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Petroleum Geoscience

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Predicting brittle cap-seal failure of petroleum traps: an application of 2D and 3D distinct element method

¹ Australian School of Petroleum (Engineering Discipline) University of Adelaide , Adelaide S.A. 5005, Australia (Present address: Beach Petroleum, 25 Conyngham Street, Glenside, S.A. 5065, Australia)
² Santos Ltd, 60, Flinders Street, Adelaide , S.A. 5000, Australia
³ GINKO ENPGNG Adelaide , Australia

^* Corresponding author (e-mail: Bronwyn.Camac{at}beachpetroleum.com.au)

Previous research shows the importance of understanding the relationship between fault geometry and current applied tectonic stresses in the prediction of critically stressed faults and their propensity for fluid flow via generated fracture networks along and/or around the fault plane. Data collected from the Penola Trough, onshore Otway Basin, South Australia shows that a more complex 3D failure mechanism may be active, whereby the cap-seal may fracture preferentially to fault failure.

An emerging application for a two- and three-dimensional distinct element numerical modelling technique, at the play and prospect level, respectively, which assists in assessing pre-drill seal integrity, is presented by way of two case studies from the Penola Trough. Sensitivity studies at the prospect scale show how (1) fault rock strength, (2) fault zone width and (3) the interaction of two fault sets generates local perturbations in the regional stress field. At the play scale, the depth to which a younger active fault set propagates can be explained by the distribution of stress within the rock mass generated by the present-day far-field stress acting on older regionally significant faults. This work offers a workflow and an additional technique to predict cap-seal integrity prior to drilling.

KEYWORDS: stress modelling, faults, distinct element method, fractures, fault rock, Otway Basin, Penola Trough, stress rotation, UDEC, 3DEC