Up to 1.143bn.t of shale oil could exist within the Weald Basin in Kent, Surrey and Sussex according to a new report undertaken by the British Geological Survey (BGS) for the Department of Energy and Climate Change (DECC).
However due to geological uncertainty, the BGS has warned that the volume could be as low as 273M.t and that it is not known what percentage of the oil present could be extracted commercially. The central estimate for the resource is 591M.t.
The BGS study looked at five units within the Jurassic deposits of the Weld Basin, which contain organic-rich marine shales. The units covered by the report are the Mid and Upper Lias Clays (Lower Jurassic) and the Oxford Clay, Corallian Clay and Kimmeridge Clay (Upper Jurassic).
The report said: “Conventional oil and gas fields in the basin attest to the capability of some of these units to produce hydrocarbons. It is possible that oil could have been generated from any or all of the five shales, but in the current model even the deepest Jurassic unit is not considered to have been sufficiently deeply buried to have generated significant amounts of gas. Some gas has been generated in association with oil and shallow biogenic gas may also be present.
“Organic-rich shales occur at two levels in the Lias (Lower Jurassic) of the Weald; these have direct equivalents in the Paris Basin, although in the Weald they fail to reach the richness found in France. In a third Lias unit, the Blue Lias (Lower Lias), total organic carbon (TOC) reaches 8% further west in shales in the Wessex Basin, where it sources the Wytch Farm oilfield, but organic carbon contents are typically well below 2% in the equivalent limestones and shales of the study area. This contrast in organic content may result from differences in palaeogeography and organic input or preservation between the basins. The most significant organic-rich shales in the Weald Basin occur in the lowermost Oxford Clay (TOC up to 7.8%) and middle Kimmeridge Clay (TOC up to 21.3%) and these represent potential ‘sweet-spots’ worthy of further investigation.
“None of the Jurassic shales analysed by Rock-Eval methodology in the Weald Basin has an ‘oil saturation index’ (S1*100/TOC) of greater than 50, so much of the ‘oil’ may be physically associated with kerogen, rather than present in pore space.”
According to the BGS, the study has shown that there is potential for shale oil to exist within the Weald Basin but extraction would be subject to securing licences. Given the environmental sensitivity of the area, the BGS has suggested that shale oil exploration and potential development should progress cautiously to ensure the activity is safe and the environment is properly protected.
The BGS is also working on a study of the Carboniferous shales of the Midland Valley of Scotland, which is expected to be published later this year.