Incompatible stress regimes from geological and geomechanical datasets: Can they be reconciled? An example from the Carnarvon Basin, Western Australia

- Geomechanical analysis of the Carnarvon Basin's in-situ stress regime is undertaken.
- Neotectonics and contemporary seismicity disagree with geomechanical interpretations.
- Traditional LOT interpretation yields a strike-slip to normal fault stress regime.
- A new method of LOT interpretation reconciles the geologic and geomechanical data.
- A strike-slip fault regime is broadly interpreted for the Carnarvon Basin.

Knowledge of the in-situ stress field is essential in petroleum basins, as it exerts a prime control over seal integrity, fracture stimulation, wellbore stability, and fluid flow. Previous geomechanical analysis of Australia's Carnarvon Basin, the continent's premier hydrocarbon region, highlights a normal to strike-slip fault stress regime. However, neotectonic evidence and contemporary seismicity suggest the possibility of a strike-slip to reverse-fault stress regime. We attempt to reconcile these conflicting datasets through new analysis of the in-situ stresses; Principal stress magnitudes and orientations were defined with data from 76 previously unanalysed petroleum wells. Wellbore image logs yield a maximum horizontal stress orientation of 113°N, consistent with modelling of the stress-field throughout the Indo-Australian Plate. Vertical stress magnitudes are estimated to range from 20 MPa km− 1 to 22.4 MPa km− 1. Leak-off tests (LOT) from 42 wells are analysed using two methodologies (one assuming tensile failure as traditionally accepted, the other assuming shear failure as outlined by Couzens-Schultz and Chan, 2010) and result in two different sets of horizontal stress magnitude estimates. Traditional interpretation of LOTs results in a minimum horizontal stress gradient of 16.8 MPa km− 1 and a maximum horizontal stress gradient of 21.8 MPa km− 1. The new method for LOT interpretation results in a minimum horizontal stress gradient of 18.1 MPa km− 1 and a maximum horizontal stress gradient of 25.4 MPa km− 1. Inclined natural fractures observed at LOT depths supports application of the new method of LOT interpretation. Traditional interpretation of stress magnitudes implies a normal to strike-slip fault stress regime, while the new method implies a strike-slip fault stress regime. This latter interpretation is favoured by the authors, as it allows for a reconciliation of the geological and geomechanical datasets. The assessment undertaken herein allows for reinterpretation of stress magnitudes in seismically-active basins hosting differing geomechanical and neotectonic regimes, potentially altering existing understanding of seal integrity, fracture stimulation, wellbore stability, and fluid flow.