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Fault Healing and Fault Sealing in Impure Sandstones
David N. Dewhurst,1 Peter J. Boult,2 Richard M. Jones,3 Stuart A. Barclay4
1Commonwealth Scientific and Industrial Research Organization, Division of Petroleum Resources, Australian Petroleum Cooperative Research Center, Kensington, Western Australia, Australia
2Australian School of Petroleum, University of Adelaide, Australia and also Department of Primary Industries and Resources South Australia, Adelaide, Australia
3Woodside Energy Ltd., Perth, Western Australia, Australia
4Commonwealth Scientific and Industrial Research Organization, Division of Petroleum Resources, North Ryde, New South Wales, Australia
This work was performed as part of the Australian Petroleum Cooperative Research Center Research Programme on Hydrocarbon Sealing Potential of Faults and Cap Rocks. Samples were provided by Origin Energy, Primary Industries and Resources of South Australia, and Woodside Energy Limited. Sponsors of this program (Woodside Energy Limited, Origin Energy, OMV, Japan National Oil Corporation, Globex (now Marathon), Santos, Exxon/Mobil, BHP-Billiton, Chevron-Texaco, Conoco-Phillips, Anadarko, and Statoil) are thanked for permission to publish. Helpful discussions with Bronwyn Camac, Dave Cliff, Andrew Davids, Quentin Fisher, Richard Hillis, Suzanne Hunt, and Scott Mildren are duly acknowledged. We also thank the three reviewers, Hege Nordgard Bolas, Quentin Fisher, and Alton Brown, for their clear, insightful, and constructive comments, which significantly enhanced this chapter.
Clay content is a first-order control on the mechanical and fluid-flow properties of fault rocks. The effects of deformation and also diagenesis are modified by the presence of clay in impure sandstones, although our understanding of the results of such changes is not well constrained. Because a lack of data for fault rocks in impure sandstones limits our ability to assess fault seal risk, a study was undertaken to investigate the effects of physical and diagenetic processes on these parameters in the Otway Basin on the southern margin of Australia. Fault rocks formed in impure reservoir sandstones from the eastern and western Otway Basin exhibit distinct geomechanical and capillary properties caused by differing clay content and distribution, overprinted by regional differences in diagenesis and geohistory. In the eastern Otway Basin, grain mixing and shear-induced clay compaction have increased fault capillary threshold pressures relative to host reservoir strata. These processes have led to a greater proportion of rigid framework grain contact, generating increased fault friction coefficients relative to the host reservoir rocks. Fault strands tend to form dense clusters as a result of strain hardening and preferential localization of new faults in weaker reservoir sandstone.
Mechanical and diagenetic processes in fault rocks in impure sandstones from the western Otway Basin have significantly altered physical and geomechanical properties as a result of increased quartz dissolution and precipitation aided by lower clay contents. Here, faults exhibit increased friction coefficient and capillary threshold pressures because of more efficient grain packing, suturing of quartz grains, and fracture healing likely resulting from local diffusive mass-transfer processes.
Phyllosilicate framework fault rocks from both regions appear significantly stronger than their host reservoirs as a direct result of syn- and postdeformational physical and diagenetic processes. These findings have direct implications for understanding the micromechanics of deformation in impure sandstones, for physical property evolution during and postfaulting, and for geomechanical prediction of fault reactivation. In a regional context, the regeneration of fault strength influences stress distribution in regional top seals through localized rotation of stress trajectories and increased differential stress, which has resulted in fracturing and loss of hydrocarbons.
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