Shale Gas Potential in Triassic Strata of the Deep River Basin, Lee and Chatham Counties, North Carolina, USA
The Deep River Basin is a 150-mile long northeasttrending half-graben with a steeply-dipping eastern border fault in central North Carolina. The basin is filled with ~7,000 feet of Triassic strata, which are divided into the following three formations in descending stratigraphic order: (1) Sanford Formation (red and gray siltstone and shale); (2) Cumnock Formation (black shale, with some beds of gray shale, sandstone, and coal); and (3) Pekin Formation (gray sandstone and shale). The Cumnock Formation includes a ~400 foot thick interval of Upper Triassic (Carnian) organicrich black shale. This shale extends across ~25,000 acres, at depths of less than 3,000 feet in Lee and Chatham counties. Organic geochemistry and thermal maturation analyses indicate that the black shale in the Cumnock Formation is gas-prone, and that values of total organic carbon (TOC) exceed 1.4% in places. The Cumnock Formation contains systematic fractures that are observable in outcrop, in drill cores, and on 1:24,000-scale geologic maps superimposed on LiDAR data. The primary fractures trend northwest, whereas the conjugate fractures trend northeast. In some places along the west side of the basin, the primary fractures are filled with diabase dikes (that locally heated the Cumnock Formation), although mapping in underground coal mines (now closed) has shown that the diabase dikes do not extend far into the basin. Six of the 28 wells that have been drilled in the Cumnock Formation have reported natural gas and oil shows, and two shut-in wells have reported pressures of 900 psi and 300 psi.
AAPG Search and Discovery Article #90095©2009 AAPG Eastern Section Meeting, Evansville, Indiana, September 20-22, 2009