Probabilistic Determination of the Role of Faults and Intrusions in Helium-Rich Gas Fields Formation

Abstract

Natural gas fields with economic helium (>0.3 He %) require the radioactive decay of crustal uranium (U) and thorium (Th) to generate He and tectonic/structural regimes favorable to releasing and concentrating He. An unknown is determining the role of faults and structural features in focusing deep-seated He sources on shallow accumulations. We tested the correlation between high-He wells (n = 94) and structural features using a new high-resolution aeromagnetic survey in the Four Corners area, USA. A depth-to-basement map with basement lineaments/faults, an intrusion map, and a flattened basement structural high map were created using Werner deconvolution algorithms by combining magnetic, gravity, and topography data with magnetic and gravity depth profiles. We show quantitatively (via analysis of variance) that a non-random process controls the relationship between He (>0.3%) and both basement faults and intrusions: 88% of high-He wells occur <1 km of basement faults; and 85% of high-He wells occur <1 km of intrusions. As He % increases, the distance to the structural features decreases. Strong spatial/statistical correlations of He wells to both basement faults and intrusions suggest that advective transport via faults/intrusions facilitates He migration. The role of gas phase buoyancy and structural trapping is confirmed: 88% of high-He occurs within basement structural highs, and 91% of the remaining wells are <1 km from intrusions (potential structural high). We present a composite figure to illustrate how a probabilistic approach can be used as a predictive model to improve He exploration success by targeting zones of intersection of basement faults and intrusions within basement structural highs.