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Title: Deformation band development near meso-scale faults in porous sandstones : implications for fault seal prediction
Author: Skamvetsaki, Angela.
ISNI:       0000 0001 3416 4968
Awarding Body: Royal Holloway, University of London
Current Institution: Royal Holloway, University of London
Date of Award: 2002
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Seismic-scale faults are generally associated with clusters of subresolution faults, and the issue of how to predict the latter's numbers and distribution has been the subject of much recent debate due to its bearing on accurate fault seal evalution. One important class of subseismic-scale faults are deformation bands, which are tabular shear zones commonly formed in reservoir-quality aeolian and fluvial sandstone successions. Relevant outcrop and oil field case studies suggest such structures can reduce host rock permeability by up to four orders of magnitude, yet there is little published infonnation on the controls on their localisation and their relationship to larger-scale fault growth processes. This study addresses this knowledge gap and reports results of integrated structural, statistical, probe permeability and hydromechanical test investigations on fault-controlled deformation band arrays from two areas, Cullercoats Bay in NE England and the Clair Field, offshore NW UK continental shelf. Key aims are to examine the mesoand microscopic architecture of these band networks, establish their general evolution and elucidate their relationship to large fault development. Ancillary concerns include the expansion of the still limited database of deformation band spatial attributes, and assessment of the main areas of sensitivity in the analytical and statistical techniques used to describe these and other similar fault systems. Deformation bands at Cullercoats occur within the aeolian Yellow Sands of Permian age in the hanging wall of the Ninety Fathom fault, a major normal fault episodically active from the Carboniferous until at least Permian times. Structural analysis suggests that this band population was initiated as a result of dextral or oblique-dextral slip on the underlying Carboniferous Ninety Fathom fault, and was then progressively modified during the propagation of this fault into the overlying sediment cover and attendant development of fault-related folding and second-order faulting. In Clair, deformation bands are associated with arrays of calcite-filled veins and are inferred to have formed in response to fault-triggered fluid redistribution processes within the variably lithified aeolian-fluvial sandstones of the Clair Group. Statistical and mechanical evidence from both areas indicates that deformation band growth preceded major fault formation there, a finding that is consistent with the predictions of post-yield fracture mechanics models for process zone development at fault tips. A further common result from the two localities is that deformation band development and permeability character appears to be primarily controlled by the porosity and loading history of the faulted sandstones; therefore, assessment of the time of faulting should be a first step for determining whether a given subsurface fault is likely to be associated with deformation bands. Specific conclusions are: (1) The damage zones of band-related faults in porous sandstones scale linearly with fault displacement. (2) Deformation band densities decay quasi-exponentially with increasing distance from the faults within whose damage zones they occur. (3) Deformation band spacing distributions depart from strict self-similarity owing to the confinement of the bands within discrete mechanical horizons, yet their overall statistical character attests to multifractal scaling and Levytype stable behaviour. (4) Because of the observed deviations of deformation band statistics from simple power-law scaling laws, extrapolation of seismic-scale fault populations down to the deformation band level may give incorrect estimates of band numbers and/or size attributes. (5) Deformation bands display a broad range of microstructures and permeability signatures depending on host rock lithology, degree of compaction, previous stress history and local deformation details. (6) Despite their low-very low static permeabilities, deformation bands may act as fluid pathways during their development or reactivation in a subsequent tectonic event. (7) Application of standard statistical and probe permeability approaches to deformation band characterisation should be approached with caution due to problems inherent in the nature of deformation band systems itself. (8) Based on microstructural evidence and diagenetic and mechanical considerations deformation in the two study areas may have taken place at - 1.5-2 km, under maximum effective confining pressures of around 30 MPa.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Geology