Early Tertiary heat flow along the UK Atlantic margin and adjacent areas

Abstract

The sedimentary basins of the UK North Atlantic margin are characterized by extensive igneous activity, mostly of Early Tertiary age, related to continental rifting which led to the separation of Europe from Greenland. Given this setting, it might be expected that elevated basal heat flow during the Early Tertiary would have exerted a critical control on the thermal histories of potential hydrocarbon source rocks. Apatite fission track analysis and vitrinite reflectance data have been used to identify, characterize and quantify significant palaeothermal events which have affected the region. This approach allows reconstruction of thermal histories based on directly measured data, rather than relying on theoretical models.Results from the West of Shetland region show significant Tertiary heating, but this is often characterized by low palaeogradients or non-linear palaeotemperature profiles, suggesting heating caused by fluid movements. No evidence has been observed of enhanced Early Tertiary heat flow, and little evidence has been found to support significantly deeper burial during the Early Tertiary. Results from Hebridean basins also argue against a significantly increased Early Tertiary heat flow, while showing strong evidence for Late Tertiary uplift and erosion.Consistent evidence for increased Early Tertiary heat flow is seen to the south, in the Lake District and Solway basins. This may reflect the ability of extensive igneous activity to release heat from the base of the crust to shallower levels throughout the Atlantic margin and the Tertiary igneous province, whereas in the absence of igneous activity, the lack of a pathway for heat to escape results in elevated basal heat flow.Models based on high Early Tertiary heat flow are likely to over-predict maturity levels in Jurassic source rocks, and predict an earlier timing for hydrocarbon generation. Both factors could lead to false assessment of regional hydrocarbon prospectivity.