Concentric hydrocarbon accumulations in deep rift basins: A case study of Jizhong and Huanghua depressions, Bohai Bay Basin, China

Deep rift basins are geological environments that can contain large resources of petroleum and can be particularly rich in unconventional oil and gas reserves. However, due to deep burial and complex geological conditions, the occurrences and distributions of hydrocarbon are not easily delineated, which seriously hampers the exploration process. Based on comprehensive analyses of the exploration process and hydrocarbon accumulation characteristics in the deep basin of the Jizhong and Huanghua depressions in Bohai Bay Basin, it was found that the hydrocarbon accumulations appear to be distributed in a concentric pattern. The interplay among tectonics, sedimentation, and hydrocarbon generation-migration in the deep basin determines the distribution of hydrocarbon accumulation. The inner tectonic zone formed by the deep trough area is mainly defined by the deposition of (semi) deep lacustrine mudstone, which forms retained shale reservoirs. In the outer tectonic zone, prodelta (fan) and (fan) delta front fine-grained strata deposited within the low-middle slope area and shallow lake and the subsequent deposition of (fan) delta plain sandstone bodies form intercepted stratigraphic and lithologic reservoirs. Conventional and unconventional reservoirs are distributed in a concentric order. The mechanisms and patterns of concentric hydrocarbon accumulation in deep basin settings can provide useful analogs for oil and gas exploration in deep basins with similar structures, particularly for unconventional oil and gas resources.