A Regional Analysis of Fault Reactivation and Seal Integrity Based on Geomechanical Modeling: An Example from the Bight Basin, Australia

T he Bight Basin is a major frontier basin of Jurassic–Cretaceous age, which is currently undergoing renewed exploration interest. Although only limited data is available for understanding the petroleum systems in the basin, several observations indicate that poor fault seal integrity may represent a key exploration risk. The presence of a paleo-oil column in the Jerboa-1 well, interpreted gas chimneys, oil slicks, and asphaltite strandings indicate that seal failure caused by fault reactivation is potentially a significant issue in the Bight Basin. Thus, in this study, we investigated the likelihood that faults in the Bight Basin will undergo sufficient structural reactivation to induce fault seal failure, under the regional in-situ stress field. Fault reactivation risk was assessed for two sets of faults that represent extensional events of Late Jurassic (Sea Lion faults) and Late Cretaceous age (Tiger faults). Analysis of in-situ stress data suggests that the region is currently under a strike-slip or normal stress regime. Interpretation of borehole breakouts from six wells indicates the average maximum horizontal stress orientation is 1308N. Although the magnitudes 5 Reynolds, S. D., E. Paraschivoiu, R. R. Hillis, and G. W. O’Brien, 2005, A regional analysis of fault reactivation and seal integrity based on geomechanical modeling: An example from the Bight Basin, Australia, in P. Boult and J. Kaldi, eds., Evaluating fault and cap rock seals: AAPG Hedberg Series, no. 2, p. 57 – 71. 57 Copyright n2005 by The American Association of Petroleum Geologists. DOI:10.1306/1060756H23162 of the three principal stresses could not be unequivocally constrained, plausible ranges of values were determined based on well data. Pore pressure in wells in the region is hydrostatic except in Greenly-1, where moderate overpressure occurs. This study assesses the risk of fault reactivation using the fault analysis seal technology (FAST) technique. The FAST technique evaluates the increase in pore pressure ( P) required to cause reactivation as a measure of fault reactivation risk. In all cases investigated, faults striking 40(±15)8N of any dip are the least likely to be reactivated. Thus, traps requiring such faults to be sealing are the least likely to be breached. Fault reactivation risk for the strike-slip and normal stress regimes have been plotted in map view on a series of fault orientations for the Sea Lion and the Tiger faults using a range of hypothetical dips. The results for these hypothetical dips clearly demonstrate the importance of knowing both the strike and dip of a particular fault when conducting a three-dimensional fault seal analysis, because the risk can range from relatively low risk at 258 dip to relatively high risk at 708 dip, with differences being more significant for certain fault orientations.