Geochemical Characteristics And Gas Bearing Analysis Of Lower Cambrian Black Shale In Western Hunan

Western Hunan and its surrounding areas are the key areas for shale gas exploration and development in southern China.Black shale widely occurred in the Niutitang Formation of Lower Cambrian,which is the main research target layer in this area.The geochemical characteristics and gas-bearing analysis results of the Lower Cambrain black shale show that the Niutitang Formation black shale samples have high organic matter abundance,and the total organic carbon content is high and varies greatly,ranging from0.47% to 19.69%,with an average of 5.32%.The equivalent vitrinite reflectance ranges from 4.00% to 4.58%.The maturity is very high,and the evolution degree is in the over mature stage.The kerogen type is I,and the ratio of Pr/Ph is 0.04-0.51,indicating the reduced sedimentary environment;the source of organic matter is mainly algae,and the geochemical characteristics indicate that the XAD1 well has good shale gas production conditions.However,the measured data shows that the Niutitang Formation has a very low gas content,and even some intervals' data cannot be detected,indicating that the combined effect of geology and geochemistry has caused a large amount of shale gas to be lost.In this paper,Illite and type I kerogen were selected as the research objects to study the effects of black shale on methane adsorption and diffusion under different temperatures and pressures with combining Molecular Dynamics simulation and Monte Carlo methods.The simulation results show that when the temperature is lower than200?,the main control factor is pressure,and when the pressure and temperature increase,the adsorption capacity increases;when the temperature is higher than 200?,the main controlling factor affecting the adsorption capacity changes to temperature,and the adsorption capacity decreases with the increase of temperature.The adsorption heat of black shale for methane is about 10 k J/mol,which is physical adsorption.In the aspect of diffusion,temperature is the main controlling factor,the higher the temperature is,the larger the diffusion coefficient is.Considering that there was no more methane source after the end of hydrocarbon generation,as well as that there was water in the rock,methane continued to diffuse and structural movement destroyed the reservoir,the originally higher methane adsorption amount gradually tends to 0 in calculation,which is consistent with the current production gas measurement value of XAD1 well,which is 0,and it provides a good explanation for the qualitative and quantitative description of the gas-bearing problem of black shale in XAD1 well.