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