NOBLE GASES RECORD DEPTH OF GAS PHASE FORMATION IN N2/HE SEEPS

Commercial helium (He) discoveries rely on successful identification of subsurface traps where He-rich fluids have formed a gas phase. This may happen if an external gas phase (e.g. methane or CO2) migrates through groundwater stripping He which becomes concentrated in the gas phase. These types of He occurrences can be economical, however, the addition of external gas phase dilutes the He contents and increases the carbon footprint of exploration. The highest He concentrations (1-10%) are observed where He primarily occurs with nitrogen (N₂), co-produced in the continental crust (1–3). In N₂/He systems, gas phase formation occurs when N₂ in the fluid reaches a saturation limit (bubble point), which could be triggered by pressure or temperature change (e.g. uplift or a thermal event). To produce a commercially viable gas formation, bubble point must be exceeded at or below the reservoir depth. Using a global data compilation of N₂/He-rich surface seeps, we propose an inverse modelling technique to reconstruct the depth of gas phase formation. Atmospheric noble gases (²⁰Ne, ³⁶Ar, ⁸⁴Kr, ¹³⁰Xe) occur in water in predictable ratios, dependent on temperature conditions at the time of recharge. During phase separation in the subsurface, atmospheric noble gases are partitioned into the gas phase based on relative gas/water volumes. We use inverse modelling to determine the best fit water volumes for the observed noble gas ratios, including additional parameters for excess air and Kr/Xe enrichment. Comparing water volumes to N₂ concentrations, we determine the depth of gas phase formation. The bubble point depth derived from surface seeps can be used to identify whether proximal geological traps are above or below the point of gas phase formation, critical in determining their viability in forming a primary N₂/He gas field.

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