New insights into the role of system sealing capacity in shale evolution under conditions analogous to geology: Implications for nanopore evolution

Abstract

Closed and semi-closed pyrolysis of a lacustrine shale sample was conducted using the same instrument characterized by lithostatic pressure and limited reactive space in order to unravel the impacts of the system sealing capacity on organic matter occurrence and nanopore development in shales under conditions analogous to natural processes. The solid residues were subjected to organic geochemical measurements, low-pressure gas (N2 and CO2) adsorption tests, and field emission-scatter electrical microscope analysis. The results revealed that solvent extraction played positive and negative roles in the pore development of solid residues from both systems at different thermal maturities. Inorganic and organic pores were primarily filled or shielded by soluble bitumen at low and high temperatures, respectively. The occurrences of organic matter in solid residues from the two systems were different at lower temperatures but gradually evolved similarly at higher temperatures and were dominated by nanoscale interparticle organic matter. Organic pore occurrence in solid residues from the two systems was approximately comparable. However, differences in pore volume and specific surface area of solid residues from the closed and semi-closed systems increased measurably below 550 °C. At 550 °C, a strong mechanical compaction and overpressure environment caused deformation and closure of pores surrounding rigid minerals and pores associated with ductile minerals surrounding organic matter in solid residue from the closed system, while only a few pores surrounding rigid minerals from the semi-closed system deformed. Consequently, similar pore parameters of solid residues from the two systems were obtained at this temperature. Additionally, the variation in organic porosity of solid residues from the two systems has obvious stages based on mass balance calculation. The organic porosity evolution is collectively affected by system sealing capacity and thermal maturity at lower matured stages. In contrast, organic porosity evolution in solid residues is independent of system sealing capacity at high- and overmatured stages. Although the depositional environment is different, this study provides direct evidence that the difference in system sealing capacity is not the predominant factor leading to different organic pore developments in the Silurian and Cambrian shales in South China.