Constraints of palaeoenvironment on organic matter of Benxi Formation shale and discussion on enrichment mechanism under different facies

As a hydrocarbon-rich sedimentary basin in China, the Ordos Basin has enormous potential for shale gas resources. The shale of the Upper Carboniferous Benxi Formation is rich in organic matter, however, its palaeoenvironment and organic matter enrichment mode are yet to be revealed. In this study, the geochemical characteristics of the shale of the Benxi Formation in the east-central part of the Ordos Basin were analyzed to investigate its palaeoenvironment. At the same time, the organic matter enrichment modes in different sedimentary facies were compared and analyzed. The results indicate that: 1) the shale of the Benxi Formation was mainly deposited on the continental margin and strong terrestrial clastic input; 2) the deposition period of the Benxi Formation shale had a hot and humid climate with high palaeoproductivity and local volcanic hydrothermal fluid, and a high sedimentation rate with the strong stagnant environment. The bottom water was in dysoxic conditions and a semi-saline deposition environment; 3) multiple factors, such as palaeoproductivity, volcanic hydrothermal, redox conditions, and palaeosalinity interact to influence the enrichment of shale organic matter in Benxi Formation; 4) the organic matter enrichment modes of continental, marine-continental transitional, and marine shales can be classified into three types: “production mode”, “hybrid mode of preservation and production”, and “preservation mode”, respectively. This study provides a reference for the organic matter enrichment mode, shale gas formation conditions, and core area evaluation in these marine-continental transitional shales, and also offers new guidance for exploration ideas for shale gas in different sedimentary facies.