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Title: Prediction of Natural Gas Composition in Ultradeep Sandstone Reservoirs
Author(s): Colin Barker and Nicolas E. Takach
Annotation: Petroleum generation is kinetically controlled and depends on time and temperature but is essentially independent of rock type. Ultimately, it leads to a thermodynamically stable assemblage with a composition that is controlled by temperature, pressure, and local mineralogy. We used a free-energy minimization computer program to calculate the composition of gases in thermodynamic equilibrium with various mineral assemblages in clastic reservoirs from 10,000 to 40,000 ft (3048-12,192 m). In clean sand reservoirs, methane concentration falls with increasing depth and is eliminated by 25,000 ft (7620 m). If a graphitic residue is present, additional methane is generated, and methane is still present at 40,000 ft (12,192 m). It appears that gas composition is influenced by the balance between the reaction of graphite with water to give methane, and the oxidation of methane by water to form carbon dioxide and hydrogen. In systems with iron minerals, hematite decreases and magnetite increases as methane is oxidized. High temperatures that accompany increasing depth of burial lead to the thermal decomposition of calcite cements, and the carbon dioxide generated dilutes any methane to give uneconomic gas compositions. Hydrogen sulfide is generated when the reservoir mineral assemblage contains sulfur-bearing minerals, such as pyrite and anhydrite. This becomes significant in the deeper part of the depth range studied. Thermodynamic calculations show that equilibrium gas composition in the deep subsurface is a strong function of reservoir mineralogy.
Bibliographical description: Prediction of Natural Gas Composition in Ultradeep Sandstone Reservoirs //Colin Barker and Nicolas E. Takach. - The American Association of Petroleum Geologists Bulletin V. ?6, No. 12 (December 1992), P, 1859-1873, 12 Figs., 6 Tables.
Publication's type: статья
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