SYNOPSIS OF NATURAL CARBON DIOXIDE OCCURRENCE IN THE EASTERN MID-CONTINENT, UNITED STATES

Low-BTU gases can be a significant component locally of the natural gas reservoirs in some basins in the United States. Of the various non-hydrocarbon gases in these systems, carbon dioxide (CO₂) is frequently the most prevalent, and in some instances, of sufficient quantity and quality to be the primary commodity produced from the gas field. On-going research by the United States Geological Survey (USGS) on geologic storage for CO₂ includes investigations on these natural CO₂-rich reservoirs, in that they may be natural analogs for understanding long-term CO₂ storage systems. Recognizing critical attributes in low-BTU gas systems such as residence time, diagenetic alterations, and geochemistry may serve as pathways for predictive engineering of anthropogenic CO₂ storage reservoirs. The recently completed national geologic CO₂ storage resource assessment by the USGS ranks the Illinois, Michigan and Appalachian Basins as the 4^th, 8^th, and 13^th of the 36 basins assessed, in terms of mean technically accessible storage resources in the nation. In these basins, natural gas is a major commodity and existing geochemical data from these areas notes occurrences of elevated levels of CO₂ within the natural gas. Improved understanding of natural CO₂ reservoir systems may facilitate refinement of potential geologic storage resources of CO₂ within these basins. At present, preliminary research into natural CO₂ in these basins suggest CO₂ exists mainly at low levels in individual wells in the Illinois Basin. In the Michigan Basin CO₂ associated with Antrim Shale biogenic gas production is present (and is removed and used for tertiary CO₂-flood oil recover locally). In the Appalachian Basin, two CO₂-rich gas fields are known: the Furnace in Kentucky, and the Indian Creek in West Virginia, as are a few wells in other portions of the basin with elevated CO₂ content. A summary of the current research on natural CO₂ systems in these basins will be presented.