MAPPING AND TESTING THE SOUTHERN MARGIN OF THE MOUNT SIMON SANDSTONE (CAMBRIAN), A MAJOR TARGET FOR CO2 STORAGE IN THE MIDWEST

The Cambrian Mount Simon Sandstone is a regional saline aquifer with the largest projected CO₂ storage capacity in the Midwest. Mapping along the southern margin of the formation in Kentucky was undertaken to better constrain CO₂ storage estimates and reservoir characteristics in this important unit along the Ohio River corridor. In this area, the top of the sandstone is 3,000 to more than 12,000 ft (900 to more than 3,600 m) deep. Porosity is known to decrease with depth, so the sandstone is a likely CO₂ storage target at depths of less than 7,000 ft (2,100 m).

The recently completed Battelle No. 1 Duke Energy well injected CO₂ into the Mount Simon on the Cincinnati Arch along the Ohio River. In that well, the sandstone is 300 ft (91 m) thick and 3,200 ft (985 m) deep. Eastward into the Appalachian Basin, the sandstone thins and pinches out north of the Kentucky River Fault System and Rome Trough, a Cambrian rift graben. Thin sandstones in the Maryville Limestone Formation south of the faults may represent Mount Simon sands that crossed structural highs along the faults and entered the trough, or may be locally reworked Mount Simon. In the Rome Trough, a deeper, basal sandstone, not the Mount Simon, overlies the Precambrian.

Structures also influence the southern margin of the Mount Simon Sandstone in western Kentucky. Previously, the Mount Simon was interpreted to gradually thicken toward the Rough Creek Graben, a Precambrian rift in the southernmost Illinois Basin. As in eastern Kentucky, however, sandstones above the Precambrian in the rift graben south of the Rough Creek faults are not the Mount Simon. The Mount Simon pinches out north of the faults in most areas. A series of NE-SW–oriented faults bisect the northern shelf of the rift graben. The recently completed Kentucky Geological Survey No. 1 Blan well, a carbon storage test of the Knox Formation that was drilled to the Precambrian, is located on a horst of these faults. The Mount Simon was absent in this well, and the overlying Eau Claire shales were thin (187 ft, 57 m). In contrast, the southernmost Mount Simon Sandstone occurs along the eastern margin of the Rough Creek Fault System in a subsidiary graben north of the fault system. Southward thinning is locally accompanied with interbedding of Eau Claire shales, which thicken into the Rough Creek Graben.