Pore Evolution and Formation Mechanism of Organic-Rich Shales in the Whole Process of Hydrocarbon Generation: Study of Artificial and Natural Shale Samples

Shale pore evolution has a dramatic influence on gas occurrence and is of great significance for the evaluation of gas shale reservoirs. To better understand the nanopore evolution during the whole hydrocarbon generation process, thermal simulation experiment, gas adsorption (N-2 and CO2), and organic geochemistry experiments were carried out. The results show that total pore volume (PV) and specific surface area (SSA) exist in two favorable development periods during the whole hydrocarbon generation process with corresponding vitrinite reflectance (R-o) values of similar to 1.5-1.8% and similar to 2.5-3.2%, respectively, which are two peak periods of porosity development. In the two hydrocarbon generation intervals, primary organic matter (OM) and secondary OM crack to gases accompanied by the formation of OM-hosted pores in large quantities, resulting in the increase of porosity. Shale pore formation and evolution are dominantly controlled by hydrocarbon generation as well as diagenesis. Compaction and cementation exert destructive influences on mineral associated pores, especially in the relatively early diagenesis process, resulting in a great decrease of total porosity. Infilling of secondary OM to inorganic interparticle pores also results in the destruction of porosity. OM-hosted pores are documented as a function of thermal maturity, which is significantly related to kerogen and secondary OM cracking to hydrocarbon.