Fracture Characterization of Greater Ekofisk Area Chalks from Core Analysis

H. E. Farrell

Fracture characteristics are one of the input parameters for both waterflood design and subsidence analysis of the Ekofisk field. The fracture parameters of Maastrichtian age (Tor Formation) and Danian age (Ekofisk Formation) reservoir chalks have been determined from detailed description and analysis of 18 cores. Fracture pattern, mineralization, orientation, and intensity were all logged for the major groups of natural fractures. The fracture characteristics vary with chalk sedimentology. As a result, the fracturing in the Tor and Ekofisk Formation reservoirs shows some fundamental differences. The Tor Formation contains two fracture types. One set, stylolite-associated fractures, form an anastomosing network of nearly vertical extension fractures that extend between 5 nd 20 cm from individual stylolites. These fractures occur predominantly in thin debris-flow facies chalks that are typical of the Tor Formation. Pilot waterflood results indicate that this fracture set imparts a permeability anisotropy to the Tor Formation reservoir in the pilot waterflood area. The second fracture set affects both Tor and Ekofisk Formation rocks. These tectonic fractures are conjugate, through-going, and planar, and they dip between 60° and 70°. This group of fractures occurs predominantly in massive debris-flow facies chalks that are common in, but not restricted to, the Ekofisk Formation. Fracture orientation data indicate that a radial pattern of tectonic fractures may be present in this field. The field waterflood is being designed to accommodate this hig ly varied fracture pattern and analyses are under way to determine the possibility of fracture slip and closure during subsidence.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.