Characteristics of accumulation dynamic fields and orderly distribution pattern of hydrocarbon reservoirs in the whole petroleum system of a continental faulted basin: A case study of the Nanpu Sag in the Bohai Bay Basin

Widespread shale, tight, and conventional hydrocarbon reservoirs coexist in continental faulted basins in eastern China, as characterized by complex reservoir distribution patterns. Based on the whole petroleum system theory and the dynamic field model for hydrocarbon accumulation, we investigate the Nanpu Sag in the Bohai Bay Basin. By analyzing representative hydrocarbon reservoirs, we identify the dynamic boundaries of varying hydrocarbon reservoir types, delineate the dynamic fields for hydrocarbon accumulation, and examine the hydrocarbon accumulation characteristics within each dynamic field. Accordingly, the dynamic field evolution-controlled hydrocarbon accumulation model in the whole petroleum system is established with the genetic mechanisms of the hydrocarbon reservoirs elucidated. The results indicate that the Nanpu Sag exhibits three dynamic boundaries: the buoyancy-driven hydrocarbon accumulation depth (BHAD), the hydrocarbon accumulation depth limit (HADL), and the active source-rock depth limit (ASDL). These boundaries delineate a free, a restricted, and an irreducible dynamic field for hydrocarbon accumulation, which govern the conventional, tight, and shale hydrocarbon reservoirs, respectively that are distributed in a regular pattern from top to bottom. The BHAD corresponds to a critical porosity of 10.0%, a permeability of 1 × 10^-3 μm², and a pore-throat radius of 0.380 μm. The HADL is characterized by a critical porosity of 2.0% and a pore-throat radius of 0.037 μm. For the ASDL, the critical porosity is predicted to be 1.0%, corresponding to a vitrinite reflectance (R_o) of approximately 3.3%. These dynamic boundaries differ across varying areas primarily due to the influence of lithology and geothermal gradient. Specifically, a coarser-grained lithology and a lower thermal gradient are associated with a deeper BHAD. During the late depositional stage of the Dongying Formation, conventional hydrocarbon reservoirs are primarily formed due to the presence of the free hydrocarbon dynamic field. During the late depositional stage of the Minghuazhen Formation, tight hydrocarbon reservoirs were principally formed under the action of the restricted dynamic field. At present, the free dynamic field predominates in the shallow to intermediate stratigraphic levels with conventional hydrocarbon reservoirs occurring, the restricted dynamic field is primarily distributed in the intermediate to deep layers with tight hydrocarbon reservoirs forming, and the deep irreducible dynamic field contributes to the formation of shale hydrocarbon reservoirs in the Gaoliu area and the Linque Sub-sag. Therefore, the Linque Sub-sag—Gaoliu area in the Nanpu Sag exhibits a distribution and evolutionary sequence of shale, tight, and conventional hydrocarbon reservoirs within the Paleogene-Neogene whole petroleum system from sub-sags to the basin margin.