New Constraints on the In Situ Stress Tensor in the Eastern Offshore Otway Basin, South-eastern Australia: Implications for the Reactivation Potential of Sealing Faults

A detailed understanding of the in situ stress tensor is integral for evaluating the reactivation potential of sealing faults within fault-bound hydrocarbon accumulations. The Shipwreck Trough’s Pecten High in the Otway Basin, south-eastern Australia, hosts a vast majority of economic gas fields in this basin, with the distribution of significant hydrocarbon accumulations principally controlled by spatial proximity of actively generating source kitchens, complex migration pathways and fault sealing integrity.

This study combines petrophysical wireline data, drilling data and image log data from recently drilled wells along the Pecten High in the Shipwreck Trough with contiguous, high-fidelity 3D seismic datasets to interpret the geometry of fault systems and to evaluate the likelihood of fault reactivation based on a newly determined in situ stress tensor for this region.

Breakouts interpreted from image log data reveal a NW-SE maximum horizontal stress orientation and formation pressure tests have confirmed near-hydrostatic conditions for all the Pecten High wells. Newly available leak-off test results constrains the minimal horizontal stress magnitude for offshore Pecten High wells to be ~20-22 MPa/km, which is slightly greater than the vertical stress magnitude for depths shallower than ~2.5-3 km. This implies a reverse to strike-slip faulting stress regime, which is consistent with neotectonic faulting evidence. Frictional limit theory assuming μ = 0.6 estimates the upper limit of the maximum horizontal stress magnitude gradient to be ~45 MPa/km. Seismic interpretation reveals ~W-E to NW-SE trending faults dipping moderately to steeply towards the north and south, and indicates that the likelihood of fault reactivation is moderate to high based on the newly constrained in situ stress tensor in the vicinity of the recently-discovered Halladale and Black Watch natural gas fields. Faults of these orientations that were previously believed to be at low risk of reactivation in a normal fault or strike-slip fault stress regime are now considered to be at high risk of reactivation, with major implications for the overall fault seal integrity and the prospectivity of the Shipwreck Trough. Our study emphasises the value of integrating structural geological and geomechanical datasets when evaluating natural gas reservoirs in neotectonic regions.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California