Depositional Environment Variation and Organic Matter Accumulation Mechanism of Marine–Continental Transitional Shale in the Upper Permian Longtan Formation, Sichuan Basin, SW China

Abstract

Marine–continental transitional shale gas exploration is one of the hottest topics in the academic and industrial community. Frequent sedimentary environment changes lead to substantial heterogeneity and complex organic matter enrichment mechanisms, and it is of great practical significance to clarify the paleoenvironment of marine–continental transitional shale and reveal the enrichment mechanism of organic matter for subsequent shale gas exploration. However, strong heterogeneity creates divergence for our understanding of organic matter enrichment mechanisms and resource potential. This paper selected the marine–continental transitional shale of the Permian Longtan Formation in the southeastern Sichuan Basin as an example. Through TOC analysis, XRD measurements, and major and trace element analyses, we identify the depositional cycle and lithofacies, characterize the paleosedimentary environments of different lithofacies, establish depositional models, and clarify the organic matter enrichment mechanism. Four lithofacies can be identified in the Longtan Formation in the study area: organic–lean carbonate shale (LC), organic–lean mixed shale (LM), organic–lean argillaceous shale (LCM), and organic–rich argillaceous shale (RCM). The terrestrial detrital influx of the LCM and RCM shales is obviously strong, while the RCM shale was deposited in a relatively dysoxic and humid environment. For the Longtan shale, the terrestrial detrital influx and redox environment determine the enrichment of organic matter. In the lagoon sediment system, due to the high terrestrial detrital influx, the argillaceous shale in the transitional shale gas system should be given more attention in future exploration. For different kinds of shale gas systems (marine, marine–continental transitional, and lacustrine), the effect of the terrestrial detrital influx is the main difference. In marine–continental and lacustrine shale gas systems, shales with high TOC values can still exist in the context of oxic sedimentary environments. The research results of this paper can provide a theoretical basis for future marine–continental transitional shale gas sweet spot prediction and exploration from geochemical perspectives.