Shale Gas

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shale gas, natural gas obtained from sheetlike formations of shale, frequently at depths exceeding 1,500 metres (5,000 feet). Shales are fine-grained sedimentary rocks consisting of silt- and clay-sized particles that were laid down hundreds of millions of years ago as organic-rich mud at the bottom of ancient seas and tidal flats. Over time the mud layers were buried by further sedimentation, and the resulting heat and pressure transformed the mud into shale and the organic matter into natural gas. Over long stretches of geologic time, gas that was generated in the shales migrated into more-permeable rock layers, forming today’s so-called conventional reservoirs—gas deposits that are easily tapped through conventional drilling. However, much gas is still contained in the shale “source rocks.” The problem for producers is that it diffuses at an extremely slow rate and must be extracted through unconventional means. The most productive method is usually horizontal drilling through the shale seam, followed by hydraulic fracturing, or fracking, of the rock by the injecting of fluid at extremely high pressure.