Abstract

Shallow gas accumulations associated with Cretaceous shales on the eastern margin of the Williston basin have historically provided supplies for local consumption. Increased demand for natural gas and new exploration concepts can make this historical production into modern reserves.

Shale gas, like coalbed methane, is the result of biogenic and/or thermogenic processes. Although thermogenic gas systems have had wide application in gas shale exploration, biogenic gas systems are also important. Early-generation biogenic gas that formed during deposition of the host rock is present on the eastern Williston basin margin. However, it is late-generation biogenic gas that holds the greatest resource potential.

Late-generation biogenic gas forms long after deposition of the host rock and may even be forming at the present day. On the northern margin of the Michigan basin, the Antrim Shale (Devonian) hosts an economic accumulation of late-generation biogenic gas. This area of production provides an exploration model that consists of four main components: 1) a “pasture” of organic matter, 2) a “plumbing system” of fractures, 3) favorable water chemistry, and 4) methanogenic microbes. All of these components are present in Cretaceous shales on the eastern margin of the Williston basin. However, aside from very recent limited drilling, the area is mainly untested.

Total organic carbon (TOC) measured in outcrop and core samples of Cretaceous shales from eastern South Dakota, indicate that the shales range from about 2% to 8% TOC. The Niobrara Formation is a representative unit that is a particularly attractive TOC “pasture”. The fracture “plumbing system” varies in scale from regional lineament zones down to faults and fractures visible in outcrops and cores. Favorable water chemistry exists in areas where sulfate values are relatively low and bicarbonate values are relatively high.

A glacial aquifer near Dolton, South Dakota, is locally in contact with fractured Cretaceous shale and has an area of methane shows associated with favorable water chemistry. In addition to historical anecdotes, methane has been measured in headspace over water samples drawn from the aquifer. Methane is also directly detected in the air column above the water level in well bores, using equipment commonly employed in environmental monitoring.

Samples of water from the Dolton Aquifer incubated in a series of microbiology experiments, demonstrate that real-time methanogenesis does take place. The methanogens “graze” the TOC “pasture” in favorable water environments. Exploration for late-generation microbial methane must employ biological and agricultural ideas, as well as geologic and hydrologic concepts. In addition, operators willing to experiment with new drilling and completion techniques are needed. Cretaceous units on the eastern margin of the Williston basin are currently an unassessed potential gas shale resource.