TEXTURAL AND PERMEABILITY CHARACTERISTICS OF FAULTED, HIGH-POROSITY SANDSTONES

Faults in high porosity, Lower Permian sandstones from north-west Engl and and south-west Scotland occur as millimetre-wide bands of cataclas tic deformation, across which shear displacement is limited to a few m illimetres or less. These cataclastic slip bands occur either singly o r in sets and may be associated with ordinary slip surfaces with metre -scale shear displacements. Detailed petrography, including backscatte red scanning electron microscopy and cathodoluminescence imaging, show s the cataclastic slip bands to be characterized by decreases of as mu ch as four grain size classes and complementary changes in sorting, wi th respect to the host sandstones. Grain and overgrowth fracturing, to gether with minor cementation, are the main processes identified. Crys tal plastic deformation mechanisms are not important. Core analyses sh ow that textural changes correlate with decreases in permeability and porosity of up to four orders of magnitude across cataclastic slip ban ds. Although the cataclastically deformed zones themselves are the sit es of porosity loss, enhanced permeability and porosity are often deve loped adjacent to and on both sides of these zones. Dilation of the po re volume adjacent to the cataclastic slip bands accounts for these in creases. Removal of iron oxide grain coatings, together with minor cem entation, suggests that the fault zones may have been sites of enhance d fluid flow during faulting. The area over which cataclastic slip ban ds are developed is as much as 40 m wide for aggregate displacements o f no more than a few metres. Such zones may be seriously detrimental t o potential reservoir quality through local reductions in permeability and by partitioning of the sandstone body with respect to fluid movem ent. However, the small aggregate displacement means that cataclastic slip bands cannot be identified by seismic imaging. Further, although the fault orientations follow the basin-wide trends of larger faults, their precise locations are not so related, making prediction of their position difficult.