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It has been documented that the degree of biological degradation of the petroleum in the Brent Group reservoir in the Gullfaks Field (quadrant 34/10, Norwegian North Sea) varies laterally. The most severe degradation has taken place in the western part of the field where he Brent Group sediments are not eroded. Lack of erosion in the West suggests that this part of the reservoir has always been buried at greater depths than the eastern part of the field. The geochemical description with the most severe biodegradation in the West, seems therefore to conflict with the most obvious geological model with easiest access for bacteria with meteoric water from the East. This suggests that we either do not understand the geology of the Gullfaks Field area or we do not understand the biological degradation process, or perhaps both. It is possible to give a reasonably good approximation of the amount of carbon that has been removed from the reservoired petroleum by bacterial degradation by comparing degraded and non-degraded petroleum with a fill-spill relationship in the area. By assuming that the supply of an oxygenating agent is the rate limiting factor in biological degradation of petroleum within a reservoir, it is possible to simplify the very complex processes and mechanisms involved in petroleum degradation, and to reduce the number of variables to three. These are: the thickness of the supply zone below the reservoir from which an oxygenating agent can be supplied, the flow of water within this supply zone below the oil water contact (o.w.c.) and the time available to degrade the petroleum in the reservoir. Geochemical and geological observations and modelling have demonstrated that it is unlikely that the degradation involves sulphate reducing bacteria but that degradation is probably aerobic bacterially mediated and a supply of oxygen ''rich'' water with nutrients or is related to a yet unidentified process. This mass balance based on the supply and demand of oxygen dissolved in meteoric water indicates that several thousand reservoir volumes are required to degrade the petroleum within the Brent Group to is present stage. This is difficult to explain with the current geological model of the field, however the biological degradation must be accounted for when establishing a realistic geological model in the Gullfaks area. The main advantage of our model is that it is quantitative and although some inaccuracies are inevitable it gives an order of magnitude estimate of the amount of water which must have been supplied into the reservoir.
Author(s): Horstad I, Larter SR, Mills N
Publication type: Article
Publication status: Published
Journal: Organic Geochemistry
Year: 1992
Volume: 19
Issue: 1-3
Pages: 107-117
Print publication date: 01/12/1992
ISSN (print): 0146-6380
ISSN (electronic): 1873-5290
Publisher: Pergamon
URL: http://dx.doi.org/10.1016/0146-6380(92)90030-2
DOI: 10.1016/0146-6380(92)90030-2
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