Abstract

North Sea chalk fields have moderate to considerable contribution to production from natural fractures. In the Albuskjell Field, gas/condensate hydrocarbons and contained in Upper Cretaceous (Maastrichtian) and lower Tertiary (Danian) chalks. The field is a large halokinetically induced dome on the northern limits of the productive chalk region known as the Greater Ekofisk area. Two types of fractures. The first appear to be early, are commonly compacted, predominantly healed, and resemble conjugate shear fractures. The second type is related mainly to the tips of stylolites; these are vertical, preferentially open, and are interpreted as tension fractures. Tension fractures form when the minimum effective stress is reduced to the tensile strength of the chalk, as a result of increased pore-fluid pressure and/or decreased total confining stress due to relative extension. High pore-fluid pressures alone could not have formed the tension fractures in Albuskjell. Seismic sections show that halokinesis occured throughout chalk deposition and continued, in pulses, until early Miocene time. Incremental stress values, associated with halokinesis, predict optimum conditions for tension fracture formation at 4,500-ft (1,370-m) burial, owing to a combination of halokinetic doming and overpressuring, with an important contribution from source rock maturation. Post-hydrocarbon emplacement ( approximately 3,000-ft [1,000-m]) burial identical with mid-Oligocene), preserved by hydrocarbon invasion. Healed, shear fractures formed prior to significant pressure solution and hydrocarbon emplacement and are thought to be associated with extra-dome processes, graben tectonics or halokinetic reactivation of earlier northwest-southwest-trending major faults. Distribution is expected to be fairly uniform over the field, with broadly northwest-southeast orientations. Shear-fracture density is moderately constant between the Danian and Maastrichtian chalk sequences, but is significantly reduced, to absent, in the argillaceous base-Danian. Open tension fractures are preferentially absent from tight (early-cemented or argillaceous) chalks. They are also poorly developed in "soft" lithologies and predominantly occur in chalks with moderate to high porosities with stylolites. The spacing of open tension fractures is assumed to be constant over the field, whereas fracture width decreases from crest to flank; this is in line with the concept of lower horizontal stresses in the structural crest. Although significant halokinesis ceased during the early Miocene, residual strain may still exist in Albuskjell, with larger horizontal effective stresses occuring in the flanks of the structure. Tension fractures are probably open under initial field conditions to a potential depth of 14,500 ft (3,500m).--Modified journal abstract.

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