Formation and Development of Pore Structure in Transitional Shale─Insights from Thermal Simulation Experiments on the Shanxi Formation Shale in the Ordos Basin

Transitional shale gas reservoirs have become a new field of shale gas exploration in China. However, the formation and evolution of shale pores, particularly the organic matter (OM) nanopores, in this type of shale are still unclear. Simulation experiments with a typical transitional shale were conducted, and field emission scanning electron microscopy (FE-SEM) and low pressure CO₂ and N₂ adsorption (CO₂GA/N₂GA) were used to characterize the shale pore structure. Results show that the development of OM pores is controlled by the hydrocarbon generation process, and meanwhile, the evolution model in the transitional shale is established. At the oil generation stage (0.5% < Ro < 1.1%), volumes of micropores and mesopores decrease rapidly, with a minimum value of micropore volume at Ro = 1.02%, due to the infilling of extractable OMs. At the post-oil generation stage (1.1% < Ro < 1.5%), micropore volume increases due to the cracking of extractable OM, and mesopore volume decreases owing to partial extractable OM migrating into mesopores and the effect of compaction. At the wet gas generation stage (1.5% < Ro < 2%), volumes of shale pores slightly increase due to the release of the occupied pores. At the dry gas generation stage (Ro > 2%), volumes of micropores, mesopores, and macropores increase rapidly when Ro is below 2.99%, due to the porous solid bitumen formation and new pores generated in kerogen. When Ro is above 3.31%, micropore and macropore volumes decrease due to the graphitization of OM and strong compaction. The continuous increase in mesopores is probably due to the combination of micropores. This study can provide a scientific guide for transitional shale gas exploration and resource evaluation.