Iain C. Scotchman¹ and Andrew D. Carr²

¹Statoil (UK) Ltd., 11a, Regent Street, London, SW1Y 4ST , UK
²Advanced Geochemical Systems Ltd., 1 Towles Fields, Burton on the Wolds, Leicestershire, LE12 5TD, UK

ABSTRACT: Multiple Hydrocarbon Charges Generated by Cretaceous – Tertiary Rifting and Overpressure Development in the UK Faroe-Shetland Basin

The petroleum system in the UK Faroe-Shetland Basin (FSB) is enigmatic. Hydrocarbons discovered to date are predominantly mixed fresh and biodegraded heavy oils with minor amounts of associated gas, reservoired in the Mesozoic and Tertiary (Scotchman et al., 1998). The main Jurassic source rocks occur in kitchens at depths below 6000m and basin modelling, based on present-day heatflow and a kinetic vitrinite reflectance maturation model (e.g. Easy%Ro of Sweeney and Burnham, 1990), predicts these source rocks to be either gas-mature or overmature (Holmes et al., 1999; Jowitt et al., 1999). Attempts to resolve this conundrum have resulted in ‘whoopie-cushion’ (Iliffe et al., 1999) or holding tank models (Lamers and Carmichael, 1999), whereby oils were generated in the late Cretaceous and stored at depth before release at intervals to produce both degraded and fresh oil charges to Tertiary reservoirs.

The Mesozoic sediments in the FSB are frequently overpressured and, using an overpressuredependent vitrinite reflectance basin model (Carr, 1999), a multiple rifting heat flow history was developed based on both the tectonic and the volcanic history of the basin. Predictions obtained from this model shows that oils were generated from about 100 Ma until the development of overpressure prevented further maturation. Inversion associated with the base Tertiary unconformity reduced overpressure and enabled further hydrocarbon generation to occur, allowing charging of shallow Tertiary structures. Biodegradation of these oils was followed by further charging with more mature, fresh oils. Rapid subsidence during the Paleogene resulted in increased overpressure, thereby preventing generation of large volumes of gas. Multiple charges of successively more mature oils were therefore generated, controlled by the tectonic history of the source kitchen. The model therefore removes the need for holding tanks to account for the multiple charging of oils and the absence of gas.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado