Occurrence and Origin of Mississippian Gas in Four Corners Region

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In the Four Corners region (junction of Utah, Colorado, New Mexico, and Arizona), uncommon amounts of carbon dioxide, nitrogen, and helium occur in carbonates of Mississippian age. About 80 per cent of the reservoir beds are gray, tan to brown, crystalline and sucrose, microcrystalline to very fine-grained, secondary dolomite. Vuggy, intercrystalline, fracture, earthy, and interoolitic porosity are present in the dolomite intervals. Vuggy and intercrystalline porosity are the most common; fracture porosity is probably more prevalent than has been noted. Tan to brown, partly dolomitic, microcrystalline to very fine-grained limestone beds constitute about 20 per cent of the reservoir intervals. Carbon dioxide ranges from 0.3 per cent to 97.5 per cent; nitrogen from a trace to 89.03 per cent; and helium from a trace to a maximum of 7.5 per cent. The amounts of nitrogen and helium decrease with distance from the Defiance uplift in the southern half of the area. There is a general increase in the percentage of nitrogen and helium with depth in individual boreholes in both Mississippian and Pennsylvanian rocks. A plot of helium and the sediment thickness from the top of the Mississippian to the top of the Precambrian shows high helium in the thinner stratigraphic intervals (450-650 feet). Nitrogen and argon were not separately plotted against the sediment thickness, but a similar relationship is believed to exist. A comparison of carbon dioxide and the same stratigraphic interval shows that seven of eight carbon dioxide occurrences in the 60-100 per cent range are 800-1,600 feet above the top of the Precambrian. In both Mississippian and Pennsylvanian rocks the N2/He ratio is nearly 10 : 1, the N2/A ratio nearly 100 : 1, and the ratio of He to A nearly 10 : 1. The relatively good correlation between the abundance of these gases suggests strongly that they were derived from a common source. It is concluded that this source was Precambrian basement rocks, particularly those of the Defiance uplift. The decay of nitrogenous material has undoubtedly contributed a little nitrogen to the Four Corners gases. The origin of the hydrocarbon gases in the Mississippian reservoirs has not been established with certainty. They are probably indigenous to the Mississippian reservoirs, but may have migrated into the Mississippian sediments from Devonian, or, more likely, from Pennsylvanian beds. Though there are a number of possibilities for the origin of the carbon dioxide, the writer favors: (1) an indirect intrusive source (by metamorphism of Mississippian carbonates during Laramide or younger intrusions); and (2) a contribution of environmental carbon dioxide. Methane, ethane, and carbon dioxide are not in chemical equilibrium. This disequilibrium state is attributed to either kinetic factors in the production of the gases and (or) to a mixing of gases at a temperature sufficiently low so that equilibrium has not been attained since the time of mixing.