Sedimentary Environment and Organic Matter Accumulation of Wufeng-Longmaxi Shales, Southwest Yangtze Platform: Insights from Geochemical and Petrological Evidence

Abstract

Upper Ordovician-Lower Silurian Wufeng-Longmaxi Foration is the most developed strata of shale gas in southern China. As several shale gas wells were drilled in the mountain-basin transition zone in the Southwest Sichuan basin, the attraction was drawn by this area. While, due to the complex sedimentary environment adjacent to the Kangdian Uplift, the area favorable for organic-rich shale development is still in fog. Further study is necessary to obtain a more complete understanding of organic matter enrichment in the marginal region around Sichuan Basin. We therefore focus on the mechanism of accumulation of organic matter and the characterization of the sedimentary environment of the Wufeng-Longmaxi Shales. In this study, a combination of methods was applied, including thin section identification, scanning electron microscopy (SEM) observations and X-ray diffraction (XRD), and elemental analysis on outcrop samples. According to the mineralogical and petrological analyses, five lithofacies have been defined, including mudstone, bioclastic limestone, silty shale, dolomitic shale and carbonaceous siliceous shale. And for the geochemical characterization, the detrital influx, redox conditions and paleo-productivity have been studied. The results showed that Wufeng period is generally a suboxic environment and the early Longmaxi period is a reducing environment.