Variation of vertical stress in the onshore Canning Basin, Western Australia

Abstract

Vertical stress is one of the three principal stresses and is an important parameter in geomechanical studies that are focussed on the prediction of pore pressure, fracture gradients and wellbore stability. Variations of the vertical stress magnitude can be attributed to variations in lithology or diagenetic history, localised uplift and overpressures caused by disequilibrium compaction. This study uses wellbore data from 102 open-file petroleum wells to characterise vertical stress within the onshore Canning Basin of north-western Australia. Vertical stress magnitudes are interpreted from density logs and checkshot data and at 1 km depth below the ground surface range from 20.5 to 25.0 MPa km–1 with a mean value of 22.1 MPa km–1 (s.d. = 1.0 MPa km–1). Significant variation is evident within the calculated stress magnitudes, and when presented spatially, three regions of elevated vertical stress are identified: the Barbwire Terrace, the Devonian reef complexes of the northern Lennard Shelf and the Mowla Terrace. Lithology, abnormal pore pressures and tectonic uplift are investigated as potential mechanisms of the observed variation. Although abnormal pore pressures are identified, no direct correlation between overpressured areas and elevated vertical stress magnitudes is observed. The Canning Basin has an extensive history of uplift; however, there is little evidence for significant recent inversion. While uplift is likely to exert some influence over vertical stress magnitudes in the Canning Basin, the primary cause is interpreted to be lithological: areas of elevated vertical stress magnitude are also areas where thick intervals of carbonate sediments are present.