The Pore Structure Characteristics And Its Relationship With Shale Gas Capacity In Organic-rich Mudstone Of Wufeng-Longmaxi Formation,Sichuan Basin

Pore structure characteristics for 32 fresh outcrop shale samples collected from Shuanghe Town,Changning County and 17 core samples collected from the Qianqian 1 core well in southeast Chongqing,Sichuan Basin were investigated by low pressure nitrogen adsorption and nuclear magnetic resonance(NMR).The main factors controlling pore development and gas accumulation in shales were discussed by integrating total organic carbon(TOC),mineralogy and shale gas content.The results show that:(1)Open slit-like and parallel plate structure are major pore types with an average pore diameter of 3.76?12.08 nm,and 2?30nm mesopores are dominated.The high BET surface area and total pore volume in the Wufeng Formation and lower part of the Longmaxi Formation are benefit for gas adsorption and gas storage.(2)The porosity measured by nitrogen adsorption method ranges from 4.04%?10.39% with an average of 6.76%,which is greater than the NMR porosity,varing from 2.6% to 6.25% with an average of 4.34% in the Wufeng Formation and Longmaxi Formation shales in Sichuan Basin.The reservoir conditions of gas shales characterized by low porosity and low permeability appear to be mainly affected by nanopores development.(3)A positive correlation between the BET surface area,micropore volume and mesopore volume with the TOC values reveals that organic matter(OM)pores are primarily composed of micropores and mesopores which are also observed by scanning electron microscopy images,and OM acts as a key controlling factor of shale gas reservoir evaluation.(4)Mineralogical compositions have an important influence on the development and preservation of mudrock pores.The clay-mineral-hosted pores are present in the studied shales,showing that the samples with higher content of clay minerals but similar TOC content have larger specific surface area.Whereas brittle minerals,including carbonate,quartz and so on,are favourable for the preservation of small pores in organic-rich mudstone by forming supporting frame to protect the organic matter filling in it and restraining pore collapse.(5)A positive correlation between the total pore volume,porosity,total gas content and adsorbed gas content with the TOC values reveals that high OM richness can produce more gas and provide source material primarily.Also the OM-hosted pores enhance the pore network connectivity,which provide space and path for shale gas preservation and migration.