Paleoenvironmental modes and organic matter enrichment mechanisms of lacustrine shale in the Paleogene Shahejie Formation, Qikou Sag, Bohai Bay Basin

Abstract

The Paleogene Shahejie Formation (Shahejie Fm) of Bohai Bay Basin has ignited unprecedented interest among scholars owing to another breakthrough achieved in shale exploration besides Kongdian Fm. However, this breakthrough, particularly high output submember—the third Member of the Paleogene Shahejie Formation (Mbr 3) lacks elaborate study on paleoenvironment and its covariations with organic matter (OM) concentration for shales, which impedes the further acquaintances of OM distribution and shale oil–gas possibility. The present study adopts advanced approaches to gain insights into the deposition process of shales in the Mbr 3, Qikou Sag, Bohai Basin. Petrological characteristics, organic geochemistry, and an abundant element database offer a solid foundation for performing master controlling factor analysis and establishing deposition modes for OM accumulation and preservation. Our results indicate that the entire Mbr 3 has potential. Shales in the Mbr 3 of the Shahejie Fm were deposited under a temperate environment with weak weathering and sediment influx surrounded by suboxic freshwater. However, the organic-enriched shale in the upper Mbr 3 is a more desirable exploration layer. Disparate paleoproductivity support and clastic detritus sediment afforded a diminished consequence of OM accumulation between the upper Mbr 3 (L1–L2 in this study) and middle Mbr 3 (L3 in this study). Our investigation suggests that the OM concentration in the shales was dominated primarily via paleoproductivity and secondarily via redox reactions. Clastic sediment influx sets the primarily passive effect, while effect of salinity was minimal. Two sedimentary modes were established to interpret the variability between L1–L2 and L3 during the two depositional stages. The lake expansion period of L3 resulted in shales being deposited mainly on a gentle slope with a shallow water column containing insufficient OM susceptible to redox condition compared to the optimal state in the semideep lake in L1–L2 stage. The results provide an integrated reconstruction of paleosedimentary environment for the target layer and offer a solid basis for further exploration through coupling relationship between sedimentary environment and OM enrichment in the study area.