Fault facies and its application to sandstone reservoirs

The concept of fault facies is a novel approach to fault description adapted to three-dimensional reservoir modeling purposes. Faults are considered strained volumes of rock, defining a three-dimensional fault envelope in which host-rock structures and petrophysical properties are altered by tectonic deformation. The fault envelope consists of a varying number of discrete fault facies originating from the host rock and organized spatially according to strain distribution and displacement gradients. Fault facies are related to field data on dimensions, geometry, internal structure, petrophysical properties, and spatial distribution of fault elements, facilitating pattern recognition and statistical analysis for generic modeling purposes. Fault facies can be organized hierarchically and scale independent as architectural elements, facies associations, and individual facies. Adding volumetric fault-zone grids populated with fault facies to reservoir models allows realistic fault-zone structures and properties to be included. To show the strength of the fault-facies concept, we present analyses of 26 fault cores in sandstone reservoirs of western Sinai (Egypt). These faults all consist of discrete structures, membranes, and lenses. Measured core widths show a close correlation to fault displacement; however, no link to the distribution of fault facies exists. The fault cores are bound by slip surfaces on the hanging-wall side, in some cases paired with slip surfaces on the footwall side. The slip surfaces tend to be continuous and parallel to the fault core at the scale of the exposure. Membranes are continuous to semicontinuous, long and thin layers of fault rock, such as sand gouge, shale gouge, and breccia, with a length/thickness ratio that exceeds 100:1. Most observed lenses are four sided (Riedel classification of marginal structures) and show open to dense networks of internal structures, many of which have an extensional shear (R) orientation. The average lens long axis/short axis aspect ratio is about 9:1.

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