Relationship between Geological Structures and Hydrocarbon Enrichment of Different Depressions in the Bohai Bay Basin

AbstractThe Bohai Bay Basin contains many depressions with varying degrees of hydrocarbon enrichment associated with the geological structures of different depressions. This study discussed the relationship between the geological structures and hydrocarbon enrichment of the depressions in the Bohai Bay Basin. Based on the Paleogene strata distribution and the length to width ratio of different depressions, their geological structures are divided into three types in plan‐view: open (length/width < 2), narrow (length/width > 4) and transitional types (length/width 2–4). In cross section, the geological structures can be divided into dustpan I, dustpan II and double‐faulted types. Based on tectonic evolution and sedimentary characteristics, the depressions are classified into early‐formed, inherited and late‐formed categories. Generally, narrow depressions are mainly located in the northeast and southwest of the Bohai Bay Basin, while open depressions are dominantly distributed in the central area of the basin; late‐formed depressions are mainly around the Bohai sea area, and early‐formed depressions are mostly located in the periphery of the basin. Geological structures of the depressions control the formation of the source, reservoir and cap rocks as well as hydrocarbon accumulation setting, and further influence the pay zones and oil‐bearing sequence. In detail, dustpan II and double‐faulted depressions mainly have A‐type sags, which often possess better hydrocarbon generation conditions than dustpan I ones; hydrocarbons in open dustpan II depressions tend to accumulate in the central uplift areas or buried hill, while those in narrow dustpan I depressions always accumulate in gentle slope belts. The oil‐bearing sequence for different evolutional depressions corresponds well with the sedimentary strata of the main development stages of depressions. In early‐formed depressions, hydrocarbons are mainly enriched in deeply buried reservoirs, while in late‐formed depressions hydrocarbons are abundant in the relatively shallow traps. In summary, most inherited and late‐formed dustpan II depressions are enriched in hydrocarbons due to their extensive source rocks and good source–reservoir–seal assemblages, whereas dustpan I and early‐formed depressions are relatively poor in hydrocarbons.