A novel method for quantitatively identifying driving forces and evaluating their contributions to oil and gas accumulation

Different driving forces govern the formation of distinct types of oil and gas accumulation and yield diverse oil and gas distributions. Complex oil and gas reservoirs in basins are commonly formed by the combination of multiple forces. It is very difficult but essential to identify driving forces and evaluate their contributions in predicting the type and distribution of oil and gas reservoirs. In this study, a novel method is proposed to identify driving forces and evaluate their contribution based on the critical conditions of porosity and permeability corresponding to buoyancy-driven hydrocarbon accumulation depth (BHAD). The application of this method to the Nanpu Sag of the Bohai Bay Basin shows that all oil and gas accumulations in the reservoirs are jointly formed by four driving forces: buoyance (I), non-buoyance (II), tectonic stress (III₁) and geofluid activity (III₂). Their contributions to all proven reserves are approximately 63.8%, 16.2%, 2.9%, and 17.0%, respectively. The contribution of the driving forces is related to the depth, distance to faults and unconformity surfaces. Buoyancy dominates the formation of conventional reservoirs above BHAD, non-buoyant dominate the formation of unconventional reservoirs below BHAD, tectonic stress dominates the formation of fractured reservoirs within 300 m of a fault, and geofluids activity dominates the formation of vuggy reservoirs within 100 m of an unconformity surface.