LAUSR

Abstract This research analyzed two low-maturity Mesoproterozoic Xiamaling marine shale samples with different TOC contents in Zhangjiakou of Hebei, China. Artificial anhydrous pyrolysis experiment simulated the thermal evolution process of the shale sample from low maturity to over maturity. The evolution characteristics of geochemical parameters and mineralogy of shales with the increase in maturity were investigated. A general evolution model of nanoscale pore structure and methane adsorption content (MSC) was established. Results show that hydrocarbon generation and mineral conversion promote the generation, development, combination, and rearrangement of nanoscale pores in shale samples with the increase in thermal maturation. The maturity increase helps facilitate the generation and evolution of pores in shales. Heating can promote the generation, development, and evolution of nanoscale pores, and this condition can contribute to forming large pore volume and surface area. The formation and evolution of the nanoscale pores in shale can be roughly subdivided into adjustment, development, combination, and rearrangement stages. Nanoscale pore structure parameter and MSC decrease and reach the minimum value as the temperature reaches 350 °C in the adjustment stage. Then, they rise and reach the maximum value at approximately 650 °C during the development stage. Finally, they decline again as the pyrolysis temperature rises at the combination and rearrangement stages. The general evolution model of nanoscale pore structure and MSC was proposed and subdivided into three stages by combining the hydrocarbon generation, mineral conversion, nanoscale pore structure, and MSC evolution of the thermal simulation shale samples. The differences in organic geochemical index, mineral characteristics, nanoscale pore structure, and MSC evolution characteristics of the simulated samples are mainly due to the difference in their TOC content.