In Situ Stress Field, Fault Reactivation and Seal Integrity in the Bight Basin, South Australia

Abstract

The in situ stress field and consequent risk of reactivation has been evaluated 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°N maximum horizontal stress orientation. The large variation in water depths across the Bight Basin required the use of effective stress magnitudes. A depth-stress power relationship is used to define the effective vertical stress based on density log data from 10 wells. The effective minimum horizontal stress was estimated at 6 MPa/km using effective pressures from leak-off tests. An upper bound (18.7 MPa/km) for the effective maximum horizontal stress was determined using frictional limits to stress. Pore pressure in wells in the region is hydrostatic except in Greenly 1 where over pressure occurs below a depth of 3600 m. The risk of fault reactivation in the Bight Basin was evaluated using the FAST technique. 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 for three different in situ stress cases. In all three cases faults striking 40°N (±15°) of any dip are the least likely to be reactivated.