Pore Structure Characteristics in the Organic-Rich Shale of the Silurian Longmaxi Formation in the Southeast Sichuan Basin

The pore structure of shales plays an important role in the evalution of shale gas resources,exploration and development.High mercury injection,low pressure liquid nitrogen adsorption and air-ion mill-field-emission environmental scanning electron microscope were conducted to investigate the characterization of microscopic pore structures of the Silurian Longmaxi Formation shale,and analyse the factors development of micro-pores.Research shows the shale is of relatively high threshold pressure,poor-sorting pore throats,and low efficiency of mercury withdrawal,indicating that the pore-throat are heterogenous.The shale of specific surface area ranges from 12.330 to 29.822m²/g,with a mean of 20.132m²/g,while the distribution of pore volume is between 0.015 9and 0.094 7cm³/g,with an average mean of 0.044 5cm³/g.The average pore diameter ranges from 3.484 to 12.473 nm,with a mean value of 7.400 nm.The mesopores are main pores,while some micro-and macro-pores also exist.The main pore morphology is ink-bottle and silt pores.Pores in shales are divided into six types:organic pores,residual pores,secondary dissolution pores,pyrite inter-crystalline pores,clay minerals inter-crystalline pores,micro-fractures.The diameters of residual pores and secondary dissolution pores can reach micron level.The development of micro-pore is influenced by different degree of organic content,quartz content,clay minerals,thermal maturity.Specific surface area and pore volume increases with the increase of content of organic carbon,quartz.With the increasing content of clay mineral,specific surface area and pore volume showed a trend of decrease.Appropriate degree of thermal evolution is one of the important factors affecting the nanoscale pore development.