The diagenesis of tertiary sands from the Forth and Balmoral fields, Northern North Sea
The Palaeocene and Eocene Forth Field is located in Quad. 9, Block 23/b, adjacent to the East Shetland Platform. The Fourth reservoir consists of a series of massive well sorted, medium to fine grained, turbidite sands which contain biodegraded oil and gas. The textural homogeneity of the Forth sands suggests that sedimentary facies was not a major diagenetic control. The timing of oil migration and the periodicity of oil leakage controlled the relative paragenesis in different sand units. Pervasive ferroan and non ferroan calcite cemented sand horizons dominate the Forth paragenetic sequence. Bitumen filled inclusions within these cements indicate oil emplacement and carbonate cementation occurred simultaneously. Calcite oxygen isotope results suggest East Shetland Platform meteoric water, flushed the reservoir, biodegrading the migrated oil and displacing the original seawater. Biodegradation of oil took place at the palaeo-oil water contact, producing a laterally extensive cementation zone. Frequent oil leakage may have produced a series of different palaeo-oil water contacts which became preferential cementation sites. The Palaeocene Balmoral Field is located approximately 100kms to the south of Forth in Quad. 16, Block 21. To a large extent, the distribution of non-carbonate diagenetic phases in the Balmoral Field is controlled by lithoclast composition and the relative abundance of interbedded shales. Non ferroan and ferroan calcite concretions preferentially precipitated where there were localised accumulations of organic matter. The concretions precipitated at < 500m burial depth, sourced by bacterial oxidation and sulphate reduction of organic matter in meteoric pore fluids. 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 flushing. Laterally extensive carbonate cements, formed in association with oil biodegradation, have the potential to compartmentalise a reservoir. The distribution of these cements within Tertiary reservoirs adjacent to the East Shetland Platform is likely to be controlled by the relative timing of meteoric flushing and oil migration.