New Mexico Geological Society Annual Spring Meeting — Abstracts

Helium (He) and Nitrogen (N₂) gases occur together in reservoirs but have different sources. He occurs as two isotopes, ³He and ⁴He. ³He is primordial and is derived from the mantle. In gases in crustal reservoirs ⁴He is dominant and is derived primarily from alpha decay of ²³⁸U, ²³⁵U and ²³²Th. ⁴He is generated primarily in granitic basement rocks. The ⁴He utilizes deep-seated faults and fracture systems to migrate into crustal reservoirs. N₂ in crustal gases is derived from the mantle and also from sedimentary sources including maturation of sedimentary kerogens, maturation of coals, and from several processes in red-bed sequences. Mantle derived N₂ is introduced into the crust through plutonic magmatic activity and volcanism and utilizes the same migration pathways as helium to migrate into crustal reservoirs. It is possible to differentiate between Mantle-derived N₂ can be distinguished from sedimentary N₂ by cross plotting He vs N₂ contents of gases.

The ⁴He sources can be directly assessed via the drilling of deep wells through thousands of feet of basement with U and Th concentrations measured either through analysis of continuous core or drill cuttings or with spectral gamma-ray logs. However, such deep wells are extremely rare and are prohibitively expensive to drill for He exploration. The work summarized in this talk investigates if the generative capability of the basement may be indirectly assessed through analysis of He and N₂ contents of gases. Concentrations of mantle-derived N₂ increase linearly with He concentrations. The ratio of He to mantle N₂ varies across New Mexico and can be determined by plotting He vs N₂ contents of reservoir gases. The He:mantle N₂ratio is determined by the slope of a best-fit linear regression line Areas with higher He:N₂ ratios indicate greater contents of He relative to mantle-derived N_2­.

He:N₂ ratios and He contents of natural gases vary across New Mexico. In the commercially produced Abo (Lower Permian) gases in central Chaves County in the northwestern part of the Permian Basin, the slope of the He:N₂ plot is 0.076 and the average helium content of the Abo gases is 0.486%. In southeastern Chaves County Lower Pennsylvanian gases have an He:N₂ ratio of 0.034; average helium content of these gases is 0.036%. Siluro-Ordovician gases in this area have an He:N₂ ratio of 0.039 and an average He content of 0.037%. Further to the east in Lea County, the He:N₂ ratio is 0.014 in Lower Permian gases, 0.018 in Lower Pennsylvanian gases, and 0.039 in Siluro-Ordovician gases; average helium contents are 0.026% (Lower Permian), 0.027% (Lower Pennsylvanian), and 0.108% (Siluro-Ordovician). In northwestern New Mexico on the Four Corners Platform where He has been commercially produced since the 1940’s, the He:N_2­ ratio is 0.083 in Pennsylvanian reservoirs and 0.072 in Mississippian reservoirs; average helium contents are 3.23% (Pennsylvanian) and 4.21% (Mississippian).

He: mantle N₂ ratios vary substantially from area to area. The variation is considerably less among different stratigraphic intervals within a given area. Furthermore, higher He concentrations in the gases coincide with higher He:N₂ ratios. The larger He:N₂ ratios may be the result of enhanced He migration upward through basement fractures and therefore increased generating capacity of helium in the basement. Alternatively, the higher He:N₂ ratios may be the result of subdued migration of N₂ from the mantle. That the larger ratios result from enhanced helium generating capacity in the basement is suggested by the substantially larger helium concentrations in the gases with larger He:N₂ ratios. This concept may aid in the indirect assessment of helium source rocks in helium gas systems.

References:

1. 1. Broadhead, R.F., 2023, Helium - Relationships to other reservoir gases and some implications for exploration: The New Mexico example: The Mountain Geologist, v. 60, no. 4, p. 141-158. https://doi.org/10.31582/rmag.mg.60.3.141

   2. Broadhead, R.F., 2025, He:N2 ratios of gases and what they can tell us about basement generating capacity of helium: Clues from New Mexico: The Mountain Geologist, v. 62, no. 3, p. 199-213 .https://doi.org/10.31582/rmag.mg.62.3.199