HYDROGEOCHEMICAL ANALYSIS OF LEAKING CO2-CHARGED FAULT ZONES: THE LITTLE GRAND WASH AND SALT WASH FAULT ZONES, EMERY AND GRAND COUNTIES, UTAH

The Little Grand Wash and Salt Wash fault zones in the Colorado Plateau near Green River, Utah, leak CO₂-rich gases to the surface through several saline springs, seeps, and geysers (12865 to 21189 mg/l TDS). These fault zones are an analogue for CO₂ sequestration that has failed to contain the gas and is therefore an engineering failure analysis of a system that cannot contain pressurized fluids. This hydrogeochemical study of the fluids emanating from the faults aims to locate the sources and determine the chemical evolutions of the fluids. d²H and d¹⁸O isotopic data show that the ground waters are meteoric and have not circulated deeply enough to experience an oxygen-isotope shift. Solute chemistry indicates that the Wingate Formation aquifer may be contributing to the emanating waters. d¹³C data and P_CO2 values indicate that the gas is external to the ground water systems (i.e., not from soil zone gas or dissolution of carbonate aquifer material alone). A ³He/⁴He ratio of 0.302 from a geyser in the Little Grand Wash fault zone and a 0.310 ratio from a bubbling spring in the Salt Wash fault zone indicate that the majority of the gas is crustally derived and contains a minimal component of mantle or magmatic gases. Sulfate reducing bacteria and methanogenic bacteria are probably not major contributors of the gases. The gas is likely sourced from depths of 1 km or greater, and possible sources include the thermal degradation of carbonates or organic matter, the oxidation of oil by mineralized waters, or diagenetic reactions in the siliceous-carbonate environment. Calculation of saturation indices by computer modeling indicates oversaturation of the waters with respect to aragonite, calcite, dolomite, and quartz. Numerous tufa, travertine, and carbonate vein deposits are located in both fault zones. d¹³C values of 4 to 5‰ from the veins indicate the possible carbon sources of dissolution of isotopically heavy marine carbonates or the thermal decarbonization of carbonates. Thus, our conceptual model is that gases from 1 km or greater in the basin are migrating upwards along the faults and charging shallower ground water systems, which then leak to the surface. Carbonate precipitation in the form of tufa, travertine, and veins near the surface indicate that CO₂ migration also resulted in CO₂ sequestration at high stratigraphic levels.