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1 Department of Geology and Petroleum Geology, Meston Building, Kings College, University of Aberdeen, Aberdeen AB9 2UE, UK
3 Nederlandse Aardolie Maatschappij B.V., Postbus 28000, 9400 HH Assen, The Netherlands
2 Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU, UK
The Tertiary reservoirs of the Forth and Balmoral Fields are sand-rich turbidites, which were deposited in a deep marine setting in the vicinity of the Fladen Ground Spur and East Shetland Platform. The Forth Field reservoir contains biodegraded oil and consists of Palaeocene and Eocene unconsolidated to semi-consolidated, well sorted, medium- to fine-grained sub-arkoses at a maximum present burial depth of 1.8 km. Pervasive ferroan and non-ferroan calcite cements dominate the Forth paragenetic sequence. Bitumen-filled inclusions within these cements indicate that oil emplacement and carbonate cementation occurred simultaneously, probably within the first 1000 m of burial. Carbonate
The Balmoral Field is located c. 100 km to the south of Forth. The reservoir consists of a series of poorly sorted subfeldspathic arenites with infrequent spherical carbonate concretions. These non-ferroan and ferroan calcitic concretions preferentially precipitated around localized accumulations of organic matter at < 500 m burial depth. Pore fluids were meteoric, and carbonate was provided by both bacterial oxidation and sulphate reduction of organic matter. Meteoric water is thought to have been derived from the East Shetland Platform to the north of Balmoral. Oil migrated into Balmoral during the Oligocene, post-dating meteoric flooding.
Laterally extensive carbonate-cemented sand horizons, formed in association with oil biodegradation, have the potential to compartmentalize a reservoir. The distribution of these cements within Tertiary deep water sandstone reservoirs adjacent to the East Shetland Platform is likely to be controlled by the relative timing of meteoric flushing and oil migration.
18O data suggest that meteoric water from the East Shetland Platform flushed the reservoir, biodegraded the migrated oil and displaced the original sea water. Biodegradation of oil took place at the palaeo-oil-water contact, producing a complex sequence of laterally extensive non-ferroan and ferroan calcite concretions. Frequent oil leakage during shallow burial may have produced a large number of concretionary cement layers. The cemented layers have the potential to act as major barriers to vertical fluid movement and to significantly reduce the net to gross ratio of the reservoir. Pore-filling ankerite post-dates non-ferroan and ferroan calcite. Ankerite precipitation marks the final stages of bacterial influence within the sediment column.
