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Physical character and fluid-flow properties of sandstone-derived fault zones

Abstract Evaluation of hydrocarbon entrapment and production patterns in faulted sandstone reservoirs requires understanding of the nature and fluid-flow properties of sandstone-derived fault zones. This study documents the interrelationships between sandstone composition, deformation mechanisms, fault-zone character, and fluid-flow properties (permeability and capillary properties) using a global sandstone dataset. Quartz-rich sandstones deform by cataclasis (most commonly), diffusive mass transfer, or a combination of these processes to form deformation bands. The fluid-flow properties of these zones depend on deformation mechanism(s). Faulting of mineralogically immature sandstones results in the formation of clay-matrix gouge zones by a combination of processes, including cataclasis, intergranular sliding in clay-rich materials, and diffusive mass transfer. Clay-matrix gouge zones generally have lower permeabilities and higher capillary displacement pressures than deformation bands. Most deformation bands have capillary properties sufficient to maintain hydrocarbon column-height differences of less than 75m across them, whereas clay-matrix gouge zones can potentially seal hydrocarbon columns with heights of several hundred metres. Both low-permeability deformation bands and clay-matrix gouge zones are likely to influence production patterns, although the magnitude of these effects will depend on the spatial distribution and abundance of faults and the permeabilities of the fault zones and undeformed sandstone.