Helium in Natural Gas and Thermochemical Sulfate Reduction (TSR) Influence
Helium is present in many natural gases however the provenance of Helium is not the same as that which generated the hydrocarbon gases. It is proposed that the source and concentration of Helium in hydrocarbon reservoirs may be the result of a two-phase model. In Phase 1, Helium in reservoirs may be independently trapped by the same subsurface mechanisms that also trapped the hydrocarbons. All economic Helium accumulations require a migrating gas phase (e.g. Natural gas, CO2, N2) in contact with pore waters to enable Helium to be efficiently ‘stripped’ or partitioned from pore waters into the gas phase or other understaurated condition. In the case of a typical hydrocarbon field this migrating phase includes a ‘charge’ of hydrocarbon gas travelling along permeable migration conduits until focused towards a geological reservoir structure and allowed to accumulate (‘trapped’). This therefore explains why the greatest helium concentrations are typically found in natural gas accumulations. In Phase 2, secondary post-accumulation reservoir alteration processes such as thermochemical sulfate reduction may significantly affect the Helium concentrations, albeit indirectly. Concentrations of Helium in affected fields may be elevated 2-4x times higher than that regionally observed. The presence of elevated H2S and N2 in these fields coupled with the fact that some reservoirs are in association with anhydrite and are at subsurface temperatures > 125 °C suggests thermochemical sulfate reduction geochemical processes are responsible. The TSR reactions serve to chemically destroy and reduce hydrocarbon concentrations which has the effect of enhancing and thereby increasing the original ‘inert’ Helium concentrations in a field. This happens to be a positive for Helium concentrations.
AAPG Datapages/Search and Discovery Article #90337 ©2019 AAPG Middle East Region GTW, Regional Variations in Charge Systems and the Impact on Hydrocarbon Fluid Properties in Exploration, Dubai, UAE, February 11-13, 2019