Sedimentary Environment and Enrichment of Organic Matter in the Shahejie Formation, Huanghekou Depression, Bohai Bay Basin, China

Abstract

As a hydrocarbon-rich depression within the Bohai Bay Basin, the Huanghekou Depression is a focal region for exploring hydrocarbons in the eastern China Sea. Previous studies have insufficiently examined the correlation between the enrichment of organic matter and the environments in which it is deposited. Herein, the hydrocarbon potential, palaeoclimate, sedimentary environment, organic matter sources, and organic matter enrichment of the source rocks of the Shahejie Formation in the Huanghekou Depression were investigated using organic and inorganic geochemical indicators. The organic matter type of the source rock in Huanghekou Depression’s Shahejie Formation was predominantly Type II, with a minor presence of Type III. Furthermore, the source rock had a poor-to-good comprehensive evaluation grade in E3s1–2, whereas E2s3 and E2s4 had medium-to-good comprehensive evaluations in their source rocks. In terms of maturity, E3s1 was in an intermediate position between the immature and mature stages and E3s2 and E2s3 were between the low-maturity and mature stages, whereas E2s4 transitioned into full maturity. Biomarkers and sensitive element indicators indicated that the organic matter in E3s1–2 was primarily derived from lower aquatic organisms and algae. This palaeoclimate was characterised by aridity, a water body containing saline and semi-saline water, and a strongly reducing environment resulting from water body stratification, leading to oxygen deficiency. The organic matter in E2s3 was primarily derived from aquatic organisms and algal inputs; these deposits were formed in a reduced environment characterised by relatively low salinity, ranging between semi-saline and freshwater conditions. The organic matter enrichment model of the Shahejie Formation was established based on sedimentary environment, palaeoclimatic, and organic matter source analyses, utilising E3s1–2 as preservation models and E2s3 as the productivity model. This study provides a basis for in-depth exploration and advancement of oil and gas reserves.