Petroleum seepages and impregnations in Great Britain

Abstract

This paper documents 173 occurrences of surface petroleum seepages and impregnations in Great Britain. A brief description is given of the seven main areas where they are found. The descriptions range from pore fillings of bitumen to steady flows of oil and gas. The principal areas in which seepages are found are as follows: the Orcadian basin of north-east Scotland, the Midland Valley of Scotland, the flanks of the Pennines, the Welsh Borderlands, Cornwall, the Dorset coast of the Wessex basin and the south-eastern Weald. Stratigraphically the seepages range from Caledonide basement to Holocene peat. The principal sources for the petroleum are Devonian oil shales (Orcadian basin), Carboniferous shales and coals (Midland Valley, Pennines, Welsh Borderlands, Devon and Cornwall) and the Liassic (Wessex and Wealden basins). Several anomalous seepages are recorded from Welsh and Scottish ‘basement’. There are three clearly defined styles in which the seepages occur. Many are found where permeable carrier beds unconformably overlie impermeable ‘basement’ around the margins of basins (Orcadian basin, Midlands Carboniferous basin); others are fault related, especially in the Wessex and Wealden basins. The seepages in the Cornish tin mines are best explained as sourced from the Culm series and drawn by the convective flow of connate waters into the thermal chimneys of the granite intrusions.