川东南地区志留系龙马溪组页岩孔隙结构特征

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

页岩储层的孔隙结构对页岩气资源评价和勘探开发具有重要意义。通过高压压汞法、低压氮气吸附法、氩离子抛光-场发射扫描电镜对川东南龙马溪组页岩微观孔隙结构特征进行了深入的研究,分析了微观孔隙发育影响因素。研究表明,页岩排驱压力比较高,孔隙分选差,退汞率极低,说明孔隙与喉道非常不均一;页岩比表面积为12.330~29.822 m2/g,平均为20.132m2/g;孔体积为0.015 9~0.094 7cm3/g,平均为0.044 5cm3/g;平均孔径为3.484~12.473nm,平均为7.400nm;主体孔隙为中孔,存在一部分的微孔和大孔,氮气吸附-脱附曲线表明孔隙形态以墨水瓶形孔和狭缝状孔为主。孔隙类型可分为有机质孔、原生残余孔、次生溶蚀孔、黄铁矿晶间孔、黏土矿物晶间孔、裂缝6种类型,其中原生残余孔、次生溶蚀孔可达微米级。有机碳含量、石英含量、黏土矿物含量、热演化程度均会影响微观孔隙发育,比表面积和孔体积随有机碳、石英含量的增加而增加;而随黏土矿物含量的增加,比表面积、孔体积呈减小趋势;适宜的热演化程度是纳米级孔隙发育的重要影响因素。

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 of0.044 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.