Prior to 1970, shallow gas production was established in "sweet spots" where the reservoirs are best developed in the northern Great Plains. Recent advances in completion technology coupled with higher gas prices have led to the expansion of these areas through the development of submarginal, low-permeability reservoirs. The development is concentrated in two main areas which cover more than 22,000 sq km. The gas occurs at depths less than 600 m, and recoverable reserves average 2 Bcf of gas per section.

The reservoirs are of Late Cretaceous age and generally consist of siltstone and sandstone laminae, a few millimeters or less in thickness, enclosed in organic-rich silty shale that serves as a seal and was the source for the biogenic gas. The laminae are discontinuous because of depositional processes and/or biologic activity. Coarsening-upward sandstone cycles are locally developed. Although these cycles display the best reservoir properties, they are volumetrically minor. Porosity is confined to small passageways within the laminae, among randomly oriented allogenic clay platelets, and to well-sorted sandstone near the top of coarsening-upward cycles. Diagenesis has reduced permeability and resulted in the formation of fluid-sensitive clays and carbonate cement. However, dissolutio has enhanced porosity and permeability in well-sorted lithologies.

The reservoirs are stimulated with sand proppant, carbon dioxide, and water to provide economic flow rates. Typical wells have initial potentials of 300 Mcf of gas per day. Production declines rapidly the first year, but levels off to about 100 Mcf of gas per day. Wells are difficult to evaluate because conventional logs cannot distinguish pay zones in sequences of thin, discontinuous, low-permeability reservoirs.

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