Mechanisms of Differential Enrichment of Oil and Gas in the Paleogene of the Bohai Exploration Area: Controlled by Sag Types and Source–Reservoir Coupling

As of 2019, 119 oil and gas fields were involved in the Paleogene of the Bohai exploration area, accounting for more than 85% of the total number of oil and gas fields in the Bohai Bay Basin. Previous studies have confirmed that there are significant differences in the degree of oil and gas enrichment in the Paleogene of the Bohai exploration area. However, how these differences affect the hydrocarbon generation potential, and oil and gas resource distribution in each region has not been thoroughly studied; the controlling factors contributing to these differences have also not been fully elucidated. In view of this, based on the research results of projects on oil and gas exploration in the Bohai Bay Basin over the past decade, this paper statistically analyzed a large amount of data on source rocks, sedimentation rates, etc., and explored the differential enrichment of oil and gas in the Bohai exploration area and its genetic mechanisms. The research shows that from the offshore to the center of the marine area, the Paleogene oil and gas enrichment horizons have the characteristics of changing from old to new; the Paleogene source rocks in the offshore and marine areas have significant differences in thickness, total organic carbon content, organic matter type and maturity, and these differences determine the hydrocarbon generation potential and oil and gas resource distribution in each area; the sedimentary filling process in the Bohai exploration area is mainly controlled by tectonic uplift and subsidence and changes in lake level, and it has experienced stages such as rifting and lake transgression, thus showing obvious differences in provenance and sedimentary reservoirs; three sets of source–reservoir–caprock combinations are developed in the Bohai exploration area, and oil and gas are mainly enriched in the source rock strata and adjacent reservoirs. The middle and upper source–reservoir–caprock combinations have the best configurations, and from the offshore to the marine area, oil and gas show a trend from enrichment in Es3 to enrichment in Es1–Ed. Due to the differences in sag types, there is differential enrichment of oil and gas in the Bohai exploration area. According to the sedimentation rate, the sag types in the study area can be divided into early-developed type, inherited-developed type and late-developed type. The sag types in the Bohai exploration area show the characteristics of gradually changing from the offshore to the late type in the marine area, and there is a good corresponding relationship between the sag types and the vertical enrichment strata of oil and gas. The research results can provide a reference for discussing the oil and gas accumulation laws and future exploration and development of the Bohai exploration area.