Variations of organic matter content and type within the sequence stratigraphic framework of the lacustrine deep-water Dongyuemiao formation, Sichuan Basin, Western China

The organic matter (OM) enrichment patterns and mechanisms in lacustrine deep-water shale successions were controlled by complex geological and biological processes, rather than any single factor. The integration of sedimentology, petrography, geochemistry, and sequence stratigraphy in the study of shale successions is vital to the development of robust paleoenvironmental interpretations and OM enrichment model. Based on a systematic sedimentological, petrographic, and geochemical analysis of three continuous cored wells of the Lower Jurassic Dongyuemiao Formation, Sichuan Basin, within a sequence stratigraphic framework, this study reports the complex geological responses in detrital input, sedimentation rate, benthic redox conditions, primary water column productivity, and OM enrichment pattern that caused by lake-level fluctuation. The results indicate that none of the proxies for detrital input, redox conditions, or productivity exhibit a significant correlation with total organic carbon (TOC) content in sediments. The OM enrichment was regulated by a complex interplay of these three factors, which is a function of relative lake-level fluctuation. In general, detrital input was decreased during lake-level transgression; whereas increased during lake-level regression. Reducing conditions and primary pro-ductivity were generally enhanced during lake-level transgression; whereas diminished during lake-level regression. Coupled relationships between lake-level fluctuation, bulk TOC content, and OM types were observed. The bulk TOC content and liptinite proportion in sediments increased gradually during the lake-level transgression. In contrast, the bulk TOC content and liptinite proportion in sediments gradually decreased during the lake-level regression. This general principle applies to both second-order and third-order lake-level fluctu-ation. Our study demonstrates that the development of a reliable OM enrichment model in lacustrine shale successions requires a comprehensive sequence stratigraphic framework.

Automated summary

The patterns and mechanisms of organic matter (OM) enrichment in lacustrine deep-water shale successions are controlled by complex geological and biological processes. The integration of sedimentology, petrography, geochemistry, and sequence stratigraphy is crucial for accurate paleoenvironmental interpretations and the development of OM enrichment models. This study analyzes the geological responses caused by lake-level fluctuation in detrital input, sedimentation rate, benthic redox conditions, primary water column productivity, and OM enrichment pattern, based on a systematic analysis of three continuous cored wells in the Sichuan Basin. The results demonstrate that the correlation between proxies for detrital input, redox conditions, or productivity and total organic carbon content is not significant. The enrichment of OM is regulated by the interplay of these factors, which is influenced by relative lake-level fluctuation.