An evaluation of the prospectivity of the North Gjallar Ridge, Norwegian Sea

Abstract

The North Gjallar Ridge is a structural high in the deepwater Cretaceous Vøring Basin in the Norwegian Sea. The high is characterized by the NE-SW striking fault complex defined at Base Tertiary level. The North Gjallar Ridge displays a number of structural closures, and three of these closures are identified as prospects and is the target of prospectivity evaluations in the presented Master Thesis.

Concerning the petroleum system, an effective source rock is not yet proven in the Vøring Basin. Both a Late Jurassic and a Late Cretaceous source rock have been theorized by researchers. Maturity and timing of hydrocarbon expulsion are associated with significant risk since a possible source rock in the basin will be deeply buried and the reservoir sections on the North Gjallar Ridge have experienced Late Cenomanian to Early Paleocene rifting. The reservoir sequences are of the Late Cretaceous Springar Formation and are documented by studies based on the interpretation of core samples and paleo-reconstructions. The sandstones are deposited in a sub-marine deepwater fan system sourced from East Greenland. The trap/seal combination is that of rotated fault blocks sealed by an intra Springar Formation shale layer.

Hydrocarbon accumulations on the North Gjallar Ridge will most likely be in gas phase. This assumption is based on the depth at which a possible source rock is buried in the Vøring Basinand adjacent gas discoveries. The identified prospects on the North Gjallar Ridge have been informally named; the Kliningen Prospect, Merraskaret Prospect and the Hårskallen Prospect. Recoverable resources have been calculated to 5.42 x 109 Sm3 of gas/2.62 x 109 Sm3 of gas/0.68 x 109 Sm3 of gas, respectively. The volumes are relatively modest, but could be of interest if nearby discoveries are developed, allowing prospects on the North Gjallar Ridge tobe tied back to existing installations.

The probabilities for technical discoveries for the Kliningen Prospect, Merraskaret Prospect and the Hårskallen Prospect are calculated to be 0.43/0.08/0.08, respectively. The high probability of discovery of the Kliningen Prospect is due to the assumption that an observed seismic flatspot in the prospect is regarded as an actual direct hydrocarbon indicator. Issues regarding retention and petroleum charge are reflected in the low probabilities of discovery of the Merraskaret and the Hårskallen prospects.