ノルウェー領北部北海 Vigdis 油田

落差の小さな断層によるシーリングについて

抄録

The Vigdis field is located approximately 200km north-west of Bergen in Norway. The water depth ranges from 230m in the south to 300m in the north.
The hydrocarbon is accumulated in a westerly tilted fault block in the Tampen Spur area, where numerous fields also have been discovered.
The main reservoir rock in the field is the Middle Jurassic Brent Group deltaic sandstone. The Brent delta prograded from south to north with its thickness of around 200m in the field. The Brent Group is traditionally subdivided into five formations in this area from the youngest to oldest: Broom, Rannoch, Etive, Ness and Tarbert. Although each formation basically has a high sand/shale ratio, the lowest sand/shale ratio or relatively high shale content is observed in the Ness Formation which behave very important role in the fault seal in this field.
The field, consisting of three structural elements, called Vigdis West, Vigdis Middle and Vigdis East is bounded to the east by a truncation of the Base Cretaceous Unconformity, to the north and west by OWC and bounded to the south by the E-W trending cross fault. The displacement of E-W trending cross faults is so small that the Brent Group is juxtaposed each other across the faults. Thus the faults might be considered to be non-sealing, which was a main exploration risk for Vigdis Middle.
After drilling of exploration/delineation wells, the fault plane analysis along each boundary fault revealed that the Ness Formation, richest in shale content enhanced the sealing capacity of fault. The OWC in the field vary among the structural elements. It becomes shallower from Vigdis West to Vigdis Middle. The relative positions of the Ness Formation and other sandstone rich formations along the faults control the OWC discrepancies.