RESUMO
Although previous studies have yielded valuable insights into shale gas reservoirs, a comprehensive understanding of the organic geochemical characteristics and organic matter enrichment of marine-continental transitional shale has yet to be achieved. The Longtan Formation transitional shales were extensively deposited in Southern Anhui Province, South China, during the Late Permian. Our analysis of twenty-two rock samples from one core (Gangdi-1 well) and two outcrops (Daoshanchong outcrop and Changqiao outcrop) revealed that the Longtan Formation shale extracts exhibit a wide range of C11-C35n-alkanes and acyclic isoprenoids, with unimodal, bimodal, and multimodal distributions. The carbon peak ranges from nC15 to nC24, with high quantities of medium-chain n-alkanes (nC22-nC25), indicating that the organic matter in Longtan Formation shale originates from a mixed source of higher plant debris and lower aquatic organisms. Our conclusion is supported by the ternary diagram of C27-C28-C29 regular steranes and the variations of the δ13C values of C15-C32n-alkanes, which is higher than the corresponding value (<1.6) of n-alkanes from a single source. Furthermore, thermal maturity proxies based on organic petrography (Ro and Tmax) and biomarkers, such as the ratios of C31 22S/(22S + 22R), C29 20S/(20S + 20R), and C29 ßß/(αα + ßß), suggest that organic matter is in a mature stage of hydrocarbon generation. By analyzing the Pr/Ph ratio and pyrite morphology combined with a plot of total organic carbon (TOC) versus total sulfur (TS) and the Pr/nC17-Ph/nC18 diagram, we speculate that the Longtan Formation shales were chiefly developed in a dysoxic-to-oxic water environment. Finally, we establish depositional models of organic matter enrichment in deltaic and tidal flat-lagoon environments, emphasizing that the abundant mixed-sourced organic matter can significantly enhance primary productivity, and a higher sedimentation rate can distinctly shorten organic matter exposure time in the oxidized water environment, thereby promoting organic matter accumulation in such a setting.
