Fault reactivation in the Port Campbell Embayment with respect to carbon dioxide sequestration, Otway Basin, Australia

Abstract

The Naylor structure in the Port Campbell Embayment, Otway Basin, South Australia is proposed as a demonstration site for the subsurface geological storage of carbon dioxide (CO2). The Naylor structure is a fault-bounded high with normal faults to the north and west to SW. Seismic interpretation shows evidence of recent fault reactivation in the Otway Basin. It is postulated that residual hydrocarbon columns (accumulated and leaked prior to present day) in the Otway Basin leaked due to fault reactivation. Thus, a critical issue in the geological storage of CO2 in the Port Campbell Embayment is the potential for the reactivation of faults bounding the Naylor structure. The propensity of faults to be reactivated is assessed by determining the in-situ stress field, the mechanical properties of the fault rock and the orientations of the existing faults. The in-situ stress field lies on the boundary of a strike-slip and reverse faulting regime in the Port Campbell Embayment. The vertical, minimum horizontal and maximum horizontal stress gradients are 21 MPa km–1, 19 MPa km–1 and 38 MPa km–1 respectively and the pore pressure gradient is hydrostatic. The maximum horizontal stress in the Port Campbell Embayment is oriented at 150°N. One planar and two curviplanar faults were identified within the Naylor structure. Two fault segments act to trap accumulations at the crest of the structure. These fault segments have relatively low propensities to reactivate near the crest of the structure. The intended migration pathway of the CO2 plume does not intersect the identified faults until it reaches the crest of the Naylor structure. However, reservoir heterogeneities such as sub-seismic faults may cause the migrating CO2 plume to move towards identified fault segments which are not intended to trap the injected CO2 and have a relatively high propensity to reactivate.