DIAGENESIS AND PORE EVOLUTION FOR VARIOUS LITHOFACIES OF THE WUFENG-LONGMAXI SHALE IN SOUTHERN SICHUAN BASIN

Mineralogy differences in clastic sediments have a major impact on diagenetic pathway and pore evolution between various lithofacies during burial. There is, however, a lack of studies on shale diagenesis since its fine-grained particles are hard to directly observe. In this study, a quantitative pore counting was used to assess the effect of diagenesis on pore evolution for Wufeng-Longmaxi shale with siliceous, argillaceous, and calcareous lithofacies. The siliceous shale was formed under the anoxic reducing condition which favored organic matter (OM) enrichment, while argillaceous and calcareous shales were formed under dysoxic-to-oxic conditions. Authigenic quartz, of mostly biogenic origin, accounts for about 60% and 12% of the total quartz content in siliceous and calcareous shales, respectively, whereas quartz in argillaceous shale is almost detrital origin. The diagenetic sequence has the following order: compaction, cementation, clay transformation, dissolution, as well as OM maturation and its accompanying pore filling. The quantitative calculation reveals that the initial porosity rapidly declines due to compaction and cementation, while dissolution and OM maturation increase porosity to some extent; the dominant factor resulting in porosity reduction varies in different lithofacies. Specifically, the porosity decline induced by cementation is greater than that by compaction in siliceous shale, and it just the reverse in argillaceous shale, while it in calcareous shale is caused by compaction and cementation which show a similar extent. In addition, the matrix composition controls the dominant diagenetic events in different lithofacies during the early diagenetic stage, as indicated by the cementation of authigenic quartz for siliceous and compaction of clays for argillaceous shales. Overall, diagenesis determines the final status of the pore space during the middle diagenetic stage, which further controls the abundance and distribution of migrated OM, and the development of OM pores in the entire pore spectrum.

Acknowledgements: This project was supported by the National Natural Science Foundation of China (No. 41690134).