Sedimentary environment controls on the lacustrine shale lithofacies: A case study from the Nanpu depression, Bohai Bay Basin

Abstract

Organic-rich shale of the Paleogene Shahejie Formation in the Nanpu Sag Bohai Bay Basin was analyzed using petrographic and elements analysis. These analyses were used to identify the lithofacies, reconstruct the sedimentary paleoenvironment and their relation. This paper adopted the division scheme and naming method of “Total organic carbon (TOC) + sedimentary structure + mineral composition” to carry out lithofacies identification and division. The petrographical characteristics indicated that there are 15 types of lithofacies. According to the lithologic changes, we divided the study interval from the fourth sub-member of the third member of the Shahejie formation into three parts from the bottom up. The lower part has an evident fluctuation in paleoclimate, salinity, productivity, and redox condition. The middle part has cold and arid climate conditions, the weakest weathering, the highest paleo-productivity, and anoxic conditions. The upper part was deposited under humid and warm climate conditions, strongest weathering, brackish-fresh water, lowest paleo-productivity, and dominant oxic condition. Paleo-productivity and redox conditions are the basic conditions for the enrichment and preservation of organic matter, the paleo-climate and paleo-salinity were the critical factors controlling the mineral composition and laminae development. Under warm and humid climate conditions, the shallow lake mainly develops organic-poor massive felsic shale and siltstone; the semi-deep lake is mainly developed with organic-medium laminated/layered felsic shale; the deep lake mainly develops organic-rich laminated/layered clayey shale and mixed shale. Under dry and cold climate conditions, the shallow lake area mainly develops organic-poor felsic shale, and the semi-deep lake mainly develops organic-rich layered carbonate shale and organic-rich massive mixed shale; the deep lake mainly develops organic-rich laminated/layered clayey shale.