Numerical simulation of extensional fault reactivation and fluid flow: Generic models related to the Timor Sea

Abstract

Structural analyses and interpretation of seismic data from the Timor Sea indicate that most of the faults in the region have been reactivated during the Late Tertiary extensional event. These reactivated faults and the associated fluid flow represent a major risk to trap integrity and hydrocarbon preservation. To explore the controls of extensional fault reactivation on fluid flow in petroleum basins, this numerical modelling study has investigated a group of 3D coupled mechanical deformation and fluid flow models characterized by various simple fault geometries and reservoir stratigraphic architecture. The models simulate a regional-scale block (50×50 km in planview and 10 km in depth) with a larger fault and a small fault (either parallel or cross-cutting). The stratigraphic sequences defined for these models include four rock units. They are, from top to bottom, a carbonate layer, a thin shale layer, a sandstone unit and a relatively soft basal rock unit providing a more ductile domain for fault movement and for reducing the edge effects at the bottom model boundary. Such fault and stratigraphic architecture is a simplified representation of regional structures and stratigraphy in the Timor Sea. Model mechanical and fluid flow properties are based on rock property data for the region, in combination with the general rock property data from literature. To consider rock permeability enhancement with faulting, we have incorporated a simple scheme that allows fault permeabilities to increase with accumulation of shear strain, from their initial values (identical to those of host rocks) to various maximum values depending upon host rock type. Greater shearinduced permeability increase is allowed for the shale unit. Initial fluid pore pressures are important boundary conditions for fluid flow modelling. The current models have investigated the effects of four different initial pore pressure (or hydraulic head) distributions, based on data from the Timor Sea: 1) a slightly over-pressured system in the rocks below the shale; 2) a higher over-pressured system in the rocks below the shale; 3) a slightly over-pressured system but with a small hydraulic head difference between the left and right hand boundaries