Oil & Gas Geology ›› 2022, Vol. 43 ›› Issue (3): 682-695.doi: 10.11743/ogg20220316

• Petroleum Geology • Previous Articles     Next Articles

Characteristics and geochemical indication of over-mature source rocks in the Paleozoic, Yingen-Ejinaqi Basin

Zhijun Chen1,2(), Chunming Zhang1(), Yonghong He2, Zhigang Wen1, Fangxia Ma2, Wei Li3, Yiwen Gao2, Yiguo Chen2, Huiyuan Zhang3, Dongtao Wei3   

  1. 1.College of Resources and Environment,Yangtze University,Wuhan,Hubei 430100,China
    2.Research Institute of Yanchang Petroleum (Group) Company Limited,Xi’an,Shaanxi 710075,China
    3.Xi’an Center of Geological Survey,China Geological Survey,Xi’an,Shaanxi 710054,China
  • Received:2021-04-19 Revised:2022-03-08 Online:2022-06-01 Published:2022-05-06
  • Contact: Chunming Zhang E-mail:chenzhijun2203@aliyun.com;Zhangcm1991@163.com

Abstract:

The Paleozoic source rocks encountered by some wells and present on outcrops in Yingen-Ejinaqi Basin are believed to carry with them important geochemical information that will help understand the oil and gas accumulation conditions in the Paleozoic system of the basin. Geochemical characterization and hydrocarbon generation potential assessment of 25 such rock samples indicate over-mature (Ro=4.20 %) source rocks with low total organic carbon (TOC) content (1.31 %) and mixed kerogen types (typesⅡ1-Ⅱ2). They are mainly products of thermal degradation with biomarkers showing a bimodal N-alkanes, abnormally high relative abundance of pregnane (including L pregnane) in steroids, high relative abundance of tricyclic terpene in terpenoids, a predominance of low-carbon ring number compounds in aromatic compounds and other characteristics. A concept of “organic carbon composition” is presented and a method for calculating its value based on rock pyrolysis data is established. The results show that although the pyrolytically effective carbon only accounts for a fraction of organic carbon composition of the over-mature source rock, the original average TOC content of source rock is high at 2.07 %. In addition, the duration of hydrocarbon generation is long and its yield is high. Therefore, the Paleozoic over-mature source rocks in Yingen-Ejinaqi Basin are suggested to have a high original hydrocarbon generation potential.

Key words: organic carbon composition, biomarker compound, geochemical characteristics, over-mature hydrocarbon source rock, Paleozoic, Yingen-Ejinaqi Basin

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