FARNHAM DOME AND GRASSY TRAILS FIELDS, UTAH - CO2 ACCUMULATION SOURCES AND MIGRATION PATHS

Analyses of carbon dioxide (CO₂) gas samples from Farnham Dome and Grassy Trails Fields in Carbon and Emery Counties, Utah, suggest that, 1) the gas present at the fields is from a similar source, and 2) that previously proposed long distance (65-100 kilometer) migration histories in the literature are not well supported by this data. The fields are located 13 kilometers apart and contain high concentrations of CO₂ in Permian to Jurassic units. The gas samples analyzed in this study are from the Jurassic Navajo Sandstone.

Bulk gas composition, stable isotope and noble gas isotope data from 5 samples at 3 wells at the fields are tightly grouped and show that the gases in the two fields are nearly identical and therefore would share the same source and migration path. Ratios of radiogenic and nucleogenic produced isotopes (avg. ⁴He/⁴⁰Ar = 5.15, avg. ⁴He/²¹Ne = 27.7 x 10⁶) are close to calculated crustal production values. Average ratios of atmospherically derived isotopes (²⁰Ne/³⁶Ar and ⁸⁴Kr/³⁶Ar) are mainly attributed to single stage partitioning from air saturated water to the gas phase. The lack of mass fractionation exhibited in the atmospheric, radiogenic and nucleogenic noble gas isotopes demonstrates that the gases did not migrate in the fractionation-prone gas phase, but rather as dissolved constituents in groundwater exsolved in a single-stage water to gas partitioning model.

Previous theories in the literature suggest that the gas may have migrated over long distances from either Paleozoic carbonates in the Uinta Basin or distant igneous intrusions. New geochemical data collected for this analysis, particularly from noble gas isotopes, do not support those suggested migration histories. The large scale migration in fluid or gas phases would fractionate the uniform radiogenic/nucleogenic and atmospheric ratios greater than that observed in the data. In addition to the low mass fractionation observed, the gases have low He concentrations and non-crustal type ratios of ³He/⁴He compared to the ratio in air (avg. R/R_A = 0.415) that specifically preclude the Paleozoic carbonates of the Uinta Basin as a possible source where lower R/R_A and higher He concentrations would be expected. Ongoing regional sampling may resolve these theories and form the foundation for an attempted delineation of a regional CO₂ system.