An Integrated Geomechanical Evaluation of Cap and Fault Seal for Risking Petroleum Trap Integrity – Using the Distinct Element Method  

Bronwyn Camac, University of Adelaide, Australian School of Petroleum, Adelaide, S.A., Australia, [email protected]

The understanding of the geomechanical behaviour of cap rock in the Penola Trough, South Australian Otway Basin is in its earliest stages. Due to its thickness, lithotype and regional extent, the main sealing unit of the gas bearing sands of the Pretty Hill Formation, the Laira Formation, has been considered to have a low risk of cap rock failure in the past. The observation of an open conductive fracture network throughout the seal on FMI logs and associated perturbations in the regional stress field, as analysed from dipmeter data, led to the question: Can these perturbations be modelled using distinct element code and ‘ground truthed’ against field data, where these perturbations correlate with brittle seal failure. If so, can this technique be developed into a risking tool for seal evaluation, ahead of the bit?” 

This research concentrates on these questions and is showing that using existing tools and data routinely collected in the petroleum and mining industries, a more comprehensive and integrated risking analysis can be undertaken for hydrocarbon seals, incorporating both cap and fault seal. 

The selection of modelling parameters is critical to the success of the outcomes of the models. Much of the research has been directed towards the more confident determination of fault parameters, such as friction angle, stiffness and orientation. 

The developed techniques have also been applied to other areas within Australia and New Zealand. These modelling techniques have direct application to risk assessment in other highly faulted regions throughout Australia and internationally.