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Porosity, Permeability, Pore- and Geomechanical Characterization of the Triassic Cumnock Formation: A Lacustrine / Rift Continuous Gas Assessment Unit


The Deep River basin is a continuous gas assessment unit (AU) and a total petroleum system (TPS). The Cumnock Formation source rocks are grey and black freshwater shales with thin basal coals. This formation is early Norian Age of 225-227Ma, and 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). The reservoir is being characterized using: 1) high pressure air / mercury infusion capillary pressure (MICP), 2) pore characterization using scanning electron microscopy (SEM) with argon-ion beam milled samples, 3) geomechanical analyses for triaxial shear, Poisson's ratio, Young's modulus accompanied by whole rock XRD mineralogy of core through the entire vertical extent of the ‘sweet spot’ and basin center of the Cumnock Formation, and 4) light microscopy using ultrathin thin sections. Multiple porosity types were identified from argon-ion beam milled samples using SEM. Porosity types include: 1) intergranular (particularly between clay particles), 2) intragranular (especially in phosphate grains), 3) diagenetic growth of pyrite, and 4) pore development from the consumption of kerogen during hydrocarbon generation. Combinations of these porosity types were observed. The Cumnock Formation's average 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 × 10-5 md. The minimum is 0.090 × 10-5 md, the maximum is 7.11 × 10-5 md, and the standard deviation is 1.77 × 10-5. Porosity near the basin center (V.R. Groce #1) averages 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 × 10-5 md; the minimum is 0.05 × 10-5md, the maximum is 6.68 × 10-5 md, and the standard deviation is 1.99 × 10-5 md. Molecular chemistry and stable isotopes of the gas from two shut-in wells have been obtained. Helium (∼0.21%) is present in both wells. These data provide comprehensive, integrated and robust characteristics of one continuous gas reservoir of the Triassic continental rift / lacustrine basins in North Carolina.