Pore Structure Evolution And Controlling Factors Of Continental Shale Reservoir

There has not been a remarkable achievement,although the continental shale gas accounes for about 1/3 of the total shale gas resources,partly because of the weak study about pore structure which constraints the gas content in shale reservoir.In this paper,Dameigou shale of middle Jurassic series in Qaidam Basin and Yanchang shale of Triassic system in Ordos Basin are selected as the research objects.TOC analysis,rock pyrolysis analysis,XRD,FE-SEM,low pressure nitrogen/ carbon dioxide adsorption,and high pressure Hg injection were used to research reservoir characteristics,pore-structure characteristics,pore-structure evolution characteristics and its controlling factors of development in continental shale,which are carried in shale samples without any processing,like extraction.Result shows that continental shale has “two high,two low,several types” feature: high organic and shale content;low maturity and brittle mineral content;several types of organic matters,with a majority of humus-mixed type.In research areas,the argillaceous shale,siliceous shale and mixed shale are developed while the calcareous shale is not.The lamination development and TOC differ from each other in the three kinds of shales.With the help of scanning electron microscopy,intergranular pores,intragranular pores and organic pores,three kinds of pores are observed in continental shale.Comparing to marine shale,in continental shale,the diameter of intergranular pores is bigger,the diameter of intragranular pores is smaller,the organic pores are less developed and unevenly distributed.The pore diameter of continental clay-rich shale are commonly small,but the pore connectivity is not bad,with developed slit-shaped pores with parallel plates and inkpot-shaped pores;the pore diameter of mixed shale are commonly bigger,the pore connectivity is poor,wedge-silt-shaped pores are developed while inkpot-shaped pores are not.The evolution trends of specific surface area and pore volume of per TOC unit in clay-rich shale and mixed shale are different.An overall decreasing trend of pore volume is shown in clay-rich shale,the pore volume decreases at first and then decreases in mixed shale,an overall decreasing trend is only shown in micropores.It's the same in the two types of shale that pore volume is mainly provided by mesopores while specific surface area is mainly provided by micropores and mesopores.The difference is that,with the increase of the degree of evolution,the contribution of mesopores(2 to 10nm)to pore volumes and specific surface area decreases in clay-rich shale,but the contribution of mesopores(2 to 10nm)to pore volumes and specific surface area increases in mixed shale.The formation of immature hydrocarbon and organic acid corrosion promoted the development of shale porosity in immature stage,liquid hydrocarbon filling,cementation and compaction restrained the development of porosity.In oil window stage,pores were in unstable state of rapid development,a large number of liquid hydrocarbon generated and filled or clogged the pores,the pore structure of shale reservoir became more complex.Residual hydrocarbons mainly exist in micropores(dia.0.3~0.6nm)and mesopores(dia.2~20nm).The presence of residual hydrocarbons affects not only the pore structure characterization of the continental shale reservoir but also the analysis on gas content.In the condensategeneous stage,the development of organic pores came into a more stable stage.With the discharging and cracking of hydrocarbons,before filled or clogged pores exposed,and the pressure in shale reservoir decreased,the compaction enhanced.Other than that,the development of organic pores also relates to the occurrence of organic matters.The catalytic hydrocarbon generation of clay minerals and pyrite results in the difference of the amounts,sizes and distribution characteristics of pores in associated organic matters and enriched organic matters.