The threat that you refer to (National parks land faces new oil threat, campaigners warn, 16 March) actually looms over a great swathe of south-east England, not just national parks. And the plethora of promised wells will not be “conventional” as your article states – at least not in the scientifically accepted meaning of the word. A new, political definition of “conventional” was inserted into national minerals planning guidance in March 2014 by the then Department of Energy and Climate Change. It declared “conventional” all sources of oil and gas in limestone and sandstone. This is not true. Both limestone and sandstone, geologically speaking, can be conventional or unconventional. The scientific divide between the two pivots on permeability – how freely oil or gas can flow through the rocks. And, deep within the shale under the Weald, the thin, muddy limestone layers that are currently the target of oil companies have low permeability. They are unconventional.

It is convenient for the oil industry to be able to claim its drilling to be conventional. To the public, media and planners it makes oil wells seem a more minor issue. But the industry’s plans are major. Precisely because of the low permeability of the target rocks (now muddy limestone, soon no doubt the surrounding shale), there will be a need for a great many wells. You can extract oil only by getting up close to each bit of “unconventional” rock, and dissolving it with acid or cracking it open. Stephen Sanderson, CEO of UK Oil and Gas, said of his plans for Surrey and Sussex: “This type of oil deposit very much depends on being able to drill your wells almost back to back.”

In the north and the east Midlands, they call a frack a frack, in both scientific and political-speak. For the moment, here in the south-east, the presence of the limestone “impurities” within the shale, along with the devious government definition of “conventional”, allow oil companies to avoid the F-word while getting their drills into the ground across our land.

Kathryn McWhirter

Balcombe, West Sussex

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