ژئوشیمی مولکولی و مطالعه زیست نشان گرها در سازند گرو، غرب ایران

نویسندگان

1 دانشجوی دکترا، گروه زمین‌شناسی، دانشگاه شهید چمران اهواز، اهواز، ایران

2 استادیار گروه مهندسی شیمی، دانشگاه ایلام، ایلام، ایران

چکیده

سازند گرو یکی از سنگ­های منشأ نفتی مهم در حوضه زاگرس در غرب ایران است که مطالعات زمین­شناسی متعددی روی آن انجام گرفته است. با این وجود مطالعه پیش­رو سعی دارد که زیست­نشان­گرها وابسته به منشأ مواد آلی، بلوغ حرارتی و سن زمین­شناسی را در نمونه­های بیتومن استخراج شده از نمونه­های رخنمون سطحی از سازند گرو را مورد مطالعه قرار دهد. آنالیز زیست­نشان­گرها با استفاده از تکنیک کروماتوگرافی گازی و کروماتوگرافی گازی- طیف­سنجی جرمی انجام شده است. مطالعه زیست­نشان­گرهای استران (نسبت 28C به 29C) در بیتومن حاکی از این است که سازند گرو طی شکوفایی فیتوپلانکتونی دوره کرتاسه تشکیل شده است و ماده آلی آن عمدتاً متشکل از پلانکتون­های جلبکی و باکتریایی و نیز مقدار کمی مواد آلی گیاهی جابه­جا شده از خشکی است که نهایتاً در یک حوضه رسوبی کربناته ته­نشست یافته است. به­دلیل حاکمیت شرایط احیایی در محیط­رسوبی، بیتومن استحصالی غنی از ترکیبات نیتروژن-گوگرد-اکسیژن و آسفالتن است. با توجه به نتایج درصد اجزای بیتومن انتظار زایش نفتی از نوع آروماتیک-آسفالتیک می­رود. این یافته­ها مبین رخداد  فرآنید­های دگرسایی بر روی مواد آلی این سازند می­باشد. در اثر دگرسایی یک نوع بیتومن جامد و غیرقابل حل، به نام بایوبیتومن تشکیل شده است. همچنین آنالیز طیف­سنجی جرمی ایزوتوپ پایدار کربن برای بررسی فرآیندهای هوازدگی سطحی و ارتباط ژنتیکی بیتومن استحصالی با ماده آلی موجود در سازند گرو به­کار رفته است. هوازدگی سطحی و آب­شویی به واسطه حذف مقداری از اجزاء اشباع و آروماتیک سبک، باعث شده است که در ترکیب ایزوتوپ کربن در کروژن و بیتومن ناهمخوانی وجود داشته باشد. میزان بلوغ حرارتی مواد آلی تقریباً بالا است و سنگ منشأ گرو در اوایل پنجره تولید نفت قرار دارد.

کلیدواژه‌ها


عنوان مقاله [English]

Molecular geochemistry and biomarker analysis of the Garau formation, western Iran

نویسندگان [English]

  • M. Hemmati 1
  • Y. Ahmadi 2
1 Ph. D. student, Dept., of Geology, Shahid Chamran of Ahvaz University, Ahvaz, Iran
2 Assist. Prof., Dept., of Chemistry Engineering, Ilam University, Ilam, Iran
چکیده [English]

The Garau Formation is one of the important oil source rocks in Zagros basin, Western Iran, which several geological studies have been performed about it. However, current article attempts to study biomarkers related to organic matter sources, thermal maturity and geological age in the extracted bitumen samples from geological sections of the Garau Formation. Biomarkers were analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) techniques. As well as, stable carbon isotope composition analysis has been used to investigate the weathering processes and genetic association of extracted bitumen with source rock samples. The study of sterane biomarkers (C28/C29) indicates the Garau Formation has been formed during phytoplankton assemblages bloom of the Cretaceous period, and includes predominantly algal matter along with bacterial matter, and some reworked higher plant inputs. Sedimentation eventually occurred in a marine carbonate basin. Due to predominance of reduced (anoxic) conditions in the sedimentary basin, the bitumen composition is rich in nitrogen-sulfur-oxygen (NSO compounds) and asphaltenes. Based on bitumen fractions, aromatic-asphaltic oil is expected to be generated. These findings indicate the occurrence of alteration processes on the organic matter of the Garau Formation. As a result of biodegradation, a type of solid and insoluble bitumen, called biobitoman, has been formed. Stable carbon isotope analysis shows that biodegradation and water-washing have lost some more saturates and lower molecular weight aromatics in the surface bitumen samples of the Garau Formation. The thermal maturity of the organic matter is almost high and the source rock has entered to the oil window.

کلیدواژه‌ها [English]

  • Garau formation
  • Biomarker
  • Stable carbon isotope
  • Biobitumen
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