, 524 thin sections prepared from core samples of well-11 were used to achieve the desired goals. All thin sections were stained with potassium ferricyanide and Alizarin Red-S to dolomite and calcite minerals identification (Dickson, 1965). The nomenclature for carbonate rocks used in this work is a combination of the terminology introduced by Dunham (1962) and Embry and Klovan (1971), which is based on textural aspects. Facies analysis and interpretation of the depositional environment was performed using by Burchette and Wright (1992) and Flügel (2010) schemes. Based on the detailed petrographic results, forty-five limestones and thirty-two dolomites from well-11 were carefully selected for trace elemental analysis. Elemental analysis carried out using atomic absorption spectrometry (AAS) in the geochemistry laboratory in the Ferdowsi University of Mashhad for the major and trace element determinations. Forty-five powdered of the limestone samples, that were previously used for the major and trace elements, were analyzed with a VG STRA Series II for oxygen and carbon isotopes at the G.G. Hatch Stable Isotope Laboratory, University of Ottawa.
Results and discussion
The detailed thin-sections analysis of the carbonate samples resulted in the distinction of 26 microfacies types in a subsurface section of the Shadegan Oil Field along that have been deposited along a homoclinal ramp-type platform and is divisible into an inner ramp, mid ramp, outer ramp and basinal settings.
The bulk-rock oxygen and carbon isotopic analyses of the Asmari limestones are compared with similar analyses of the Asmari Formation in the Dezful Embayment (Aqrawi et al, 2006) and of other Palaeogene-Neogene carbonates (Veizer et al, 1999). It can be deduced from Figure 10 that the δ18O and δ13C values correlate well with those found by Aqrawi et al. (2006) for the Asmari Formation, but show slightly lower δ18O and δ13C values than those mentioned by Veizer et al. (1999) for the Palaeogene-Neogene carbonates. The geochemical and isotopic data allow, in combination with the petrographic data obtained from thin-section analysis, recognition of the primary aragonite mineralogy and the evolution of the rock fabric, as well as a reconstruction of the diagenetic evolution, temperature, the nature of the percolating fluids, and the water/rock ratio or diagenetic system.  The input of the δ18O-enriched samples (-0.85‰) within the Anderson and Arthur (1983) formula gives a syn-sedimentary temperature of only 23 °C. Based on textural and geochemical features four types of dolomite (D1 to D4) were identified in the sedimentary succession of the Asmari Formation. D3 (with high values of iron and manganese) is more affected by diagenetic alteration than the D1. Due to oxidizing conditions, iron and manganese values in D1 (near-surface) are lower than the burial dolomites (D3), which formed under a more reducing state at the greater depth of burial (Tucker and Wright 1990; Hou et al, 2016).

کلیدواژه‌ها [English]

  • Shadegan Oil Field
  • Oxygen and carbon isotopes
  • Oligocene-Miocene
  • Asmari Formation
  • Diagenetic system

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پژوهشهای دانش زمین
دوره 13، شماره 2 - شماره پیاپی 50
پژوهشی
مرداد 1401
صفحه 162-188

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