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Noble gas Isotopes, Major Element Isotopes, and Gas Composition From the Cumnock Formation: Sanford Sub-Basin, Deep River Basin, Lee County, North Carolina, U.S.A.


Noble gas isotopes, major element isotopes, and gas composition were obtained from the shut-in Butler #3 (API 32-105-00008) and Simpson #1 (API 32-105-00007) wells, drilled in 1998, and sample gas from the Cumnock Fm. of Late Norian age. This is the first gas chemistry compilation of these wells. The wells' gas, sampled in 2009 and in 2014, had a strong ‘fruity’ light petroleum odor, a visible condensate plume when the wells were flowed, and are in the oil and wet gas window. Molecular and stable isotopic composition are typical of thermogenic natural gases; wetness and carbon and hydrogen isotopic composition of methane suggest generation in the oil window - consistent with the Cumnock's measured thermal maturity (%Ro= 0.8 – 1.0). Shut-in well pressures were ~900psi (Butler #3), and ~200psi (Simpson #1); both had a substantial initial gas flow. Limited data are from the 1982 Dummitt-Palmer #1 CBM well (API 32-105-00002), now plugged and abandoned.

Helium concentrations were ~0.20% to 0.24% from the noble gas analysis with neon ranging from 0.11 to 0.04 ppm and argon approximately 33 ppm. The measured noble gas composition contains very low atmospheric contamination with helium isotopes (0.07 R/RA) clearly defined by a crustal origin, while neon and krypton and are mainly attributed to atmospheric origin (20Ne/22Ne ~9.8, 86Kr/84Kr ~0.3). Argon isotopes are mixed between crustal and atmospheric origins with 40Ar/36Ar values ~ 418 to 520. The F20Ne/36Ar (~0.9 to 2.6), F84Kr/36Ar (~0.8) and F132Xe/36Ar (0.6–0.7) in the gas show enrichment in the light isotope associated with multi-stage fractionation processes with gas and fluid interaction.

The methane content (range ~58–64%) is inverse to the nitrogen content from denitrification of very thin ammonium-bearing units (also rich in oil), and likely from illite in overlying strata, but ~100% in the CBM well where these strata are absent.