In situ stress field, fault reactivation and seal integrity in the Bight Basin, South Australia

Abstract

We evaluate the in situ stress field and consequent risk of fault reactivation in the Bight Basin in order to assess the risk of fault seal breach at seismically mapped prospects. Borehole breakouts interpreted from dipmeter and image logs in five wells in and around the Bight Basin indicate a 130° maximum horizontal stress orientation. The large variation in water depths across the Bight Basin requires the use of effective stress magnitudes. We use a depth-stress power relationship to define the effective vertical stress based on density log data from 10 wells. The effective minimum horizontal stress gradient is estimated at 6 MPa/km using effective pressures from leak-off tests. We determine an upper bound (18.7 MPa/km) for the effective maximum horizontal stress gradient, using frictional limits to stress. The upper bound to the effective maximum horizontal stress indicates the region is in a strike-slip faulting stress regime. However, a normal faulting stress regime cannot be ruled out. Pore pressure in wells in the region is hydrostatic except in Greenly 1 where mild overpressure occurs below a depth of 3600 m.We use the FAST technique to evaluate the risk of fault reactivation in the Bight Basin. The risk of fault reactivation and consequent seal breach is expressed in terms of the pore pressure increase that would be required to induce failure. We consider three different stress regimes. These include a strike-slip faulting stress regime, a normal faulting stress regime, and a case on the boundary of strike-slip and normal faulting stress regimes. In all three cases, faults striking 40° (±15°) of any dip are the least likely to be reactivated.