Modeling overpressure vertical transfer via faults in clastic sequences at the basin scale: Implications for overpressure generation and fluid flow

Abstract

The vertical transfer of overpressure significantly influences the subsurface fluid dynamics, fault zone stability, and geomechanical drilling safety. However, the mechanisms governing vertical overpressure transfer through faults remain poorly understood, and few quantitative assessments have clarified how key geological parameters control the magnitude and evolution of vertically transferred overpressure. In this study, a conceptual model for overpressure vertical transfer was developed, and a sensitivity analysis of geological factors controlling this process was conducted using the DMflow simulator. The results indicate that fault-zone permeability, fault activity duration, sand body permeability, the number of sand bodies connected by a fault, and the initial overpressure difference exert a strong control on both the transfer process and the magnitude of the vertically transferred overpressure. In contrast, the fault dip and sand body spacing have a relatively minor influence on the vertical transfer overpressure. In permeable formations, vertically transferred overpressure dissipates rapidly; therefore, overpressure generated by multiple fault activation events cannot be effectively accumulated. The overpressure is uniform within permeable formations connected by a fault; however, the pressure coefficient is highest in the shallow formation. Overpressure transfer alters local fluid migration pathways. During overpressure re-equilibration, the development of a strong fluid potential gradient promotes the rapid upward migration of deep fluids into shallower layers along the fault. This understanding not only provides new insights into the mechanisms of overpressure generation in sedimentary basins but also has significant implications for predicting pre-drilling formation pressure and improving the understanding of fluid flow behavior within fault zones.