Gas content evolutionin western Guizhou and differential occurrencein China of Permian shale with type III kerogen

Abstract

Permian organicmatter(OM) in China mainly comes from higher plants rich in type III kerogen. Exploring gas content evolution and occurrence mechanisms is of great significance for shale gas exploration. Shales of western Guizhou in China were collected from wells and subjected to experiments under formation conditions (e.g. thermal simulation, Rock-Eval pyrolysis, X-ray diffraction and CH4 isotherm adsorption). A thermogenic gas generation model was established based on the logistic model, a hydrocarbon expulsion model was established based on the hydrocarbon potential method, and an adsorbed gas volume model was established based on temperature, pressure and total organic carbon(TOC). Various gas content of shale with type III kerogen were calculated in a special geological history. The differences of shales between the western Guizhou and the northeastern Ordos Basin in China were compared in terms of gas generation, expulsion and adsorption capacity. The results showed that the gas generation capacity of the western Guizhou samples was stronger because of the higher TOC and hydrogen index (HI). Limited differences in hydrocarbon expulsion efficiency were observed between the two areas. The source area of the samples had no significant effect on the adsorption capacity. Due to its high TOC, low geothermal gradient, high pressure gradient and suitable burial depth (500–1500 m), the shale in western Guizhou has a stronger adsorption capacity. Obvious differences in the free gas volume and proportion in different areas were observed and should be analyzed separately in combination with the TOC, HI, geothermal gradient, pressure gradient and depth.