Quantitative characterization and main controlling factors of shale oil occurrence in Permian Fengcheng Formation, Mahu Sag, Junggar Basin

Abstract

The characteristics and controlling factors of shale oil occurrence in Permian Fengcheng Formation of the Mahu Sag, Junggar Basin are complex, making occurrence state identification and quantitative characterization difficult. It is of great significance to clarify the occurrence characteristics and controlling factors of shale oil for selecting sweet spots. In order to characterize the content of shale oil in different occurrence states, some shale samples in the central and slope areas of the sag were extracted step by step, and a quantitative characte-rization experiment method of shale oil in different occurrence states was established, obtaining the contents of free light hydrocarbon, free heavy hydrocarbon and adsorbed hydrocarbon, respectively. Based on the study of organic geochemistry and reservoir micro-characteristics, the following conclusions are obtained. Due to the low TOC and clay contents in the study area, the shale oil in the Fengcheng Formation mainly occurs in dissociative state and has the least adsorbed hydrocarbon content. Shale oil occurs mostly on mineral surface and in kerogen in oil film state, heavy hydrocarbons usually occur in shale with large micropore specific surface area, and macropores are the main occurrence space of free oil. With the increase of organic matter abundance and pore size, the shale oil content in different occurrence states shows an increasing trend, indicating that the higher organic matter abundance and larger pore size are conducive to shale oil enrichment. Maturity can greatly affect the adsorption capacity of fluid in shale, and the higher the maturity of organic matter, the higher the proportion of free hydrocarbons and the lower the proportion of adsorbed hydrocarbons. The increase of specific surface area promotes the enrichment of adsorbed oil and free heavy hydrocarbons, and restricts the enrichment of free light hydrocarbons, indicating that the specific surface area of mesoporous pores has a controlling effect on the adsorption capacity of shale oil, i.e., the larger the specific surface area, the stronger the adsorption capacity of shale oil. The relevant research and recognition can provide a basis for selecting sweet spots and evaluating the benefits of shale oil exploitation in the study area.

Keywords

• occurrence characteristics
• main controlling factor
• shale oil
• fengcheng formation
• permian
• mahu sag
• junggar basin