Comparative study of shale reservoir characteristics between the Longmaxi Formation and the Da'anzhai Member in the Sichuan Basin:Highlighting facies differences between marine and lacustrine depositional environments
Reservoir characteristic is the key focus and challenge in lacustrine shale research.To provide a theoretical basis and reference for future exploration and development of lacustrine shale gas,this study compared and analyzed the differences between marine and lacustrine shale reservoirs.Organic geochemistry experiments,whole-rock X-ray diffraction(XRD)analysis,scanning electron microscopy(SEM)with argon ion polishing,and low-temperature nitrogen adsorption experiments were conducted on lacustrine shale from the 1st Member of the Longmaxi Formation in Southeast Sichuan and the 2nd sub-member of the Da'anzhai Member in North Sichuan.This research aims to compare and analyze the organic geochemistry,reservoir characteristics,pore structure differences,and controlling factors between lacustrine and marine shales.The results show that,compared to the Longmaxi Formation shale,the Da'anzhai shale exhibits lower organic matter content.The organic matter kerogen in Da'anzhai shale is primarily type Ⅱ,with a small amount of type Ⅲ,and is in a high-over mature stage.Mineral composition in the Da'anzhai shale is characterized by lower siliceous minerals and higher clay mineral content,primarily illite and mixed-layer illite.The presence of shell fossils contributes to a higher calcium content.In terms of pore composition,the Longmaxi Formation is dominated by organic pores.In contrast,the Da'anzhai shale is primarily characterized by clay mineral interpore,accounting for over 70％of the total pores,followed by organic pores.Micro-fractures are mainly clay interlayer fractures and mineral edge fractures.The pore size distribution is dominated by mesopores ranging from 2 to 50 nm,forming a"double peak"at intervals of 4 nm and 20 nm,respectively.The dominant reservoir space in the Da'anzhai shale is primarily controlled by clay mineral and organic matter content.The effective spatial coupling of organic matter and clay mineral composition facilitates the formation of pores between oil and gas-bearing clays,creating favorable conditions for reservoir development.The striated zone of the lacustrine shale exhibits high porosity,high total organic carbon(TOC)content,high gas content,and well-developed micro-fractures,making it conducive to fracturing and fracture formation.This zone represents a high-quality reservoir development interval,conducive to oil and gas accumulation.
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