Abstract: The Jurassic Lianggaoshan Formation in the northeastern Sichuan Basin contains considerable shale oil and gas resources.In recent years,several zones in the basin have yielded high-production oil and gas wells,demonstrating favorable exploration prospects.However,reserve growth and production enhancement remain challenging because of the complex lithofacies of fine-grained sedimentary rocks,relatively low organic matter abundance,and complex pore structures.Based on geochemical data,petrographic-mineralogical analyses,and multi-scale pore characterization results,this study examined the microscopic pore structure of shale in the first member of the Lianggaoshan Formation and discusses the genesis of favorable reservoirs.The results show that six lithofacies types are mainly developed in the first member,including felsic,mixed and clay-rich shales under both low-organic-matter and high-organic-matter conditions(hereinafter referred to as low- organic matter and high-organic matter).Among them,high-organic-matter felsic shale and high-organic-matter clay-rich shale exhibit relatively high proportions of macropores and thus favorable reservoir properties.Lithofacies with higher content of brittle minerals have stronger resistance to compaction,which is conducive to the preservation of primary pores and to higher proportions of mesopores to macropores.Organic matter exerts a dual effect on the development of favorable reservoirs:an appropriate organic matter content promotes pore development,whereas excessively high organic matter content fills pore space,thereby weakening rock compaction resistance and hindering the preservation of mesopores to macropores.Through analysis of the favorable lithofacies and the main controlling factors of pore structure in lacustrine shale reservoirs,it is concluded that the shale reservoir of the Lianggaoshan Formation in the northeastern Sichuan Basin is governed by a dual-threshold control model,and that an organic-matter-mineral composite pore system is critical to the formation of favorable shale reservoirs.This study provides support for sweet-spot prediction and subsequent exploration deployment of shallow lacustrine shale oil and gas in the northeastern Sichuan Basin.

Key words: lacustrine shale, microscopic pore structure, controlling factors, Lianggaoshan Formation, northeastern Sichuan Basin