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Increasingly greater numbers of wells are being drilled below 25,000 ft, and considerations of methane stability in the deep subsurface are becoming more important. We have calculated equilibrium gas compositions corresponding to conditions down to 40,000 ft for low, average, and high geothermal gradients, for hydrostatic and lithostatic pressures, and with and without graphite. Calculations have been made for sandstone reservoirs with various amounts and combinations of feldspars, clays, carbonate cements, and iron oxides with and without graphite, and for limestone and dolomite reservoirs with various combinations of clays, iron minerals, anhydrite, and sulfur, again with and without graphite. Natural gas shows considerable stability in sandstone reservoirs under most c nditions, but its concentration in deep carbonates is much more variable and tends to a H2S-CO2 mixture except when an appreciable concentration of iron is present.
The thermodynamic predictions can (in principle) be checked by direct analysis down to the depth limit of available gas samples. In practice, considerable problems exist due to partial gas loss during sample retrieval. The analysis of gases trapped in fluid inclusions in late-stage cements offers one solution to this problem. This gas is being analyzed by thermally rupturing inclusions in the inlet system of a fast-scanning, computer-controlled mass spectrometer. Each bursting inclusion is analyzed separately, and several hundred individual inclusions can be analyzed using only 10 mg of sample. A wide variety of compositions, including water-rich, methane-rich, and H2S-rich, is found in samples from below 20,000 ft.
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