Sedimentary environment and organic enrichment mechanisms of lacustrine shale: A case study of the Paleogene Shahejie Formation, Qikou Sag, Bohai Bay Basin

Recent Chinese shale oil (or gas) exploration efforts have focused on the Shahejie Formation of Bohai Basin. However, this work has been handicapped by a lack of state-of-the-art investigations of the targeted shale. Specifically, there is a need for an improved understanding of the role that paleoenvironmental aspects of the basin played in the organic enrichment of these deposits. The present study provides the insights into the paleoenvironmental and depositional history of various of shales of the Es-1 Member of Paleogene Shahejie Formation, Qikou Sag, Bohai Basin, and addresses the controlling factors of organic matter (OM) accumulation and preservation in these deposits based on petrographic characteristics, organic geochemistry, and major and trace elements data. Our results suggest that the organic-enriched Es-1 oil shale is a desirable exploration target, especially felsic and mixed shale. The felsic shale, mixed shale, dolostone and carbonate shale accumulated under relatively humid climatic conditions but in saline lacustrine environment. Climate, salinity, and redox conditions appear to have varied at least three times during the depositional history of the studied interval. Paleoenvironmental conditions favored stable productivity levels and only a modest supply of elastic detritus sediment resulting in a diminished rate of deposition. Felsic and mixed shale deposits that have the greatest source potential accumulated under more humid atmospheric and oxygen-depleted environmental conditions than dolostone and carbonate shale. The present study suggests infers that organic carbon content of the Es-1 oil shale was controlled principally by salinity and secondarily by primary productivity and redox conditions.

Automated summary

Recent study focused on the paleoenvironment and depositional history of the Es-1 Member of Paleogene Shahejie Formation in the Bohai Basin, addressing factors controlling organic matter accumulation and preservation. Results suggest that organic-enriched shale deposits accumulated under humid conditions, with organic carbon content primarily controlled by salinity and secondarily by primary productivity and redox conditions.