Characteristics and Genesis of Immature Oil in Jiyang Depression

Hong Zhihua

Immature oil is distributed widely in the Jiyang Depression. There are various types of accumulations (most of them within the Tertiary at depths of less than 2000 meters). Most of geochemical index (such as OEP, Pr/Ph, C_29.^agr.^agr.^agr 20s/(20s+20r) etc.) of the immature oils are characterized by low maturation. According to the relative abundance of C₂₇ C₂₈ C₂₉ steranes, gammacerane content and the pristine to phytane ratio, the immature oils can be divided into three types in this depression. The oil/rock correlation results show that the hydrocarbon source beds of these three types of immature oil are respectively from immature hydrocarbon source rocks in E_S1 E_S3 and E_S4 members of Shahejie Formation.

The author discusses the genesis mechanism of immature oil based on the comprehensive study of the paleontologic assembly, paleoclimate and depositional environment of source rocks combined with the data of simulated tests. It is concluded that immature oil was generated from organic matter during early low-temperature evolution. It was the saline, strongly reducing environment which resulted in an oil source bed rich in lower planktonic algae that favored the generation of low maturity oil.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California