The Jurassic Source Rock Potential of the Celtic Sea and Western Approaches

Abstract

A working petroleum system requires the presence of mature source rocks that have generated significant quantities of petroleum along with favourable timing of generation and expulsion with respect to the reservoir, trap and seal development. The Oil and Gas Authority (OGA) is currently promoting exploration in frontier basins extensional Mesozoic half graben and graben basins of the Western Approaches and Celtic Sea regions. Despite initial exploration efforts during the 1970s to 1990s, the source rock potential and likely timing of hydrocarbon generation and expulsion in the Celtic Sea and Western Approaches remain poorly understood. The Dragon Discovery, located between the North Celtic Sea and St George’s Channel Basins is the only discovery (gas with minor oil shows). Analysis of legacy geochemical datasets suggests that the Jurassic mudrocks contain the main intervals of potential source rock.

The Lower Jurassic strata in the Western Approaches and Celtic Sea Basins are shown to have highly variable source rock potential. Regionally, Sinemurian to Pliensbachian mudrocks form a zone of correlatable source rock potential with Total Organic Carbon commonly greater than 2% and Hydrogen Index values of greater than 200 mg/g. The main limiting factors are the general immaturity to early maturity of Jurassic strata, and the limited preservation of Jurassic sediments, especially in the Western Approaches. Late Jurassic to Early Cretaceous uplift and erosion caused the truncation of Jurassic strata in the Western Approaches and South Celtic Sea Basin, and a cessation in the increase of the thermal maturity of the Jurassic interval. This Late Jurassic to Early Cretaceous uplift and erosion event is suggested as the main control on the preservation and maturity (0.4-0.6% Ro) of Jurassic source rocks in the UK sector of the Celtic Sea and Western Approaches. The potential also exists for the preservation of more deeply buried sections of Jurassic source rock in localised graben at greater thermal maturity (>0.55% Ro).