Abstract

A baseline determination of CO 2 and CH 4 fluxes and soil-gas concentrations of CO 2 and CH 4 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 CO 2 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 CO 2 flux and a maximum CH 4 flux. The CO 2 fluxes were measured in the field with an infrared spectroscopic method. The CH 4 fluxes were determined from gas-chromatographic measurements on discrete samples from under the flux chambers. The CO 2 and CH 4 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-m 2 (10.763-ft 2) chambers, and soil gas was sampled at single points at each of the 40 locations.

Carbon dioxide fluxes averaged 227.1 mg CO 2 m −2 day −1, a standard deviation of 186.9 mg m −2 day −1, and a range of −281.7 to 732.9 mg m −2 day −1, not including one location with subsurface infrastructure contamination. Methane fluxes averaged 0.137 mg CH 4 m −2 day −1, standard deviation of 0.326 mg m −2 day −1, and a range of −0.481 to 1.14 mg m −2 day −1, not including the same contaminated location.

Soil-gas CO 2 concentrations increased with depth, averaging 618, 645, and 1010 ppmv at 30, 60, and 100 cm (11, 23, and 39 in.), respectively. Soil-gas CH 4 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 CH 4 with depth reflects a slow rate of methanotrophic oxidation, even during winter conditions. The δ 13C of the soil gas CO 2 was also determined in the soil-gas samples and in the atmosphere. These data demonstrated that the increased CO 2 with depth was derived from the biological oxidation of soil organic matter.

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