Textural and permeability characteristics of faulted, high porosity sandstones

Abstract

Faults in high porosity, Lower Permian sandstones from north-west England and south-west Scotland occur as millimetre-wide bands of cataclastic deformation, across which shear displacement is limited to a few millimetres or less. These cataclastic slip bands occur either singly or in sets and may be associated with ordinary slip surfaces with metre-scale shear displacements. Detailed petrography, including backscattered scanning electron microscopy and cathodoluminescence imaging, shows the cataclastic slip bands to be characterized by decreases of as much as four grain size classes and complementary changes in sorting, with respect to the host sandstones. Grain and overgrowth fracturing, together with minor cementation, are the main processes identified. Crytal plastic deformation mechanisms are not important. Core analyses show that textural changes correlate with decreases in permeability and porosity of up to four orders of magnitude across cataclastic slip bands. Although the cataclastically deformed zones themselves are the sites of porosity loss, enhanced permeability and porosity are often developed adjacent to and on both sides of these zones. Dilation of the pore volume adjacent to the cataclastic slip bands accounts for these increases. Removal of iron oxide grain coatings, together with minor cementation, suggests that the fault zones may have been sites of enhanced fluid flow during faulting. The area over which cataclastic slip bands are developed is as much as 40 m wide for aggregate displacements of no more than a few metres. Such zones may be seriously detrimental to potential reservoir quality through local reductions in permeability and by partitioning of the sandstone body with respect to fluid movement. 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.