GEOCHEMICAL CHARACTERISTICS OF THE SKRUGARD OIL DISCOVERY, BARENTS SEA, ARCTIC NORWAY: A “PALAEO‐BIODEGRADED – GAS REACTIVATED” HYDROCARBON ACCUMULATION

• Published in: Journal of Petroleum Geology
• Main Authors: Matapour, Z., Karlsen, D.A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2017
• Subjects: Earth and Planetary Sciences (miscellaneous); Geology; Energy Engineering and Power Technology; Fuel Technology; Arctic; Alta; Barents Sea; Norway; Stø; Hekkingen; Nordmela; Loppa; Johan Castberg; Snøhvit
• Online Access: http://dx.doi.org/10.1111/jpg.12669; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fjpg.12669; https://onlinelibrary.wiley.com/doi/pdf/10.1111/jpg.12669

This paper investigates the filling history of the Skrugard and Havis structures of the Johan Castberg field in the Polheim Sub‐Platform and Bjørnøyrenna Fault Complex, Barents Sea (Arctic Norway). Oil and gas occurs in the Early Jurassic and Middle Jurassic Nordmela and Stø Formations at Johan Castberg, and both free oil and bitumen are interpreted to be sourced from the Upper Jurassic Hekkingen Formation (Kimmeridge Formation equivalent). The geochemical characteristics of the petroleum from Skrugard and Havis, including the GOR, API and facies and maturity signatures, can be understood within a complex fill history which includes a palaeo oil charge, Tertiary uplift (>2 km), dismigration, in‐reservoir biodegradation, and late‐stage refill with gas. The API and GOR of the Skrugard oil are 31° and 60m 3 /m 3 , respectively. The petroleum is geochemically similar to that in the nearby Havis structure, to that in the Snøhvit region to the south of the Loppa High, and also to the petroleum recorded as traces in well 7219/9‐1, approximately 16 km SW of Johan Castberg field. However, the petroleum differs from the oil in the Alta well 7120/2‐1, located in the southern part of the Loppa High, illustrating the complexity of the regional petroleum systems. The Skrugard oil is of medium maturity (ca. 0.8–0.9% R c ), and is significantly biodegraded despite being gas‐saturated. Evidence for biodegradation includes the reduced concentrations of C 10 ‐C 25 n‐alkanes and the presence of a prominent unresolved complex mixture (UCM) in gas chromatogram traces. However non‐biodegraded C 4 ‐C 8 range hydrocarbons are also present in the reservoir. This suggests a recent charge of gas/condensate into the structure which therefore contains a mixture of palaeo‐degraded and unaltered petroleum. Oil‐type inclusions within authigenic quartz and feldspar from reservoir sandstones at Skrugard were analysed. The results indicate that the structure (present‐day depth 1276–1395m) underwent Tertiary uplift by ca. 2–3km following an ...