Comparison of pore evolution for a Mesoproterozoic marine shale and a Triassic terrestrial mudstone during artificial maturation experiments

摘要

Semi-closed pyrolysis experiments were performed on a Mesoproterozoic Hongshuizhuang marine shale sample and a Triassic Chang 7 terrestrial mudstone sample, in order to understand the mechanisms of pore evolution in shales with similar organic geochemical characteristics in different depositional environments. The pore characteristics of the solid samples from the thermal maturation experiments were investigated by low-pressure gas (N-2 and CO2) sorption isotherms tests and Field Emission - Scattering Electron Microscopy (FE-SEM) imaging. Results indicate the marine shale samples develop more micropores than that of the terrestrial mudstone samples during the artificial maturation. Secondary organic pores are observed in the unheated samples (similar to 0.76% EqRo) for the marine shale, which is earlier than that of in the terrestrial mudstone (observed in the sample at 250 degrees C, similar to 0.8% Ro). Significant development of organic pores at 350 degrees C for both the two shales. And the transformation of smaller pores into larger pores takes place at 450 degrees C for the marine shale, as well as for the terrestrial mudstone. In highly overmatured shales, the overpressure formed by organic matter cracking to gas can make surrounding interparticle space to vanish. The high content of rigid minerals in the marine shale is more conducive to the preservation of organic pores than the terrestrial mudstone with rich in ductile clay minerals. Therefore, pore increases continuously for the marine shale but decreases for the terrestrial mudstone after 500 degrees C. Micropores would be generated if a high lithostatic pressure exists under experimental conditions, which may be largely responsible for the disagreement of pore evolution of similar shales in other publications. Finally, a conceptual model of pore evolution for type-II marine and terrestrial shales is proposed. The study reveals that the pore evolution of shales with different sedimentary environments is divergent, and this need to be separately considered in the future work.