Petrologic characteristics and genetic model of lacustrine lamellar fine-grained rock and its significance for shale oil exploration: A case study of Permian Lucaogou Formation in Malang sag, Santanghu Basin, NW China

摘要

Taking the Permian Lucaogou Formation in the Malang sag, Santanghu Basin as an example, by using petrological methods such as high resolution core image scanning, conventional thin section, scanning electron microscope and energy spectrum analysis and geochemical tests such as trace elements and biomarker compounds, the petrologic features and sedimentary origin of the lamellar organic rich fine-grained rocks of lake facies were investigated, and its significance for shale oil and tight oil exploration was analyzed. The results of the study show that there are four types of laminae, siliciclastic enrichment laminae, carbonate enrichment laminae, tuffaceous enrichment laminae and organic matter enrichment laminae, which can form three kinds of layer combinations. Organic matter is laminar enrichment or dispersed in the carbonate laminae and tuff laminae. Stratification of ancient lake water was formed in the closed saline lake sedimentary systems with insufficient continental clast supply, and the activity of warm water at the lake bottom and the monsoon climate worked jointly to control enrichment of organic matter and formation of lacustrine lamina fine-grained rocks. This kind of fine-grained rocks have higher potential of hydrocarbon generation due to high abundance of organic matter, are rich in reservoir space due to the existence of micro-pores in carbonate laminae and micro-cracks between the laminae, and suitable for fracturing because of high brittle mineral content. They have favorable conditions for shale oil and tight oil accumulation, and are significant for exploration.