The quality of reservoirs containing biogenic gas in the northern Great Plains is predictable on the basis of facies distribution. Rocks of the Upper Cretaceous Eagle Sandstone-Telegraph Creek Formation stratigraphic interval include excellent examples of several reservoir types.

Conventional reservoirs of the Eagle Sandstone are restricted to a north-south trending band of coastal sandstones in central Montana. These reservoirs are highly porous and permeable and display evidence of extensive diagenetic alteration. Conventional Eagle reservoirs are exemplified by gas fields in the vicinity of the Bearpaw Mountains, where gas is found in structural traps.

Coarsening-upward sequences of the Shannon Sandstone Member of the Gammon Shale were deposited on a shallow shelf seaward (eastward) of the coastal sandstones. Reservoir quality improves upward through each sequence, and depends upon the distribution and amount of sand accumulation. The Shannon reservoirs are transitional between conventional reservoirs of the Eagle Sandstone and low-permeability (tight) reservoirs in the Gammon Shale. The Liscomb Creek field, in southeastern Montana, produces gas from the Shannon in a structural-stratigraphic trap.

Gas reservoirs in offshore marine mudstones of the Gammon Shale consist of thin, discontinuous lenses and laminae of siltstone enclosed in silty shale. The reservoirs are tight with permeabilities of 0.1 md or less. They have high irreducible water saturations (> 80%; however, they produce little free water). The reservoirs are extremely susceptible to formation damage when exposed to water-based fluids used during drilling and hydraulitc fracturing. The Gammon reservoirs have low productivity but volumetrically make up the greatest part of rocks equivalent to the Eagle-Telegraph Creek sediments, and they probably have the greatest potential for reserves of shallow gas in the northern Great Plains. Examples of low-permeability reservoirs include pay zones in the Little Missouri fie d in southwestern North Dakota and in Gammon-equivalent rocks of the Milk River pool in southeastern Alberta.

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