The Pore Structure And Its Affecting Factors Of Wufeng-longmaxi Formation Shales With Different Lithofacies In The Southern Sichuan Basin

Using multiple technical methods to analyze the characteristics of nano-scale pore structure and its affecting factors of shale is significant for the exploration and development of shale oil and gas.Based on the mineral compositions,twelve shales from Wufeng-Longmaxi Formation are divided into three major types,which are argillaceous,mixed and siliceous shales,among the three major types five subtype can be further divided,which are S-1,S-3,M-2,M and CM-1 shales.The utilization of scanning electron microscope and QEMSCAN mineral analyses are to study the pore morphology and diagenesis of shales and calculate the surface porosities of 6 minerals.The major and minor trace element analyses were used to research on the paleoenvironment of shales.The low pressure CO₂/N₂ adsorption and high pressure mercury injection tests were used to quantify the full scale pore structure of shales and its influence factors.The characteristics of shale gas adsorption were explored through high-pressure methane isothermal adsorption experiments.The results are shown as follows.The reducing sedimentary environment,high productivity and low input of terrestrial debris ensure the high organic carbon content in siliceous shales.The organic matter pores are well developed in siliceous shales.Argillaceous shales are easier to develop the clay mineral inter-particle pores.Organic matter pores and carbonate dissolved pores are common in mixed shales.The pore volume and specific surface area of the Wufeng-Longmaxi Formation shales are mainly provided by mesopores and micropores,respectively.The pore volume of shales is mainly distributed in the range of 0.4?0.6nm,2?7nm and 10?20nm,and the pore specific surface area is mainly distributed in the range of 0.3?10nm and 1.5?7nm.The smaller the average pore size of shales,stronger the heterogeneity and larger the specific surface area for shale gas adsorption are.M-2 shales have similar pore structure parameters as the siliceous shales,such as the porosity,fractal dimension,pore volume and pore specific surface area,which indicates that M-2 shales have the same powerful storage capacity.Organic matter pores in the Wufeng-Longmaxi Formation shales can contribute more than 50%of the total porosity.The development of organic pores in siliceous and M-2 shales is an important factor affecting the parameter characteristics of pore structure of shales.In addition to organic matter,the illite in argillaceous shales also played an important role in the development of its pore space.The largest reductions in porosity are caused by compaction in argillaceous shales and cementation in siliceous shales,respectively.The increases in porosities of shales are mainly caused by the hydrocarbon generation.The methane molecules are mainly adsorbed in pores with a size range of 0?10nm.M-2 shales and siliceous shales have larger adsorbed gas content.