FARNHAM DOME AND GRASSY TRAILS FIELDS, UTAH - CO2 ACCUMULATION SOURCES AND MIGRATION PATHS
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/RA = 0.415) that specifically preclude the Paleozoic carbonates of the Uinta Basin as a possible source where lower R/RA 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.