Formation and Evolution of Nanopores in Highly Matured Shales at Over-Mature Stage: Insights from the Hydrous Pyrolysis Experiments on Cambrain Shuijintuo Shale from the Middle Yangtze Region

The Cambrian marine shales are currently over-matured in the Middle-Upper Yangtze region. Several phases of geofluids are detected within these shale reservoirs. However, the impacts and relevant mechanisms of geofluids on the occurrence of nanopores within these shale reservoirs are still unclear. A series of pyrolysis experiments were conducted on the highly matured Cambrian shale from the Shuijingtuo Fm. in the Huangling anticline within a closed hydrous system. The measurements of C-S, XRD, N₂ adsorption and FE-SEM were done on these pyrolyzed shale samples with thermal maturity of R_o=2.26%-4.01%. The results demonstrate no obvious change of TOC, continuous decrease of sulfur, inorganic carbon and clays and increase of feldspar throughout the experiments as well as the significant decrease of quartz and increase of diopside at R_o≥2.7% with increasing thermal maturity, which indicates the slight hydrocarbon generation and expulsion, the dissolution of pyrite, carbonate, clays and quartz to varying levels and the formation of feldspar and diopside under the experimental conditions. The nanopore formation within highly matured shale reservoirs is regulated mainly by the mineral dissolution with the addition of geofluids, it is feasible for the occurrence of mesopores and macropores as indicated by the strong negative correlations between the mineral contents versus the total pore and macropore volumes; the formation of feldspar and diopside restricts the occurrence of micropores and promotes the development of mesopores and macropores, further implying the predominant effects of mineral dissolution on nanoporosity under the experimental circumstances; hydrocarbon generation and expulsion have the minor impacts on nanopore development due to the limited capability of petroleum generation and expulsion at the over-mature stage. With increasing mineral dissolution or thermal maturity, the abundance, volumes and specific surface areas of micropores gradually decrease and evolve into mesopores and macropores subsequently as suggested by the negative correlations of pore volumes and specific surface areas of micropores versus those of mesopores and macropores. This study should be helpful in the better understanding of the occurrence mechanisms and relevant controlling factors of nanopores within highly matured shale reservoirs with the presence of geofluids in nature.