GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 92-4
Presentation Time: 8:50 AM


BRENNAN, Sean T., Eastern Energy Resources Science Center, U.S. Geological Survey, 956 National Center, Reston, VA 20192,

Bravo Dome is a natural CO2 reservoir in north-eastern New Mexico where CO2 is produced for enhanced oil recovery. This study involves analyses of gas from ten producing gas wells, as well as one shallow effervescent water well, all of which are west of any previously published data, to help identify the source of the CO2, the location of the charge into the reservoir, and migration within the reservoir.

Ranges of the data from these gas wells are consistent with published reports: CO2 concentrations of 98.77 to 99.86 mol% CO2, δ13C values of -6.75 to -3.7, and R/RA values ranging from a high of 3.75 in the east to 0.67 to the west. These values are similar to reported values but with an opposite geographic trend. The R/RA values reported here and in published reports appear to be directly correlated to 4He concentrations, which are highest along the margins of Bravo Dome.

The published reports of CO2/3He ratios are highest in the west (4.82), and decrease to the east (2.25), and are used to interpret greater CO2 dissolution to the east as CO2 is more soluble than 3He. The CO2/3He ratio in this study ranges from 0.48 in the west to 3.59 within Bravo Dome, which indicates that CO2 dissolution within this sample set is instead increasing to the west. These opposite trends may validate a published hypothesis that the CO2charge is located in the north-western portion of Bravo Dome, essentially between the wells of the two datasets. If that is the case, it is possible that the reservoir is filling both updip to the west and downdip to the east.

The gas from the shallow water well contained gas that was 99.24 mol% CO2, with a δ13C of -4.06, and a R/RA of 2.28, which indicates a similar source as the gas in Bravo Dome. The surficial sediments around the well are likely from the Santa Rosa Formation, which is productive within Bravo Dome, but that production is over 50 km north east of the effervescing well. The water well data indicate that gas similar to that of Bravo Dome leaks to the near surface. There is no reported evidence of leakage of CO2 to the surface at Bravo Dome, which has been used as an indication that Bravo Dome is stable and can be used as an analogue for CO2 storage projects. However, either gas from Bravo Dome is leaking to the southwest or there is a secondary migration pathway from the source that charges shallow strata southwest of Bravo Dome.