The Stable Isotope and Noble Gas Geochemistry of a Natural N2 Gas Reservoir, Northwestern Denver-Julesburg Basin, Goshen County, Wyoming
Christopher Laughrey, Dolan Integration Group
Natural gas produced from the Permian Hartville sandstone in the Samson Oil and Gas Bluff #1-11 well in Goshen County, Wyoming is composed of 97.58% N2, 2.05% CH4, 0.17% CO2, 0.151 to 0.152% He, and trace amounts of C2H6 through C6+ hydrocarbons and argon. The N2/Ar ratio is 7,933.3, a value that eliminates air contamination as a possible source of the N2. The δ15N of the produced nitrogen gas is +20.4‰, a value compatible with a crustal or magmatic source. The 3He/4He ratio of the Bluff #1-11 gas is 4.5 x 10-8 (R/Ra = 0.032), a value which eliminates a magmatic source from consideration and indicates a crustal origin for the N2. There are three possible crustal sources for the nitrogen: 1) N2 fixed as NH4+ in potassium-rich sediments; 2) N2 fixed in biotite and K-feldspar in crystalline rocks; and 3) denitrification of organic matter in hydrocarbon source rocks. The N2 concentration, δ15N of the gas, 4He/N2 and N2/20Ne ratios support an interpretation that the N2 produce d from the Bluff #1-11 well was generated by denitrification of post mature organic matter. The methane δ13C of the Bluff #1-11 gas is -32.87‰ and the δ2H of the methane is -173.7‰. These carbon and hydrogen isotopic values, in combination with the chemical composition of the Bluff gas, indicate that the CH4 is a thermogenic, post-mature gas. The respective C2H6 and C3H8 δ13C of the hydrocarbons are -27.9 and -32.3‰. The hydrocarbon gases exhibit a partial isotope reversal with respect to carbon number, i.e., δ13C1 < δ13C2 > δ13C3 which suggests mixing of wet and dry thermogenic gases in the Hartville sandstone and/or its source rocks (Pennsylvanian-age Desmoinesian black shales and marlstones) during progressive burial in the Paleogene. The CH4 and C2H6 in the Bluff well gas are co-genetic and were generated through moderate to extensive cracking of oil. The C3H8 is residual and was generated earlier from labile (oil-prone) kerogen within the late oil window. The δ13C of the CO2 in the gas is -1.6‰. The relative magnitude of isotopic offset between the CO2 and CH4 in the gas (αCO2-CH4) is 1.03215, a value consistent with a thermogenic origin. Thermal degradation of carbonate minerals in the source rocks is the likely source of CO2 in the gas. The 20Ne/22Ne ratio of 9.461 in the Bluff #1-11 well gas approximates the air ratio of 9.80. Respective 38Ar/36Ar and 21Ne/22Ne ratios of 0.190 and 0.0365 constrain an atmospheric source for the 20Ne and 36Ar components in the produced gas. R/Ra and 20Ne/4He ratios suggest mixing of radiogenic crustal-produced He and Ne with groundwater-transported radiogenic He and atmospheric Ne components. Regional groundwater degassing of 20Ne, 36Ar, and 84Kr is the source of the atmospheric noble gas components in the Bluff well gas.