Control of CO2 and CH4 Contents in High-Heat Flow Sedimentary Basins
Werner F. Giggenbach
Extensive exploration of a wide range of geothermal systems for power production has led to the development of a large array of comparatively simple and adaptable techniques for the interpretation of the chemical and isotopic compositions of high temperature water and gas discharges. By applying these techniques to natural gas deposits in areas of comparatively high heat flow, in SE Asia and New Zealand, they were found to resemble closely lower temperature geothermal systems. In both cases, relative CH4/CO2 contents approach those expected for a redox environment governed by equilibration with crustal host rock. In this case, CO2 becomes the predominant C-bearing gas at temperatures >170°C.
Variations in the isotopic compositions of coexisting CH4 and CO2, in higher temperature gas deposits (>160°C), correspond to mixing of two end member source fluids: one containing predominantly CO2, the other predominantly CH4. At 170°C, CO2 is some seven times more soluble in water as compared to CH4 thus favoring partitioning of CO2 into any coexisting aqueous liquid phase and that of CH4 into the vapor phase. Entrainment of these two fluid phases in varying proportions is likely to be the major cause of rapid and short-distance variations in relative CO2 and CH4 contents of well discharges. In addition to lower temperature (<120°C) decarboxylation processes, the other major process giving rise to high CO2 contents in natural gas, is equilibration with host rock at temperatures >160°C. Generally speaking, with i creasing temperature, kinetic processes increasingly give way to thermodynamic processes controlling gas compositions.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California