POTENTIAL CARBON SEQUESTRATION IN THE PRECAMBRIAN: THE CONTRIBUTION OF REFLECTION SEISMIC DATA TO RESERVOIR DEFINITION AND FAULTING IN THE EASTERN MIDCONTINENT

Reflection seismic data has added information on the distribution, thickness and stratigraphic relations of a Precambrian quartzarenite to sublitharenite unit in central Kentucky that may be a part of the regional Middle Run Formation of the East Continent Rift Basin. The unit may have the potential to sequester CO₂. In addition, the seismic data has provided an idea of the types, frequency, and vertical extent of the faulting that would be significant in assessing the potential for CO₂ leakage. The quartzarenite to sublitharenite unit is significant because, unlike the impervious lithic arenites in much of the Middle Run Formation, borehole data from a single well shows it to be of higher porosity. Even more important, when drilled, mud and gas flowed from the unit at 8,000 feet in the well to the surface, suggesting significant permeability. Later flow testing in the unit proved inconclusive, but cement bonding logs indicate a strong gas cut. If the interval is not gas productive, it may be suitable for carbon sequestration.

Correlation of petrophysical logs from the well with reflection seismic data showed that the thin interval penetrated by the well was only a small part of a thicker, extensive stratigraphic sequence, bounded above and below by unconformities. The sequence ranges from 0 to more than 1,000 feet, being cut out to the west by the overlying westward dipping unconformity. The sequence is likely part of a distal braid plain. At the well site, the overlying sequence is composed of impervious lithic arenites. However, nearby, seismic data indicates reflectors with bidirectional downlap, indicating that possibly porous alluvial fans cover part of the area.

Were CO₂ to be injected into the quartzarenite to sublitharenite unit, it would migrate updip in an easterly direction, likely being limited by either the crest of an anticline or ultimately trapped beneath Grenville metamorphic rocks. Currently, no seismic data are available either to the north or south of the area. To the west, where the interval is cut by the unconformity, CO₂ likely would be trapped by the overlying impervious lithic arenites or migrate into the fan facies.

Seismic data in the area show primarily normal and wrench faulting. Some cut to the surface, but many are confined to the Precambrian. A fault assessment would be required prior to any CO₂ injection.