Sedimentary environment and organic matter accumulation of Wufeng-Longmaxi shales, southwest Yangtze Plate, China: Insights from geochemical and petrological evidence

Abstract

Upper Ordovician–Lower Silurian Wufeng-Longmaxi Formation is the most developed strata of shale gas in southern China. Due to the complex sedimentary environment adjacent to the Kangdian Uplift, the favorable area for organic-rich shale development is still undetermined. The authors, therefore, focus on the mechanism of accumulation of organic matter and the characterization of the sedimentary environment of the Wufeng-Longmaxi Shales to have a more complete understanding and new discovering of organic matter enrichment and favorable area in the marginal region around Sichuan Basin. Multiple methods were applied in this study, including thin section identification, scanning electron microscopy (SEM) observations and X-ray diffraction (XRD), and elemental analysis on outcrop samples. Five lithofacies have been defined according to the mineralogical and petrological analyses, including mudstone, bioclastic limestone, silty shale, dolomitic shale, and carbonaceous siliceous shale. The paleo-environments have been reconstructed and the organic enrichment mechanism has been identified as a reduced environment and high productivity. The Wufeng period is generally a suboxic environment and the early Longmaxi period is a reducing environment based on geochemical characterization. High dolomite content in the study area is accompanied by high TOC, which may potentially indicate the restricted anoxic environment formed by biological flourishing in shallower water. And for the area close to the Kangdian Uplift, the shale gas generation capability is comparatively favorable. The geochemical parameters implied that new favorable areas for shale gas exploration could be targeted, and more shale gas resources in the mountainbasin transitional zone might be identified in the future.