Compartments in Shallow, Basin Margin Gas Accumulations in the Rocky Mountains and Adjacent Great Plains

Shurr, George W.
GeoShurr Resources, LLC, Ellsworth, MN

Shallow, basin margin gas accumulations are arranged in compartments. These compartments, like those found in deep basin center gas accumulations, are useful guides for exploration and exploitation. However, the nature of the compartments depends upon the type of shallow natural gas system.

Early generation biogenic gas forms during and shortly after deposition of the host rock. It is relatively “old” biogenic gas that has been trapped in the host rock since its generation. Consequently, closed compartments are extremely important attributes of economic accumulations. Compartments are physical entities that relate to specific depositional features in the host rock or to discrete geologic structures, such as fault blocks. Examples are found in Cretaceous reservoirs along the western margin of the Williston basin, including Bowdoin dome and Cedar Creek anticline, and on the eastern margin of the Denver basin.

Thermogenic gas is generated in the deep basin center and migrates long distances to accumulate on a shallow basin margin. Compartments in this regional flow system are dependent upon the relative timing of migration and accumulation. The Hugoton Embayment on the northwestern margin of the Anadarko basin is an example of an accumulation of migrated thermogenic gas that is reported to have compartments.

Late generation biogenic gas forms in the relatively recent geologic past by microbial activity in aquifers with dynamic flow systems. Compartments in this open system are complex and may involve hydrodynamic partitioning, as well as physical barriers. Coal bed methane produced on the northwestern margin of the Powder River basin is late generation biogenic gas and has reports of compartmentalization.