Numerical modelling of strain localisation and fluid flow during extensional fault reactivation: Implications for hydrocarbon preservation

A numerical modelling approach is used to investigate the main controls of strain localisation on faults and fluid flow in fault-bound reservoir rocks during extensional reactivation. A series of 3D coupled mechanical deformation and fluid flow models were performed to simulate the response of simple fault and reservoir-seal geometries to extensional reactivation. The model results demonstrate that initial fault length is a primary control on strain distribution between faults and on fluid transport from the reservoir through the top seal. During extensional reactivation, longer faults in the population tended to accommodate greater shear strain and accumulate greater throw than smaller faults. With greater length contrast, there is an increased throw contrast for the same amount of extension. This behaviour can be attributed to the mechanism of strain sharing among fault elements. Spacing between adjacent faults is a secondary factor affecting strain partitioning. For a given fault length contrast, increasing the spacing increases the freedom of adjacent faults to move and leads to a more even distribution of strain. Fault strike overlap styles also affect strain distribution along faults. This is mainly expressed as the reduction of throws in the overlapping fault segments and partially overlapped or adjacent fault ends. Our models further demonstrate that fault lengths are likely to control fluid flow during extensional reactivation. Large faults in the models are the main conduit for fluid transport, expressed as strong upward fluid flow along these faults from the sandstone reservoir horizon through the shale seal.