Microscopic oil occurrence in the Permian alkaline lacustrine shales: Fengcheng formation, Mahu Sag, Junggar basin

Alkaline lacustrine shale is highly heterogeneous, and the complex relationship between the organic-inorganic porosity network and hydrocarbon occurrence restricts the effectiveness of shale oil exploration and development. Herein, we investigated the Fengcheng Formation (P₁f) in Mahu Sag. This study integrated geochemistry, Soxhlet extraction, scanning electron microscopy, gas adsorption, and nuclear magnetic resonance T₁-T₂ spectroscopy to elucidate the microscopic oil occurrence mechanisms in shales. Results indicate the presence of felsic shale, dolomitic shale, lime shale, and mixed shale within the P₁f. Matrix pores and microfractures associated with inorganic minerals are the predominant pore types in P₁f. Adsorbed oil primarily resides on the surfaces of organic matter and clay minerals, while free oil predominantly occupies inorganic pores and microfractures with larger pore sizes. Variations exist in the quantity and distribution of shale oil accumulation across different scales, where free oil and adsorbed oil are governed by dominant pores with diameters exceeding 10 nm and ineffective pores with diameters below 10 nm, respectively. Shale oil occurrence characteristics are influenced by organic matter, pore structure, and mineral composition. Felsic shale exhibits a high abundance of dominant pores, possesses the highest oil content, predominantly harbors free oil within these dominant pores, and demonstrates good mobility. Fluid occurrence in dolomitic shale and lime shale is intricate, with low oil content and a free oil to adsorbed oil ratio of 1:1. Mixed shale exhibits elevated clay mineral content and a scarcity of dominant pores. Moreover, ineffective pores contain increased bound water, resulting in medium oil content and limited mobility predominantly due to adsorption. Presently, shale oil mainly occurs in the dominant pores with a diameter larger than 10 nm in a free state. During the exploration and development of alkaline lacustrine shale oil resources, emphasis should be placed on identifying sweet spots within the felsic shale characterized by dominant pores.