Organic geochemistry of petroleum seepages within the Jurassic Bencliff Grit, Osmington Mills, Dorset, UK

Occurrences of oil within the Bencliff Grit at Osmington Mills were studied through an integration of organic geochemistry and a consideration of the geological setting. Oil-stained sandstones dominate the cliff outcrop with localized regions of particularly concentrated oil impregnation. A second ‘live’ seep of oil occurs where the Bencliff Grit beds pass below high tide level at Bran Point. Organic geochemical analyses showed both oils to be at least moderately biodegraded, with the oils in the cliff outcrop showing enrichment in polar constituents compared with the active seep. Multivariate statistical analysis of the molecular composition identified an enrichment in diasterane biomarkers in the oils of the live seep; this difference is ascribed to source and/or maturity differences. The oil within the outcrop is considered to represent the residual staining of an unroofed oil field, whilst the live seepage at Bran Point represents a migration pathway towards the eroded anticline.