METHANE OCCURRENCE IN GLACIAL BURIED-VALLEY AQUIFER SYSTEMS IN NORTH DAKOTA

Shallow glacial buried-valley aquifer systems are widespread throughout the glaciated portions of North Dakota and are a primary ground-water resource for irrigative, livestock supply, and commercial use. Historically, methane has been found in wells screened within glacial buried-valley aquifers and was produced for residential and commercial use in north-central and southeastern North Dakota. This study has revealed the presence of methane in wells completed in buried-valley aquifers through reconnaissance field screening using portable flame-ionization detector (FID) instrumentation. These aquifers are dominantly buried-valley systems comprised of glaciofluvial sands and gravels commonly laden with detrital lignites originating from coal-bearing strata of the Paleocene Fort Union Group. These aquifers are commonly underlain by Cretaceous marine shales of the Pierre Formation and sandstones of the Fox Hills Formation and are overlain by low-permeability subglacial diamictons deposited during Pleistocene glaciation. Methane detected is commonly, but not exclusively, associated with lithologic intervals where detrital lignites are described suggesting methane of biogenic origin. Ground-water geochemistries correlate with and may be predictive of aquifer intervals where methane is found. These shallow aquifers host zones of localized shallow methanogenesis and potential production and provide possible reservoirs for methane that has migrated from deeper sources. Aquifers hosting methane can be grouped by depositional environment and aquifer architecture and include bedrock incised buried-valley aquifers and aquifers encapsulated in overlying glacial sediments.