The Study On Microscopic Characteristics Of High-Yield Reservoirs Of Longmaxi Formation In Zhaotong Demonstration Area

The marine shale in the south of China has superior geological conditions and high reserves.However,the shale is characterized by wide occurrence of nano-scale porosities,complex pore origins,low physical properties,and strong heterogeneity.Therefore,the exploration and exploitation of shale gas is very difficult,and it is necessary to study the microscopic characteristics of shale reservoirs in detail.In this paper,the gray-black shale of Longmaxi Formation in Zhaotong Demonstration Area is taken as the research object.After field profile measurement,core observation,outcrop and core samples are collected.X-ray diffraction analysis,nuclear magnetic resonance,argon ion polishing-field emission scanning electron microscopy,optical microscopy and nitrogen adsorption are performed to characterize the microscopic pore structure of the Longmaxi Formation shale reservoir.Afterwards,the main controlling factors of pore structure development were analyzed and the reservoirs were evaluated.The research shows that:The mineral composition of the Longmaxi Formation shale is mainly composed of quartz,carbonate minerals,clay minerals,and also contains minerals such as feldspar and pyrite;TOC value is higher at the bottom of the shale than the top of the shale,the organic types are type I and type II₁and the evolution of heat is moderate;Based on the combination of organic and inorganic components,six types of lithofacies are identified:Rich organic matter?siliceous shale,high-calcium mixed shale,low-calcium mixed shale?and containing organic matter?calcium shale,siliceous shale,low-calcium mixed shale?;Matrix pores include intergranular pores,intragranular pores and organic pores,and fracture pores include structural fractures,interbedded seams,bedding slip joints,diagenetic shrinkage micro-cracks,and abnormal pressure seams of organic matter evolution;The nitrogen adsorption experiment shows that the pore morpholohy is dominated by parallel plate-like pores,tubular pores and ink-bottle type pores.According to the BET and BJH theory,it is found that the total pore volume and total specific surface area of the Longmaxi Formation shale are both large.The mesopores provide most of the pore volume,the micropores contribute the most to the total specific surface area,and the pore size distribution is dominated by micropores and narrow mesopores below 10 nm;TOC is the main controlling factor affecting the development of pore structure.The influence of organic matter maturity on pore structure is controlled by many factors.The mineral composition has little effect on pore structure.