Detailed compositional analysis of gas seepage at the National Carbon Storage Test Site, Teapot Dome, Wyoming, USA

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 No. 3 (NPR-3) in Wyoming, USA. This was done in anticipation of experimentation with CO 2 sequestration in the Pennsylvanian Tensleep Sandstone underlying the field at a depth of 1680 m. The baseline data were collected during the winter, 2004 in order to minimize near-surface biological activity in the soil profile. The baseline data were used to select anomalous locations that may be the result of seeping thermogenic gas, along with background locations. Five 10-m holes were drilled, 3 of which had anomalous gas microseepage, and 2 were characterized as “background.” These were equipped for nested gas sampling at depths of 10-, 5-, 3-, 2-, and 1-m depths. Methane concentrations as high as 170,000 ppmv (17%) were found, along with high concentrations of C 2H 6, C 3H 8, n-C 4H 10, and i-C 4H 10. Much smaller concentrations of C 2H 4 and C 3H 6 were observed indicating the beginning of hydrocarbon oxidation in the anomalous holes. The anomalous 10-m holes also had high concentrations of isotopically enriched CO 2, indicating the oxidation of hydrocarbons. Concentrations of the gases decreased upward, as expected, indicating oxidation and transport into the atmosphere. The ancient source of the gases was confirmed by 14C determinations on CO 2, with radiocarbon ages approaching 38 ka within 5 m of the surface. Modeling was used to analyze the distribution of hydrocarbons in the anomalous and background 10-m holes. Diffusion alone was not sufficient to account for the hydrocarbon concentration distributions, however the data could be fit with the addition of a consumptive reaction. First-order rate constants for methanotrophic oxidation were obtained by inverse modeling. High rates of oxidation were found, particularly near the surface in the anomalous 10-m holes, demonstrating the effectiveness of the process in the attenuation of CH 4 microseepage. The results also demonstrate the importance of CH 4 measurements in the planning of a monitoring and verification program for geological CO 2 sequestration in sites with significant remaining hydrocarbons (i.e. spent oil reservoirs).