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Significant resources (about 100 tcf in place) of natural gas are entrapped in low-permeability, low-pressure reservoirs at depths less than 3,000 ft (900 m) in Montana and the Dakotas. The gas occurs in fine-grained sequences of Late Cretaceous age deposited on a shallow, open-marine shelf. The low-permeability reservoirs were deposited during progradations and consist of discontinuous lenses and laminae of siltstone and sandstone enclosed by silty clay shale. The siltstone and sandstone layers are porous and permeable, whereas the enclosing shale is impermeable. The large proportion of allogenic clay, including highly expansive mixed-layer illite-smectite, causes extreme water sensitivity as well as high measured and calculated water saturation values in the reservoir s quence.
Gas production has been established in "sweet spots" on structural highs where the reservoirs are best developed. Recurrent tectonic movement resulted in winnowing of coarser grain sizes on the highs by southward-flowing, geostrophic currents. In the future, exploration will occur off structure for lower quality reservoirs.
The gas was generated in the marine environment by decomposition of organic matter by anaerobic microorganisms and is referred to as biogenic gas. The most important mechanism of methane generation in marine sediments is reduction of CO2. Organic carbon values, rates of sedimentation, and products of early diagenesis indicate that biogenic gas production was widespread during deposition. Most of the gas was initially retained in solution in the pore waters because of higher methane
solubility at higher hydrostatic pressures owing to the weight of the overlying water column; the gas subsequently exsolved because of uplift and erosion during the Tertiary.
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