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ABSTRACT: Facies Remolding in Allochthonous Chalk Packages, Ekofisk and Albuskjell Fields, North Sea

Susan Juch Lutz, A. A. Ekdale

The Ekofish and Albuskjell fields in the Central Graben of the North Sea produce hydrocarbons from resedimented chalk reservoirs. Although the allochthonous nature of chalk in these fields has been recognized, the correlations of, and association between, allochthonous units has not been described. Core analysis of the Tor Formation (Maastrichtian) and the Ekofisk Formation (Danian) reveals that slump deposits have been remolded into debris flows, ooze flows, and turbidites. Packages of allochthonous sediment were deposited in slope and base-of-slope environments.

Two kinds of allochthonous packages occur. One package, 1-3-m thick, consists of a basal debris flow overlain by an ooze flow. The other package, 10-20-m thick, contains three units: a basal debris flow, an intermediate slump, and an overlying turbidite. Deposition of each type of package probably resulted from a single triggering event.

Lateral changes in facies (increased convolution and decreased clastic content) and in type of deposit (slump or debris flow to ooze flow) within the packages resulted from differing degrees of deformation as the packages moved downslope. An increase in occurrence and angularity of chalk intraclasts, and in thickness of slump units from the Albuskjell field eastward to the Ekofisk field, suggest that the graben-bounding Hidra fault zone (about 30 km away) is the source of the allochthonous deposits. Vertical changes in the type of allochthonous package (from debris and ooze flows upward to slumps and turbidites) reflect decreasing topographic relief along the fault escarpment as the graben filled.

This model of vertical (basin shallowing) and lateral (downslope) facies changes allows correlation of allochthonous chalk units, which are excellent hydrocarbon reservoirs.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990