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AAPG Bulletin

Abstract

AAPG Bulletin, V. 93, No. 7 (July 2009), P. 891917.

Copyright copy2009. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/03230908116

Fault facies and its application to sandstone reservoirs

Alvar Braathen,1 Jan Tveranger,2 Haakon Fossen,3 Tore Skar,4 Nestor Cardozo,5 S. E. Semshaug,6 Eivind Bastesen,7 Einar Sverdrup8

1Center for Integrated Petroleum Research and Department of Earth Science, University of Bergen, 5020 Bergen, Norway; present address: University Center in Svalbard, Norway; [email protected]
2Center for Integrated Petroleum Research, University of Bergen, 5020 Bergen, Norway
3Center for Integrated Petroleum Research and Department of Earth Science, University of Bergen, Norway
4StatoilHydro ASA, Stavanger, Norway
5Department of Petroleum Engineering, University of Stavanger, 4036 Stavanger, Norway
6Center for Integrated Petroleum Research and Department of Earth Science, University of Bergen, 5020 Bergen, Norway
7Center for Integrated Petroleum Research and Department of Earth Science, University of Bergen, Norway
8Dana Petroleum Norway AS, P.O. Box 128, N-1325 Lysaker, Norway

ABSTRACT

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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