Geologic Assessment of the Natural Gas Resource Potential of Lignite Coal in the Williston Basin

摘要

A bottom-up geologic evaluation was made of the coalbed natural gas resource potential of Fort Union Group lignite resources in portions of a nine-county area in the southwestern North Dakota portion of the Williston Basin. The analysis targeted the Harmon, Hansen, and Harmon lignite beds, covered areas where the overburden thickness was ≥150 feet (≥46 m), and assumed that the lignite was only 80% saturated with respect to its maximum sorbed-phase CH4 storage capacity. Various types of observations clearly indicate that natural gas is present in the Williston Basin's lignite coals. These observations include the emission of flammable gas from water wells completed in lignite-bearing aquifers, mud log gas shows when drilling through lignite seams, and gas emission from lignite drill cutting samples. The analysis results indicate that the coalbed natural gas resource estimate for the three lignite beds totals 1.7Tcf(48.1x10~9m~3). The gas-in-place resource estimate for the Harmon lignite bed is as high as 0.9 Bcf/mi~2 (9.8 x10~6 m~3/km~2), which is comparable to areas in the Powder River Basin where there are large numbers of commercial coalbed natural gas wells.