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1 Geological Institute, University of Bergen, Allégaten 41, N-5007 Bergen, Norway
2 Norsk Hydro Research Centre, Sandsliveien 90, N-5020 Bergen, Norway
3 ESSO Harge a.s., Grenseveien 6, N-4033 Forus, Norway
Spatial and size distribution data of fractures provide essential information in statistical bulk strain estimates and in predictive fracture scaling models for sediments which have undergone brittle deformation. Most fracture populations in naturally deformed sediments include structures which originated at different stages, and in response to different conditions during the deformation history of the rock. Hence, the final fracture population may contain fractures which genetically are related to gravitational (near-surface) instabilities, burial, uplift and unroofing, thermal expansion and contraction, and regional tectonism. Furthermore, grain size, bed thickness and different rheological properties may strongly influence the fracture style and frequency.
Particularly when only well cores are available, identification of fractures of different genetic origin is difficult, and fracture classification and predictive modelling can only be accomplished with data acquired from careful fracture logging. In such logging, information on position of the well relative to major structures, the geometry of nearby faults, and the general geological (sedimentary and tectonic) environment need to be included. Also, fracture frequencies should be compared to, and normalized according to, lithology and bed thickness, and the fracture frequency diagrams should be corrected for eventual well deviation before predictive fracture frequency modelling is performed.
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