Normal fault transmissibility characteristics under the transition condition of fault conduction and sealing observed in simulation experiments

Abstract

In rift basins, normal faults conducting and sealing fluids are the main mechanisms for evaluating fault-related reservoirs, and the defined threshold value of fault conduction or sealing is based on statistics. However, fault transmissibility characteristics under a transition condition of fault conduction and sealing have not been clarified. This study used structural deformation and hydrocarbon charging as a foundation to quantitatively characterize fault transmissibility under the transition condition of fault conduction and sealing. This study examined faults occurring in sandstone–mudstone interbeds and used the fault throw and mudstone content as the main variables. The physical simulation results revealed that despite all models exhibiting simple “one-layer” fault architecture with only a fault core, their internal composition undergoes a sequential change from the mudstone drag, mudstone drag and sliding surface to the mudstone breccia and sliding surface. Similarly, when the fault is in the transition condition of fault conduction and sealing under the constraints of mudstone, a normal distribution is observed in terms of the number of oil-bearing layers, oil-bearing length, and oil-bearing area of the lateral fault conduction. However, when the shale smear factor (SSF) is too high, the “one-layer” fault architecture mainly comprises the mudstone breccia and sliding surface, which increases the vertical fault transmission and reduces the lateral fault conduction. Conversely, an excessively small SSF, where the fault zone is a “one-layer” fault architecture comprising mainly the mudstone drag or mudstone drag and sliding surface, will restrict the vertical fault transmission and reduce the lateral fault conduction. When the lateral fault conduction near the inflection point of the normal distribution reaches its threshold, the superposition of mudstone along the fault and the presence of mudstone breccia in the fault zone become critical for decelerating the vertical fault transmission and accelerating the lateral fault conduction. The study of normal fault transmissibility characteristics under the transition condition of fault conduction and sealing provides a solid theoretical foundation for the risk assessment of fault-related hydrocarbon accumulation.