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A Natural Laboratory for Occurrences of Drift Gas on the Southeastern Margin of the Williston Basin

Iles, Derric L.1, Layne D. Schulz1, George W. Shurr2, Dennis W. Tomhave1, Timothy C. Cowman1
1 South Dakota Geological Survey, Vermillion, SD
GeoShurr Resources, LLC, Ellsworth, MN

Occurrences of natural gas in glacial drift are important indicators of economic accumulations on basin margins. For example, initial shows of drift gas shows preceded development of the Antrim Shale play on the northern margin of the Michigan basin. Glacial drift hosts developed reserves at the Sousa Gas Field on the northeastern margin of the Alberta basin. In general, the gas is late generation biogenic gas that formed long after deposition of the bedrock source beds. Drift gas is important because it demonstrates the presence of gas in the system.

Along the Sioux Ridge on the southeastern margin of the Williston basin, shows of drift gas are encountered in McCook and Minnehaha Counties, South Dakota. Fractured, organic rich Cretaceous rock units subcrop beneath the glacial drift. Fracture patterns in the Cretaceous bedrock are similar to fracture patterns in the underlying Precambrian Sioux Quartzite.

Near Dolton in southern McCook County, gas shows are documented within a specific glacial outwash which overlies and is locally in contact with subcrops of the Cretaceous Niobrara Formation. Gas shows are coincident with an area of low sulfate and high bicarbonate values in ground water. Analysis of gas recovered from one locality shows high methane and nitrogen content. Radiocarbon age dating of water from two wells in this area show an age range of 13,500 to 14,800 years before present. The Dolton aquifer provides a unique natural laboratory that replicates geologic conditions for drift gas occurrences in areas of commercial production.