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.

Understanding present-day stress in the onshore Canning Basin of Western Australia

“…Characterising the present-day stress state of the Canning Basin requires the determination of both orientation and magnitude of the three principal stresses, r v , r h , and r H . Image log data from 15 wells were used to determine the orientation of r h and r H , r v magnitudes were sourced from Bailey and Henson (2019). The magnitudes of r h and r H were determined through the construction of 1 D MEMs for 16 wells across the Canning Basin (after Bailey & Henson, 2018) (Table 1).…”

“…The density logs used to calculate r v are not always run to surface so are, where unavailable, estimated using the empirically derived Gardner (Gardner et al, 1974) velocity-density transform, to estimate a top-of-log stress value from well velocity survey data and allow a vertical stress profile to be constructed (King et al, 2010;Tingay et al, 2003;Zoback, 2007). Bailey and Henson (2019) used wellbore data from 102 open-file petroleum wells to characterise vertical stress within the onshore Canning Basin, interpreting r v magnitudes from density logs and checkshot data. The authors calibrated the Gardner velocity-density transform to Canning Basin stratigraphy, taking into account the complexities of variable lithology within each well (Bailey & Henson, 2019).…”

“…Although it forms one of the initial 16 stress provinces defined in the Australian Stress Map (Hillis et al, 1998;Hillis & Reynolds, 2000, 2003, limited stress data are available over the Canning Basin (Bailey & Henson, 2018, 2019. The Australian Stress Map project currently includes the Canning Basin as one of 30 defined stress provinces for continental Australia and Papua New Guinea .…”

“…A stress province is defined as a close geographic region (typically a basin) where at least four r H orientation records of suitable quality exist (Hillis et al, 1998;Hillis & Reynolds, 2000, 2003. Bailey and Henson (2019) undertook a detailed interpretation of r v magnitudes in the Canning, noting that the basin is broadly subject to a strike-slip faulting stress regime that varies based on lithology (Bailey & Henson, 2018) and hosts highly variable r v magnitudes. Bailey and Henson (2018) constructed several one-dimensional mechanical earth models (1 D MEMs) to quantify stress magnitudes.…”

“…1 D MEMs were constructed for those wells where suitable data were available, permitting the evaluation of lithological controls on stress regime within the basin. Vertical stress (r v ) magnitudes were drawn from the work of Bailey and Henson (2019), who derived values from checkshot-calibrated density logs. Calibration data for the 1 D MEMs were drawn from previous rock property testing and through modelling of borehole failure features observed in wellbore image logs.…”