Numerical modelling of the effects of fault slip on fluid flow around extensional faults

Abstract

Fluid flow and deformation in regions of fractured rock around extensional faults have been modelled using distinct element methods (UDEC code). The basic methodology is described in terms of a simple model of a planar normal fault zone, at the Earth's surface. The model is then modified to simulate deformation at greater depths and to investigate irregularities in fault shape (including dilational and anti-dilational fault jogs). The results obtained show that the deformation of a faulted region resulted in significant variation in fracture dilation (porosity), stress distribution, fluid pressure and fluid flow. The geometry of models and the applied boundary conditions had important effects on deformation and fluid flow. At shallow depth, dilation and fluid flow occurred both in the fault zone and the hangingwall, with little change in the footwall. At greater depth, the higher compressive stresses tended to close all fractures, except within the fault zone where the shear displacements caused local dilation. The presence of anti-dilational bends reduced the dilation and fluid flow in the fault zone, but promoted greater deformation in parts of the hangingwall. The dynamic response of fracture aperture, pore pressure and shear displacement to fault slip was also studied. The modelled results are in reasonable correspondence with observed natural examples and have practical significance for evaluating fluid flow and deformation in regions which exhibit normal faulting.