Fault Healing and Fault Sealing in Impure Sandstones
David N. Dewhurst, Peter J. Boult, Richard M. Jones, Stuart A. Barclay, 2005. "Fault Healing and Fault Sealing in Impure Sandstones", Evaluating Fault and Cap Rock Seals, Peter Boult, John Kaldi
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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 post deformational 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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This volume constitutes the proceedings of the AAPG Hedberg conference on seals held in Barossa Valley, South Australia, in 2002. The key driver for both the Hedberg conference and this publication was the recognition that knowledge of risk in the estimation of sealing capacity and fault-seal potential is important in making judgments at the exploration, appraisal, and development stages of the petroleum business. In addition, incorporating seal risk in the overall assessment of hydrocarbons in place can affect decisions to drill prospects and the location of appraisal and development wells, as well as reserve estimation. Improved methods to estimate seal capacity and fault integrity can lead to savings in well costs, improved recoveries through optimum placement of wells, and improved estimates of hydrocarbon in place. This volume contains 18 chapters that reflect the spectrum of presentations at the conference. The knowledge imparted by these chapters will be a window on the state of seal knowledge at this juncture of time and includes topics such as seal failure related to basin-scale processes, the role of geomechanics in seals, and the economic evaluation of prospects with a top seal risk.