Source rock and seep oil maturity in Dorset, Southern England

Abstract

Analysis of the steranes and triterpanes in the oil seeps of the Dorset coast of Southern England (Mupe Bay, Osmington Mills) shows both the oils to derive from a similar mid-late mature source rock. Measured maturity of the Jurassic source rocks at outcrop are uniformally immature (0.3–0.5% R) despite a history of Mesozoic burial and Tertiary inversion and uplift. Evidence from oil cemented sandstone clasts within oil stained Lower Cretaceous fluviatile channel sands at Mupe Bay, suggests that oil was already seeping during the Lower Cretaceous. Detailed work comparing the extracts of the oil stained clasts and the surrounding oil stained sandstones shows the clasts to contain oil derived from a slightly, but significantly, less mature source rock. This is consistent with lower maturity oil seeping first into the sand that was then incorporated as rip-up or bank collapse clasts into the channel sand. At a later stage, with further source rock burial, more mature oil entered the matrix sandstone. Locally calibrated time/temperature thermal geohistory modelling can just simulate source rock maturities appropriate to the seep oils if paleo-geothermal gradients and surface temperatures were locally higher in the Jurassic than at the present day. A volumetric balance between oil in place in the Wytch Farm field and the available area of source rock draining into the structure suggests that an anomalously thick and/or organic rich Rhaeto-Liassic section must have developed in the Bournemouth Bay basin, and that Cretaceous palaeo-geothermal gradients were higher in this specific area.