Radiogenic 4He is naturally produced in Earth's crust due to alpha decay of Uranium (U) and Thorium (Th). Helium has unique thermodynamic properties required for the medical imaging industry, aerospace and other fields of high-tech manufacturing, and currently is in increasingly high demand. Despite its economic value, the mechanisms of helium migration and retention in sedimentary basins remain poorly understood.
Oil and gas fields with economic helium (>0.3%) concentrations have been discovered in Paleozoic intervals in the Colorado Plateau, southwestern USA. Here we report new noble gas isotope and abundance data for gas samples (n = 31), from actively producing Paleozoic formations within five fields: Ratherford, Tocito Dome, Navajo Springs, Pinta Dome, and Dineh-Bi-Keyah. Helium concentrations range from 0.01% to 7.9% with varying amounts of liquid and gaseous hydrocarbons, N2, and CO2. We present multi-stage gas, water, and oil equilibration models to account for the observed noble gas elemental and isotopic signatures. Oil-dominated systems are explained by a closed system oil/water equilibration and subsequent admixture of air. He-rich dry gas samples exhibit uniform 4He/N2 ratios consistent with the regional mean values, suggesting a common crustal source and no subsequent fractionation. In contrast, air-derived 20Ne/36Ar ratios are highly fractionated. These observations are consistent with a tectonically controlled crustal gas release from the basement, groundwater saturation with 4He and N2, and subsequent degassing. Extensive gas-water interaction (i.e., migration) leads to extreme fractionation of 20Ne/36Ar, but does not affect 4He/N2 due to water saturation with crustal gases released from the basement.
We show the volume of rock required to have produced helium in the reservoir to be significantly larger than the current reservoir volume immediately beneath the field. Therefore, the reservoir helium concentration cannot be sourced by in-reservoir decay of U and Th and instead requires a process to incorporate exogenous sources of helium in the reservoir without significant dilution from hydrocarbons. For helium-rich fields, excess helium is sourced from the Precambrian granitic basement likely utilizing a large area beneath the field area (i.e., crustal gas mobilization and transport via fracture zones), consistent with the degree of water contact. Deep crustal faults in the Precambrian basement are in close proximity to the high helium fields, indicating that these structures are potentially serving as primary migration conduits via advective fluid flow.