Z. Matapour*+ and D. A. Karlsen*

* Department of Geosciences, University of Oslo, PO Box 1047, 0310 Oslo, Norway.

+ corresponding author:

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 60m3/m3, 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%Rc), and is significantly biodegraded despite being gas-saturated. Evidence for biodegradation includes the reduced concentrations of C10-C25 n-alkanes and the presence of a prominent unresolved complex mixture (UCM) in gas chromatogram traces. However non-biodegraded C4-C8 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 earlier phase of oil emplacement. The presence of the oil type inclusions, both in the current gas zone (Stø Formation) and in the oil zone (Stø and Nordmela Formations), indicates that the positions of the oil-water and gas-oil contacts have changed over time. This is consistent with a recent gas charge to the upper part of the reservoir, and also with the gas being at dew point. These observations are supported by analyses of core extracts which show an increasing bitumen content towards the OWC, and the oil-type bitumen in the present-day gas zone.

A charge history model for the Skrugard structure is proposed which integrates both the observations concerning the petroleum inclusions and the biodegraded oil together with observations of seismically-monitored gas fluxes along the rim of the Loppa High. Improved understanding of the Skrugard structure and its filling history will assist exploration in similar settings in other parts of the Barents Sea and worldwide, particularly where multiple source rocks and a multi-stage charge history have controlled reservoir filling.

Key words: fluid inclusions, microthermometry, biodegradation, uplift, Norway, Johan Castberg, Barents Sea, oil and gas, petroleum system, Skrugard, GC-MS, fill model.

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