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Weald Basin oil – is it shale oil or oil shale? and why it matters

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The UK Department of Energy and Climate Change (DECC) has published a study by the British Geological Survey of the Jurassic shales of the Weald Basin in southern England (Report). Unlike shale gas findings of a similar report published in 2013 for the Bowland-Hodder in northern England, the Weald Basin is said to contain oil:

“There is unlikely to be any shale gas potential, but there could be shale oil resources in the range of 2.2-8.5 billion barrels of oil (290-1100 million tonnes) in the ground, reflecting uncertainty until further drilling is done.”

A third study is also now said to be underway, and will apparently be completed in the summer of 2014. It will cover the Midland Valley of Scotland. Estimates will be made for both the oil and the gas “in-place” resources of the carboniferous strata of central Scotland.

Shale oil versus oil shale

The existence of shale oil (as opposed to gas) in the Weald Basin may come as less of a surprise given the existing conventional oil developments in the area (most notably the Wytch Farm oil field developed by BP). Nevertheless, extraction methods needed for shale oil (being oil produced from a shale reservoir), versus oil shale (being sedimentary rock from which Kerogen-based crude may be produced by heating), will be of interest to many. As the Report states:

“… oil shale is immature and can either be mined at or near the surface or retorted in situ at depth.”

It is understood that “retorting” involves heating oil shale in a process which causes oil and gas vapours to condense into a synthetic crude (and producing a solid coke-like residue). A heating source (generally gas or coal-fired) is needed for production. It is perhaps for this reason that the Report states that:

“Such oil shale extraction techniques make it very unlikely that it might be exploited at depth in the Weald Basin.”

Whilst this may suggest a surface or near-surface mining operation is necessary for extracting oil shale, if this is commercially and otherwise unviable, then it may be hoped that shale oil, is more abundant than currently estimated. The Report notes that:

“None of the Jurassic shales analysed …. has an ‘oil saturation index’ … of greater than 50 … [although] … it may be that some horizons within the Mid and Upper Lias, lower Oxford Clay and Kimmeridge Clay exceed the 100 required for the oil to be ‘producible’.”

In summary, unlike shale gas, where free and adsorbed gas may be extractable, with oil, only the free oil component is effectively producible. As such, the Weald Basin may only become economically attractive for unconventional oil production, if oil is found to be producible in commercial quantities as shale oil.

The Report’s findings are not therefore, perhaps likely to see the UK’s unconventional development focus shift from gas to oil just yet. Furthermore, given that most of the identified shale oil potential lies within existing licence areas, there may be limited opportunities for new entrants (unless existing licences are transferred or relinquished). It will of course be interesting to see the findings from the upcoming Scottish resource report, particularly given the significance to the Scottish independence debate. It is also interesting to note that in DECC’s publication “Underground Drilling Access” (published on the same day as the Report), that such Scottish resources are referred to as the “Oil-Shale Group” of central Scotland, which perhaps implies that the impending Scottish resource report may not flag significant quantities of hoped-for shale oil (as opposed to oil shale).

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