ABSTRACT: Micro to Macro Scale Fault Seal Investigations: Application to the Penola Trough, Offshore South Australia

Jones, Richard¹, Dave Dewhurst², and Richard Hillis³
(1) National Centre for Petroleum Geology and Geophysics, University of Adelaide, Adelaide, Australia 
(2) CSIRO Division of Petroleum Resources, Glen Waverley, Australia 
(3) University of Adelaide, Adelaide, SA, Australia

Successful fault seal analysis depends on integrating a number of predictive strategies and data amalgamated from seismic scale down to microscale. In the subsurface, open fractures constitute a seal risk, in that fractures oriented close to criticality within the in situ stress field are most likely to transmit fluids. Determination of the in situ stress field with knowledge of the failure envelope allows a more accurate assessment of fracture orientations at risk. Triaxial stress data can be combined with macroscale borehole breakout data to evaluate strikes of shear and tensile failure planes and critical failure angles for shear, assuming a normal fault stress condition. Structural analysis of cores reveals the extent and continuity of fracture connectivity throughout a cored interval. Structural densities are indicative of damage zone geometries and can be used to predict fluid transmissibility in fault zones. Microstructural analysis of fault gouge shows grain fracturing resulting in grain size reduction and decreased sorting relative to the undeformed reservoir, which is likely to inhibit fluid flow. Cataclastic faults may exhibit evidence of post-deformation quartz cementation which can result in increased threshold pressures relative to the undeformed sandstone. An example of the application of these integrated techniques from the Penola Trough is discussed. The example indicates that such an integrated study can provide a powerful analysis of seal risk in terms of both fault seal capacity, fracture orientation and fault zone fluid flow properties. This work is part of the Australian Petroleum Cooperative Research Centre's Program on "Hydrocarbon Sealing Potential of Faults and Caprocks".

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia