Pore Structure Of Transitional Facies Shale And Its Influence On Gas Bearing Property In Southern North China Basin

The pore structure of shale is a direct factor to control the gas content of shale.Based on the data of organic geochemistry and mineral composition,the author studied the shale of the Lower Permian Shanxi Formation and Taiyuan Formation in the Southern North China Basin.The porosity structure of the shale and its influence on the gas-bearing properties were quantitatively analyzed.A new method for the analysis of gas-bearing structures was proposed.The contribution of various pores to the adsorption of gas and free gas was clarified.The results show that the organic carbon content(TOC)and the mineral content of quartz and clay can affect the adsorption phase and free phase methane by affecting the specific surface and pore volume.There was a significant positive correlation between the surface area and the content of TOC and illite/smectite formation,and the porosity was positively correlated with the quartz content,but the porosity decreased with the increase of TOC and carbonate minerals.TOC is the controlling factor of shale gas content in Shanxi and Taiyuan Formation of Southern North China Basin,and other factors are weakly correlated with total gas content.In this paper,the method of calculating the free gas volume and the adsorbed gas volume using the pore volume-pore data is proposed for the difference of the mechanism of the adsorption phase and the free phase methane.Based on the characteristics of low pressure nitrogen adsorption and desorption curves,the shale porosity is reduced to two basic types of plate type and ink bottle type.Using the method of aperture ratio judgment and pore shape recognition,the model,which can be used to calculating the content of the free gas and the adsorbed gas,was proposed.The results show that the adsorption gas content is controlled by micropores,and the adsorption gas content in micropores can be 72.73% to 98.4%,with an average of 94.18%.Free gas content is controlled by the mesoporous,accounting for 19.07% to 97%,with an average of 86.72%.The macroporous has the lowest contribution to gas content,the average free gas content is 0.8%,and the average adsorption gas content is 0.005%.Based on the analysis of the conditions and characteristics of methane adsorption in shale,the control distance of single wall pore to methane was 1.52 nm,and the maximum adsorption phase thickness in the pores was 3.04 nm.Based on the data of pore volume distribution and pore size distribution obtained by low pressure carbon dioxide adsorption,low pressure nitrogen adsorption and high pressure mercury injection,the calculation methods of free gas volume and adsorption gas volume and ratio of free gas and adsorption gas parameters were proposed.Compared with the calculation results of other methods,it is shown that the method of calculating the free gas by the pore volume is more accurate than the conventional porosity calculation method,and the shale gas reservoir can be evaluated as the lower limit value.However,due to the fact that the influence of shale water and effective adsorption sites is not taken into account,the adsorption gas calculated by this method is generally too large and the ratio is too small.The research results and knowledge can be used in shale adsorption microcosmic mechanism,recoverable rate,productivity analysis and other fields.