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Porosity, Permeability, and Pore Characterization of the Triassic Cumnock Formation: A Continuous Gas Assessment Unit, Sanford Sub-Basin, Deep River Basin, Lee County, North Carolina, USA

Abstract

The Upper Triassic (Carnian) Deep River Basin is a continuous gas assessment unit (AU) and a total petroleum system. The source rocks (Cumnock Formation) are grey and black freshwater shales with thin basal coals. The Cumnock Formation was deposited in a lacustrine rift near the paleo-equator after the onset of the breakup of Pangea. The Deep River Basin Continuous Gas AU has an estimated mean gas content of 1,660 BCFG and an estimated mean natural gas liquids content of 83 MMBNGL (USGS Fact Sheet 2012-3075). In order to better characterize the potential for shale gas reservoirs in the Sanford sub-basin of the AU, twenty samples were analyzed from continuous core hole USBM DH2 (CH-C-1-45) between the depths of 1,047 and 1,178 feet. This zone is a candidate “sweet spot” for the Cumnock Formation. Fifteen additional samples (between the depths of 2,407 and 2,428 feet) were analyzed from the V.R. Groce #1 core (API 32-095-00009) located near the sub-basin center, where alluvial fan facies thin the Cumnock Formation and its coals. The majority of samples from both cores are laminated to massive, chlorite- and illite-rich mudrocks with variable carbonate and tektosilicate content and organic carbon concentrations ranging up to 6.4 weight percent. Mercury injection capillary pressure data were obtained to characterize porosity and permeability in the Cumnock Formation. A maximum pressure of 60,000 psia provided a pore aperture frequency distribution down to 0.00036 microns diameter. These data will ultimately be complimented by pore characterization using scanning electron microscopy and ion beam milled samples. The average Cumnock Formation porosity in the “sweet spot” (drill hole CH-C-1-45) is 2.28%; the minimum is 0.44%, the maximum is 6.43%, and the standard deviation is 1.46%. The average permeability in the “sweet spot” (drill hole CH-C-1-45) is 1.75 x 10-5md. The minimum is 0.090 x 10-5md, the maximum is 7.11 x 10-5 md, and the standard deviation is 1.77 x 10-5mg. Porosity near the basin center (V.R. Groce #1) is 1.58%; the minimum is 0.24%; the maximum is 3.74%, and the standard deviation is 1.03%. Average permeability near the basin center is 1.61 x 10-5md; the minimum is 0.05 x 10-5md, the maximum is 6.68 x 10-5md, and the standard deviation is 1.99 x 10-5md. In conjunction with a growing database of mineralogy and organic geochemistry, these data provide the most robust assessment available of pore space in the Cumnock Formation.