Sedimentary rocks deposited during the Upper Cretaceous Eagle-Telegraph Creek regression provide an excellent opportunity to examine shallow biogenic gas reservoirs in the northern Great Plains.

In central Montana, coastal sandstones of the Eagle Sandstone are highly porous (25%) and permeable (200 to 300 md) conventional reservoirs. Reservoir quality is adversely affected by the formation of authigenic minerals

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and enhanced by the development of dissolution porosity in silicate grains and carbonate cement.

Upward-coarsening sandstone sequences, 3 to 30 m thick, contained in the Eagle-Telegraph Creek-equivalent Gammon Shale, accumulated tens to hundreds of kilometers seaward (eastward) of the strand. These sandstones are transitional between conventional reservoirs and low-permeability reservoirs. Near the bottom of each sequence, porosity averages 15% and permeability averages 1 md. Upward through each sand accumulation, loss of allogenic clay and increasing sand content and grain size enhance reservoir properties. Porosity and permeability attain 25% and 150 md near the top of each sequence. Reservoir quality is controlled by allogenic clay content, intensity of bioturbation, precipitation of authigenic minerals, and the dissolution of cements and detrital grains.

The greatest volume of natural gas occurs in low-permeability mudstones of the Gammon Shale, which are identical to offshore equivalents of the Milk River Formation in southeastern Alberta. The reservoirs are silty shales containing discontinuous lenses and laminae of silt or very fine sand, a few millimeters or less in thickness. Effective porosity is confined to passageways within the laminae or to spaces among loosely packed clay platelets between clastic grains. Porosities range between 10 and 20%, permeabilities are commonly less than 0.1 md, pore-entrance diameters are normally 0.1 µ or less. Because of the amount and composition of allogenic clay, the reservoirs are highly water sensitive and display very high water-saturation values. Although economic flow rates are only chieved through fracturing, subsequent production has been predictable and profitable.

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