The Upper Devonian Antrim Shale is an organic-rich marine shale that provides significant unconventional U.S. gas reserves. Gas production in June, 1994, reached 315 MMCFD from approximately 3,700 wells. The productive trend, which covers 700 square miles, occurs near the formation's northern subcrop in the Michigan Basin. The reservoir is a fractured, algal-rich, highly siliceous, pyritic shale with minor interparticle porosity. The productive trend dips basinward at a rate usually less than two degrees. Three primary factors contribute to the unconventional nature of the Antrim gas play;

• methane desorption from kerogen
• orthogonal regional fractures
• a portion of the gas is self-sourced from rock of low thermal maturity

The Antrim Shale represents a desorption- type reservoir, in which gas is stored primarily on the kerogen's internal surfaces through the process of adsorption. Gas desorbs from the kerogen as pressure is reduced. This is accomplished during production by pumping water from the fracture system to lower the reservoir pressure. Desorbed gas then migrates through the shale to the fracture system. Initial water production is dominated by flow from the fractures but it declines over time to moderate rates. Initial gas production rates rise with time as reservoir pressure falls and gas saturation builds in the fractures. Gas rates may peak in a few years, then slowly decline through the expected 20- to 30-year life of the wells.

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