NOBLE GAS ISOTOPE GEOCHEMISTRY OF THE FARNHAM DOME FIELD, UTAH

U.S. Geological Survey analyses of carbon dioxide (CO₂) gas samples from Farnham Dome Field, Carbon County, Utah, have revealed two main preliminary results. First, helium (He) isotope geochemistry suggests an igneous source for the gas, however such a source is not known. Second, samples may show evidence of previously unreported internal reservoir heterogeneity and mixing. This work is an initial report from an investigation of the potential use of noble gas isotope geochemistry to establish the extent of regional CO₂ systems. Knowledge of CO₂ systems will greatly improve resource assessments and the use of natural sources as analogs in CO₂ storage.

The samples come from two wells, 1.25 miles apart, producing CO₂ from the Jurassic Navajo Sandstone at a depth of 3,200 feet. Farnham chemistry is similar to that of surrounding regional natural accumulations of CO₂. These fields exhibit gas concentrations with 80% or more CO₂. Helium isotope ratios relative to air, reported as R/Ra, of 0.409 to 0.440 point to a mixture of mantle and crustal sources in the gas. Studies of other CO₂ fields such as St. Johns Dome and McCallum Dome have explained similar ratios as evidence of magmatic sources of He. However, though the Farnham gas has similar noble gas isotope geochemistry to other southern Rocky Mountain CO₂ fields, it does not share their proximity to or association with igneous intrusions; in fact, the nearest known igneous rocks are ~60 miles away.

Previously published compositions for Navajo gas at Farnham show rather homogenous 99% CO₂ and traces of other gases. One of the wells from this study produced similar gas to published analyses. Previously unreported chemistry from the other well suggests a lack of exchange between the wells and possibly a secondary source of gas. Well #1 contains 99.8% CO2 with traces of other gases and well #2 gas compositions include 88.7% CO₂, 4.5% methane (CH₄), 6.4% nitrogen, and 0.4% He₂. A potential explanation includes mixing with down section Pennsylvanian gas of 83% CO₂ and 17% CH₄. Paleozoic faults are crosscut by the sampled wells; there is some seismic imagery evidence of minor post-Paleozoic reactivation that may have provided a mixing path. These initial results will form the foundation for an attempted delineation of a regional CO₂ system.