Key stages in hydrocarbon migration and accumulation in layers outside source rocks and the evaluation methods related: A case study of the lower member of the Minghuazhen Formation, Bozhong Sag, Bohai Bay Basin

Hydrocarbon migration and accumulation are critical for large-scale reservoir development in layers far from the source rocks. Using abundant 3D seismic and drilling data, we delve into the primary driving forces for hydrocarbon migration and accumulation in three major stages in the Bozhong Sag, Bohai Bay Basin, namely the deep vertical transport along fault zones, secondary convergence in the Guantao Formation, and lateral diversion along sand traps in the lower member of the Minghuazhen Formation. Furthermore, we predict the hydrocarbon column heights within sand bodies in the lower member of the Minghuazhen Formation. Key findings are as follows. First, the coupling between overpressure in source rocks and fault development is crucial to the vertical hydrocarbon transport along fault zones, reflecting a high vertical transport efficiency with a fractal dimension exceeding 0.7 on the condition of abundant hydrocarbon sources. Second, the buoyancy pressure of old oil reservoirs in sand-rich sequences serves as the main driving force for the secondary hydrocarbon convergence. Convergence coefficient calculated using the size and effective range of traps, is in line with the scale of reserves. The lower member of the Minghuazhen Formation has the potential for the formation of large- to medium-scale oilfields when the convergence coefficient of the Guantao Formation exceeds 700 km³. Third, fault depressurization determines the lateral hydrocarbon diversion in shallow sand bodies. The depressurization coefficient is calculated based on the fault-sand contact area in both the high and low parts of the sand bodies. The results show that the hydrocarbon column heights within sand bodies in the lower member may exceed 30 m when the depressurization coefficient is greater than 2.0. These findings underscore that the three major stages are interlocking, which is a necessary condition for the formation of high-abundance hydrocarbon reservoirs in strata outside source rocks in a hydrocarbon-rich sag.