Characterization and evaluation of shale lithofacies within the lowermost Longmaxi-Wufeng Formation in the Southeast Sichuan Basin

Abstract

Based on drilling cores, thin sections, FIB-SEM and geochemistry data, shale lithofacies in the lowermost part of the Longmaxi-Wufeng Formation were studied in the Southeast Sichuan Basin. This formation is the main shale gas producing layer in Jiaoshiba and its surrounding areas. In this article, with the combination of the three-end-member method (using mineral components) and TOC, six lithofacies, namely, organic-rich siliceous shale, organic-rich mixed shale, organic-rich argillaceous shale, organic-lean siliceous shale, organic-lean mixed shale, and organic-lean argillaceous shale, were identified. Relevant parameters that could represent reservoir quality of the different lithofacies, including porosity, permeability, pore spaces, TOC and gas content, were analyzed and compared. In the end, some of those parameters (e.g., porosity, TOC and gas content), together with special mineral contents (e.g., QMF and clay), were used to establish shale lithofacies evaluation criteria. The results show that organic-rich siliceous shale could be considered as an excellent lithofacies for shale gas and is mainly concentrated in the first section; organic-rich mixed shale, organic-lean siliceous shale and organic-lean mixed shale are assumed to be moderately promising for shale gas, while organic-rich argillaceous shale and organic-lean argillaceous shale are ranked as the least-promising lithofacies in the research area. Consequently, the first section contains the most favorable lithofacies for shale gas exploration.