Experimental Study of the PVTX Properties of Water-Methane

Hydrothermal fluids containing small amounts of methane are common in many geological environments, including sedimentary basins, submarine hydrothermal systems and low grade metamorphic rocks. To better understand the behavior of these fluids, the phase equilibrium properties of water containing small amounts of methane (< 4 mol%) were determined using the synthetic fluid inclusion technique. Methane in the experiments was generated by the reaction of aluminum carbide with water to produce methane and aluminum hydroxide. Reaction products (methane and aluminum hydroxide) were verified by Raman spectroscopy and X-ray powder diffraction analyses, respectively. Concentrations of methane in the inclusions were calculated based on the reaction stoichiometry. A series of experiments demonstrated that using fresh aluminum carbide (from newly opened container) was critical to obtaining accurate compositions. This is because aluminum carbide decomposes slowly when exposed to humid air. Error analyses based on mass balance showed that with appropriate sample handling, the errors associated with the experimental technique were < 5% (relative) for methane concentrations < 4 mol%. Synthetic fluid inclusions containing < 4 mol% methane were trapped at 500-700° C, 1-3 kilobars. For a given formation temperature and pressure, the homogenization temperature decreases with increasing methane concentration. This behavior reflects both the changing slope of the fluid isochore as well as the increasing pressure on the solvus with increasing methane. Phase diagrams summarizing the PTX properties of the water-methane system will be presented.