Baseline studies of surface gas exchange and soil-gas composition in preparation for CO2 sequestration research: Teapot Dome, Wyoming

Abstract

A baseline determination of CO2 and CH4 fluxes and soil-gas concentrations of CO2 and CH4 was made over the Teapot Dome oil field in the Naval Petroleum Reserve 3 in Natrona County, Wyoming, United States. This was done in anticipation of the experimentation with CO2 sequestration in the Pennsylvanian Tensleep Sandstone underlying the field at a depth of 5500 ft (1680 m). The measurements were made in January 2004 to capture the system with minimum biological activity in the soils, resulting in a minimum CO2 flux and a maximum CH4 flux. The CO2 fluxes were measured in the field with an infrared spectroscopic method. The CH4 fluxes were determined from gas-chromatographic measurements on discrete samples from under the flux chambers. The CO2 and CH4 were determined at 30-, 60-, and 100-cm (11-, 23-, and 39-in.) depths in soil gas by gas chromatography. A total of 40 locations had triplicate flux measurements using 1.00-m2 (10.763-ft2) chambers, and soil gas was sampled at single points at each of the 40 locations. Carbon dioxide fluxes averaged 227.1 mg CO2 m2 day1, a standard deviation of 186.9 mg m2 day1, and a range of 281.7 to 732.9 mg m2 day1, not including one location with subsurface infrastructure contamination. Methane fluxes averaged 0.137 mg CH4 m2 day1, standard deviation of 0.326 mg m2 day1, and a range of 0.481 to 1.14 mg m2 day1, not including the same contaminated location. Soil-gas CO2 concentrations increased with depth, averaging 618, 645, and 1010 ppmv at 30, 60, and 100 cm (11, 23, and 39 in.), respectively. Soil-gas CH4 concentrations averaged 0.128, 0.114, and 0.093 ppmv at 30, 60, and 100 cm (11, 23, and 39 in.), respectively. The decrease in CH4 with depth reflects a slow rate of methanotrophic oxidation, even during winter conditions. The 13C of the soil gas CO2 was also determined in the soil-gas samples and in the atmosphere. These data demonstrated that the increased CO2 with depth was derived from the biological oxidation of soil organic matter.