Cores from the Upper Jurassic reservoirs of the Witch Ground Graben area display an array of fault rock types, primarily reflecting the clay content of the host facies. Clean reservoir sands contain deformation bands whilst more impure reservoir facies are typified by PFFR faults and smears within heterolithics and intra-reservoir shales. Fault zone examples show development at different burial depths, which impacts the degree of grain fracturing, cementation and stylolitisation of pre-existing faults and generates new faults with characteristics that differ from those formed prior to significant lithification. However, subtle variations in timing of faulting and access of hydrocarbons and diagenetic brines can lead to early shallow burial cataclasites and intra-reservoir fault zones clogged with heavy oils. In general, the fault zones present in core mirror the large-scale tectonic development of the area with the majority of structures being early, pre-lithification to shallow burial, extensional faulting and related to late Jurassic rifting. Modification of these faults during deep burial and later reactivation ties in with the more limited and focused Cretaceous-Tertiary activity in the area, when hydrocarbon migration occurred: this is seen as an interplay of fault activity, complex quartz-carbonate-exotic cementation and oil staining in fault zones within Upper Jurassic cores from the area. An important observation from the cored fault zones is that reactivation was often strike- or oblique-slip in nature and that in some instances breaching (dilatancy) occurred, whilst in others, such as late, cemented zones, re-sealing was achieved. The observations of key cored fault zones has been fundamental in developing a prospect risking and fault seal evaluation toolbox for the Witch Ground Graben, which may also be applied in other exploration and production areas.


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