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The coal-bearing lower Tertiary Fort Union and Wasatch Formations in the Powder River basin of Wyoming and Montana are potentially important sources of biogenic gas. The presence of gas seeps, flowing gas wells, and gas shows in shallow drill holes indicates that these rocks contain economically recoverable methane resources. Chemical and isotopic analyses of coal-derived gas and gas produced from sandstone reservoirs in these rocks indicate that the gases are basically identical and are biogenic (^dgr13C values range from -53.59 to -60.85 ^pmil and C1-C1-5 values range from 0.97 to 0.99).
The search for shallow biogenic gas accumulations may be facilitated by the recognition of the significance of compaction anticlinal folds. The development of compaction structures occurs penecontemporaneously in response to abrupt lithofacies changes associated with specific environments of deposition. Measured relief of these anticlines is as much as 250 ft (76 m).
Compaction folds may provide early formed structural traps in cases where the overlying folded strata contain suitable sandstone reservoirs. In other cases, compaction folds may reflect the presence of stratigraphic traps in the structural core of the fold. For example, the compaction contrasts inherent in a fluvial system of lenticular channel sandstones and fine-grained overbank deposits may be the eventual site of compaction folding. In either case, compaction folds may indicate the presence of very early formed structural and/or stratigraphic traps, and these folds can be mapped on the surface and in the subsurface. They are in stratigraphic proximity to excellent gas source rock (coal), providing optimal conditions for early entrapment of biogenic gas.
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