Natural gas is currently being produced from shallow, low-permeability, low-pressure reservoirs in the Bowdoin dome area, Phillips and Valley Counties, Montana. Most of the biogenic gas is stratigraphically entrapped in thin, discontinuous siltstones and sandstones that are enclosed in a thick sequence of shales of Late Cretaceous age. There is some structural influence on the accumulations in more porous zones. The reservoirs and the associated shales were deposited on a shallow-marine shelf and thus present different mapping and recovery problems from most low-permeability reservoirs of nonmarine origin.

Early development, prior to 1960, was on the structurally high part of the dome and covered an area of about 200 sq mi (520 sq km). Production was from depths ranging from 800 to 1,000 ft (240 to 300 m). With the advent of higher gas prices and improved completion technology in the 1970s, the field now includes an area greater than 600 sq mi (1,560 sq km). The reservoirs in the expanded area are lower in permeability, occur at depths ranging from 1,200 to 1,800 ft (360 to 540 m), and require stimulation to provide economic flow rates.

Most of the recent wells have been drilled using conventional rotary rigs and mud systems designed to minimize formation damage. The wells are hydraulically fractured using sand, carbon dioxide, and water. A combination of sonic, neutron, and density logs is an aid in reservoir evaluation, although better logging tools

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are needed. Production history, which is limited, will probably prove to be the best method of estimating recoverable reserves.

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