Abstract

Results of a localized soil-gas survey conducted in the Rawhide Subdivision near Gillette, Wyoming, revealed areas where the concentrations of coal-sourced methane gas approached 1 million parts per million (100%). A map was produced that accurately displayed the areas in the subdivision where concentrations of methane gas in the vadose zone are higher than normal (background) and thus possibly hazardous to human activity.

Along the eastern edge of the Powder River Basin where Cretaceous coal deposits crop out or are near the surface, elevated concentrations of methane gas can be expected. Generally, the methane is contained by the coal or porous strata directly above the coal by a combination of overburden and groundwater pressure. Small and somewhat normal amounts of methane constantly escape to the surface even during stable conditions.

Abrupt changes (natural or man-made) of the delicate balance between overlying pressure and underlying gas concentrations can release large volumes of methane gas that migrate to the surface. Buildings (homes, schools, factory's etc.) that directly overlie these escaping gasses may act as collection centers for the methane and thus become health hazards.

The techniques used to map the methane concentrations in the Rawhide Subdivision can be utilized along the eastern Powder River Basin coal belt to more regionally identify variations in methane concentrations. This information would be helpful in;

1) future exploration programs (mapping)

2) identifying areas with already high methane concentrations (environmental)

3) preventing future poorly located housing developments (zoning)

4) monitoring present activity (mining)

5) mitigating existing problem areas (remedial)