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Potential New Method for Paleostress Estimation by Combining Three-dimensional Fault Restoration and Fault Slip Inversion Techniques: First Test on the Skua Field, Timor Sea
A. P. Gartrell, M. Lisk
Division of Petroleum Resources, Commonwealth Scientific and Industrial Research Organization, Perth, Western Australia, Australia
This work was funded through the Australian Petroleum Collaborative Research Centre Seals Program, and company sponsors to this program are gratefully acknowledged. Structural restoration software (2DMove and 3DMove) was generously provided by Midland Valley Exploration Ltd. Our gratitude also goes to Schlumberger Oilfield Australia Pty Ltd for the use of GeoFrame software, which was used exclusively for interpretation and depth conversion of seismic data in this project. The computer program TENSOR (Delvaux, 1993) was used to perform the fault slip inversion calculations. This chapter has benefited significantly from reviews and/or comments by John Walsh, Geoff O'Brien, Quentin Fisher, and Peter Boult.
This pilot study indicates that estimating paleostress orientations and magnitudes from seismic data, through analysis of fault slip data obtained using three-dimensional restoration techniques, is possible, and the results generated are consistent with regional observations. The results suggest that the stress regime responsible for late Miocene fault activity in the vicinity of the Skua oil field in the Timor Sea differs from the present-day stress regime. An extensional stress regime, having the maximum principal stress axis (1) oriented vertically, the intermediate principal stress axis (2) oriented approximately east–west, the minimum principal stress (3) oriented approximately north–south, and a stress ratio (R) of about 0.3, was calculated for the late Miocene. In contrast, measurements of the present-day stress field indicate a transtensional stress regime in which 2 is vertical, 1 is horizontal and trends east-northeast–west-southwest, 3 trends north-northwest–south-southeast, and R = 0.8. Estimation of the magnitudes of the principal stresses indicate that the differential stress operating in the late Miocene was similar to the present, but that greater mean stress in the present-day stress state results in a lowering of reactivation risk with time. These results are consistent with regional observations of widespread late Tertiary extensional faulting, with decreasing fault activity to the present day. The work also suggests that the majority of hydrocarbon leakage associated with fault reactivation in this region is less likely to be associated with the present-day stress regime than with the paleostress regime.
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