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Stable Isotope and Noble Gas Geochemistry of a High-Nitrogen Permian Sandstone Reservoir, Northern Denver-Julesburg (D-J) Basin, Southeastern Wyoming, U.S.A.

Abstract

The Samson Oil and Gas Bluff 1-11 well drilled in the northern D-J basin targeted the Permian Hartville sandstone on a seismic amplitude anomaly within a four-way structural closure. Gas kicks were noted while drilling through the target interval to TD in Precambrian basement at 8,900 ft. Density porosity measured as high as 29% and the density-neutron curve exhibited a gas effect, although the neutron response was low suggesting no hydrogen. The well flowed 7.8 mmcfd with 2,600 psi flow-test pressure for 40 hours with no decline in surface pressure. PTA indicated approximately 500 md permeability. Reservoir pressure was 3,459 psia at 7,746 ft. Cumulative production was estimated at 12.0 mmcf. However, the gas would not flare and laboratory analyses revealed this was due to a gas composition of 97.58% nitrogen with only 2.05% methane and minor to trace amounts of higher hydrocarbons, CO2, helium, and argon. The δ15N of the produced nitrogen is +20.4‰, an enriched value compatible with a crustal or a magmatic source. The 3He/4He, CH4/3He, 20Ne/22Ne, and 21Ne/22Ne ratios of the gas eliminate magmatic sources from consideration and indicate a crustal origin for the nitrogen. Three possible crustal sources for the nitrogen are N2 fixed as ammonium in potassium-rich sediments, N2 fixed in biotite and K-feldspar in crystalline basement rocks, and denitrification of organic matter in hydrocarbon source rocks. The 15N-enriched isotope composition of the gas, low 4He/N2, and high N2/20Ne indicate that the N2 was generated by denitrification of post mature organic matter in down-dip Desmoinesian black shale and marlstone source rocks. Combined 4He/CH4 and 20Ne/36Ar systematics suggest diffusive gas loss from the reservoir under low gas volume/water volume conditions and significant solubility fractionation through water-saturated crust. The gas-filled volume of the trap corresponds to the targeted Vp/Vs seismic inversion anomaly, and the seismic data indicate a down-dip gas-liquid contact. The critical question posed during evaluation of the well was, is the down-dip fluid contact a gas/oil or gas/water contact? High 20Ne/36Ar and low 132Xe/36Ar ratios in the gas samples indicate gas-water equilibrium in the Hartville sandstone reservoir; the less soluble neon was preferentially partitioned in the gas phase compared to argon, while more soluble xenon remained in the water phase. This observation supports the interpretation of a gas/water contact down-dip from the gas cap.