Synopsis

Hydrocarbon generation modelling in the Faroe–Shetland Basin (FSB) is enigmatic in that the depth of burial suggests that the main Middle and Upper Jurassic source rocks are overmature for hydrocarbon generation. However, the reservoired hydrocarbons comprise largely oil with only minor amounts of oil-associated gases, not the high maturity gas expected from the maturation modelling. The modelling also indicates that oil generation largely preceded both the deposition of the Paleocene reservoirs and the time of trap formation. This mismatch in timing has been reconciled by the use of holding tank models where the oil is stored in deeply buried Cretaceous reservoirs, before remigration after the formation of the Paleocene reservoirs and traps. The thermal histories used for this modelling are generally calibrated against geochemical parameters, such as vitrinite reflectance, but contain a number of flaws when the physical relationships between force, energy and work are evaluated. Foremost amongst the problems is the use of constant heat flows during periods of rifting, since it is impossible for a basin to be rifted under such conditions.

A thermal history that incorporates elevated heat flows during each period of rifting and volcanic activity has been derived from the tectonic history of the FSB, and was checked by calibration to measured vitrinite reflectance data in wells around the margins of the Foinaven Sub-basin. The vitrinite reflectance predictions were obtained by using the PresRo® model, in which the vitrinite reflectance is retarded by overpressures that developed during the rapid Paleocene burial in the FSB. The PresRo® model predicts late oil window maturities for the Jurassic source rocks in a pseudowell in the Foinaven Sub-basin kitchen. The results from the maturation modelling were then integrated with the fluid inclusion and hydrocarbon analyses from Westray area and Judd High wells. Three phases of hydrocarbon charging into the Foinaven Field can be identified. The first involves early and mid-mature oils that charged earlier Cretaceous sandstones during the Late Cretaceous, while the Paleocene reservoired hydrocarbons consist of two separate phases of oil charged during the Tertiary with an intermediate period of biodegradation.

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