Organic lithofacies and its reservoir characteristics of lacustrine shales of the Dongyuemiao Member in the Jurassic Ziliujing Formation, eastern Sichuan Basin

This study investigates the reservoir properties and main controlling factors of different organic lithofacies in the Jurassic Dongyuemiao Formation lacustrine shales, eastern Sichuan Basin. Based on lithofacies classification, the spatial characteristics and physical properties of the reservoirs, along with the primary controlling factors, were analyzed using core observation, scanning electron microscopy (SEM), low-temperature nitrogen adsorption, and other techniques. The results indicate that: 1) In the eastern Sichuan Basin, the Dongyuemiao Member is predominantly composed of clay minerals and quartz, which are the main mineral constituents of the lacustrine shales. Minor amounts of calcite, feldspar, dolomite, pyrite, and rhodochrosite are also present. Four main lithofacies were identified within the study area: argillaceous shale with high-medium TOC (Ⅰ 2 ), mixed shale with low-medium TOC (Ⅱ 1 ), sacrilegious-silt mixed shale with medium-low TOC (Ⅱ 2 ), and argillaceous siltstone with low TOC (Ⅲ 2 ). 2) The shale microstructure is characterized by micropores, primarily comprising intergranular pores between clay minerals and organic-matter pores. Intragranular dissolution pores, notably within shell fragments, are locally developed. Microfractures are mainly observed as intra-layer shale fractures and fractures along the edges of shell fragments. 3) High organic matter abundance (>1%), high clay mineral content (>40%), and moderate carbonate mineral content (<10%) in the shale of the study area contribute to the development of pores. In contrast, high quartz and feldspar mineral content (>50%) and excessively high (>12%) or low (<8%) carbonate mineral content inhibit shale pore development. 4) A comprehensive analysis shows that the argillaceous-silt mixed shale with medium-low TOC (Ⅱ 2 ) is the most favorable lithofacies for reservoir development, followed by argillaceous shale with high-medium TOC (Ⅰ 2 ). The mixed shale with lower-medium TOC (Ⅱ 1 ) is less favorable, and the argillaceous siltstone with low TOC (Ⅲ 2 ) is the poorest. Considering the vertical distribution of lithofacies in the Dongyuemiao Member, it is concluded that the middle-upper part of the Dong 2 submember is the most prospective reservoir interval in the study area. • Based on petrological characteristics and organic geochemical indices, this paper systematically classifies the lacustrine shale lithofacies of the Jurassic Dongyuemiao Formation in the eastern Sichuan Basin, and makes qualitative and quantitative analysis of the reservoir characteristics of each type of lithofacies. The main reservoir spaces of the Dongyuemiao Formation shale in the eastern Sichuan Basin are the intercrystalline pores of clay minerals, organic matter pores, bedding fractures, and shell edge fractures. • Through the correlation analysis of the main components, total organic carbon content, and reservoir physical properties of various types of lithofacies, not only the TOC content and mineral content range that are conducive to the development of shale reservoir pores are clarified, but also the most favorable lithofacies are revealed as the medium-low-carbon silt (argillaceous) mixed lithofacies shale (Ⅱ 2 ).