Abstract

Although the majority of the oil reserves contained in the Ninian field are being produced from tilted Middle Jurassic sandstones of the Brent Group, significant volumes of oil have also been discovered within a poorly documented, subtle structural trap that lies in the immediate footwall to the field's controlling, eastern boundary fault. Integration of well data and the interpretation of seismic surveys allow the definition of an area characterized by major footwall degradation. The data demonstrate that the 1.25-mi (2-km) wide area affected lies to the east of the zone of Brent Group truncation, along the whole 15-mi (25-km) length of the field. A combination of sedimentary and stratigraphic information shows that the degradation and products obtain a thickness that locally exceeds 450 ft (126 m). The degradation process appears to have been triggered by accelerated extension during the Callovian-early Oxfordian deposition of the Heather Formation. The development of pronounced fault terraces due to footwall collapse caused the locus of degradation to migrate westward until at least the middle Kimmeridgian. Although the present seismic data do not generally permit accurate internal resolution of the degradation complex, it is locally of sufficient quality to demonstrate the existence of fault-bound blocks above a pronounced terraced, low-angle, decollement surface with ramp-flat geometries. Core descriptions and biostratigraphic analyses suggest that the complex consists of a series of semiautochthonous to fully allochthonous Brent Group rocks now in fault-bounded, rotated blocks and stacked submarine slides. These analyses also demonstrate an important downdip decrease in the sand content and reservoir quality. Comparisons with other oil fields in the northern North Sea (e.g., the Brent and Statfjord fields) suggest that the Ninian field is not unique in having oil reserves within a structurally complicated footwall degradation complex. As production from these fields declines, a fuller understanding of their footwall degradation complexes is likely to be very important in enhancing ultimate oil recovery.

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