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Evolution of Shale Gas

Some 400 million years ago, thick shale deposits were forming as fine silt and clay particles at the bottom of bodies of water.

As plants and primitive animals died off, some of the methane gas created from the organic matter got buried with the sediments and seeped into sandy rock layers next to the shale rock, forming pockets of natural gas.

Some of that gas has remained locked in the shale layer, making it hard to bring up to the surface with traditional drilling. To access that gas, fracking has been used.

Photo: Daniel Acker | Bloomberg | Getty Images