He says the Bakken has a layer of nearly impermeable shale that sits above the oil-bearing reservoirs.

FORBES: Focus

The entire city of Fort Worth sits over a gas dense layer of black rock called the Barnett Shale that only recently became possible to access.

NPR: Drilling for Gas in Dallas-Fort Worth

The problem was that (gas) was locked in shale, the extremely dense layer of rock under Dallas and Forth Worth.

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