Organic geochemical and mineralogical characterization of the lower Silurian Longmaxi shale in the southeastern Chongqing area of China: Implications for organic matter accumulation

Abundant in organic matter, the lower Silurian Longmaxi shale in the Upper Yangtze Platform, South China, has been demonstrated to play important roles in the petroleum system as both a good source rock and a shale gas reservoir. In order to better understand the heterogeneous organic matter accumulation of such shales in the southeastern Chongqing area, the vertical variations of total organic carbon (TOC) contents, mineral concentrations and biomarker distributions were systematically analyzed in this study. The Longmaxi Formation consists of fine-grained siliciclastic rocks deposited in a restricted marine shelf environment, and it can be further subdivided into two members. The lower member is enriched in TOC, whereas a diminished TOC content is documented in the upper member. Laterally, the TOC content gradually drops southeast-ward in the study area. The mineralogical compositions, dominated by quartz and clay, and, to a lesser extent, feldspar and carbonate, show good correlations with TOC contents vertically, especially in the lower member. Saturated and aromatic hydrocarbon biomarkers indicate a dominant organic matter origin from lower hydrobionts (algae and bacteria) in a reducing marine environment. Negative correlation relationships between TOC contents and terrigenous clastic minerals indicate the predominant role of dilution of terrigenous clastic materials in controlling the organic matter accumulation. The sea level drop combined with the continuously active Xuefeng Uplift contributed to more terrigenous clastic inputs into the study area, which resulted in a significant reduction of TOC content in the upper Longmaxi member.