THE MANY ORIGINS OF NATURAL GAS
Thermodynamic calculations for the C‐O‐S‐Hsystem indicate that at a fixed oxygen fugacity methane is in a stable phase relative to carbon dioxide at high pressures and low temperatures. At a constant temperature and pressure, methane is favored at low oxygen fugacities. Volcanic gases and near‐surfa...
| Published in: | Journal of Petroleum Geology |
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| Main Author: | |
| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
Wiley
1983
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| Subjects: | |
| Online Access: | http://dx.doi.org/10.1111/j.1747-5457.1983.tb00588.x https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1747-5457.1983.tb00588.x https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1747-5457.1983.tb00588.x |
| Summary: | Thermodynamic calculations for the C‐O‐S‐Hsystem indicate that at a fixed oxygen fugacity methane is in a stable phase relative to carbon dioxide at high pressures and low temperatures. At a constant temperature and pressure, methane is favored at low oxygen fugacities. Volcanic gases and near‐surface igneous rocks exhibit high values of oxygen fugacity. However, direct measurement of the oxygen fugacity of spinels from peridotites of deep origin indicate that the oxygen fugacity of these rocks is low, corresponding to an iron‐wüstite buffer. The relative abundance of the carbon isotopes C 12 and C 13 varies widely in natural gases. Methane formed by bacterial fermentation is highly enriched in the lighter isotope, while methane from deep deposits is much less so as is the methane flowing from hydrothermal vents on the East Pacific Rise. Except in extreme cases, the carbon isotope ratio cannot be used alone to assess whether methane is biogenic or abiogenic. The carbon isotope ratio in coexisting methane and carbon dioxide can be used to estimate the temperature at which the two gases came into isotopic equilibrium. This ratio indicates a high temperature of equilibration for a number of gas deposits. The carbon and helium isotope ratios together with their geologic settings are strongly suggestive that the large quantities of methane in Lake Kivu and the gases venting along the East Pacific Rise are abiogenic. Methane associated with the Red Sea brines and various geothermal areas may also be in part abiogenic. The high abundance of carbon in the Sun, the atmosphere of the outer planets, carbonaceous chondrites and comets, suggests that carbon may be more abundant in the Earth than it is in near‐surface igneous rocks. Such a high abundance could lead to a progressive outgassing of methane at depth, which then is oxidized near the surface or in the atmosphere. Methane hydrates are stable at low temperatures and high pressures. Today, methane hydrates are found in areas of permafrost and in ocean sediments. ... |
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