HYDROCARBON GENERATIVE POTENTIAL OF MID-CRETACEOUS STRATA IN SOUTHEASTERN SOUTH DAKOTA AND NORTHWESTERN IOWA

Historically, shallow accumulations of methane associated with glacial drift have provided local energy resources in the midwestern U.S. One hypothesis suggests that the origin of the gas is contemporary methanogenesis, that is, modern communities of microorganisms feeding on organic rich bedrock.

Throughout much of southeastern South Dakota and northwestern Iowa, Cretaceous rocks lie beneath glacial drift. The Cretaceous was a time of enhanced organic carbon burial, an observation that explains the globally vast fossil fuel resources of this age. Therefore, Cretaceous rocks in the Midwest present a possible source rock for natural gas generation.

Our focus is the Middle Cretaceous Dakota Formation, Graneros Shale, and Greenhorn Formation from the Davison core, southeast South Dakota, and the Hawarden core, northwest Iowa. Total organic carbon contents (%TOC) range from ~0 to 53% in the Dakota Formation, although values are <1% in much of the formation. Thin carbonaceous shales and coals were observed in both cores. Hydrogen Index (HI) values ranging from ~0 to 200 in the Dakota Formation are indicative of dominantly terrestrially derived, gas-prone organic matter. %TOC values range from ~0 to 8% in the Graneros Shale while HI values of ~50 to 600 indicate that the organic matter represents a mix of terrestrial and marine derived organic carbon. In the Greenhorn Formation, %TOC ranges from ~0 to 10% with HI values from ~100 to 700; these values indicate a greater proportion of marine organic components with terrestrial organic matter. These observations are consistent with the overall transgressive nature of the sequence of strata.

Tmax values for organic matter in all of the study formations in both cores range from ~415 to 430, values indicative of thermal immaturity relative to oil and gas generation. The quantity, quality and maturity of Middle Cretaceous organic matter are consistent with contemporary methanogenesis. Further study of the organic matter and regional stratigraphic relationships will help to understand the formations as potential gas sources for unconventional reservoirs associated with glacial drift.