Camac, Bronwyn A.¹, Suzanne Hunt¹, Peter Boult¹
(1) University of Adelaide, Adelaide, Australia

ABSTRACT: From Borehole to Basin - 3D Numerical Modelling of the Cap Seal in the Penola Trough, Onshore Otway Basin, South Australia

Due to its thickness and lithotype, the sealing cap rock in the Penola Trough, South Australian Otway Basin 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 2001 a fault dependant gas prospect was drilled. The sealing fault of the structure was oriented to have a low risk with respect to fault seal. The discovery of a partial gas column indicates a semi-breached structure. Wellbore image data show a conductive fracture network around the main bounding fault which extends to the top of the seal and down into the reservoir unit. Borehole breakouts show that the theorised perturbations of regional maximum horizontal stress around pre-existing major disconformities exist and result in a hydraulically open fracture network or cap seal structural permeability. The fracture network may provide a conduit for migration of hydrocarbons.
This work generated questions about the failure of cap-rock seal in the Penola Trough and led to the implementation of discrete and boundary element numerical modelling techniques to fully integrate fault and cap seal studies. 3D finite difference and boundary-element methods show that predicting local stress perturbation is an important step in the risking of cap rock and fault seal in un-drilled prospects and hence increases exploration confidence in this basin. Numerical modelling techniques used in the Penola Trough for understanding cap rock and fault seal integrity and failure have direct application to risk assessment in other highly faulted regions.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.