Pore Evolution Characteristics of Marine Organic-Rich Shale Based on a Pyrolysis Simulation Experiment

The pore evolution characteristics of organic-rich shale under semiclosed and open-system conditions were investigated through a pyrolysis simulation experiment conducted on marine low-maturity shale. The results show that the experimental samples of the two systems mainly developed organic pores, clay mineral intergranular pores, and brittle mineral dissolution pores during the evolution process. The shale porosity showed a non-linear growth trend with the thermal maturity increase. However, the porosity increase rate of the semiclosed system was higher than that of the open system. The pore structure characterization parameters of the experimental samples under the two systems were quite different. The samples of the semiclosed system developed wedge- or V-shaped pores with a diameter of several tens of nanometers, while those of the open system developed more small throats with a diameter of several nanometers to ink bottle-type and closed pores of tens of nanometers. For the marine organic-rich shale, the organic matter (OM) evolution was mainly affected by the formation pressure. In summary, a semiclosed system with a higher formation pressure is more conducive to the development of organic pores and the pore structure than an open system with a lower formation pressure. The best stage of pore evolution and development is the overmature stage.