Helium in the Basement: A new journal article discusses what this can tell us about New Mexico’s subsurface geology

Emeritus Bureau of Geology researcher Ron Broadhead recently published a paper titled “He:N₂ Ratios of Gases and What They Can Tell Us About Basement Generating Capacity of Helium: Clues from New Mexico” in the journal The Mountain Geologist. His work provides a new method for understanding the geology of basement rocks across New Mexico by analyzing the ratios and isotopic compositions of helium and nitrogen gases. In addition to providing a better understanding of basement rock characteristics, this method may also provide an indirect assessment of the capacity of basement rocks to generate helium, an economic resource produced in New Mexico since the 1940s.

You can read the paper’s abstract below and contact the author directly (Ron.Broadhead@nmt.edu) for a full reprint.

Abstract

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. ⁴He is derived primarily from alpha decay of ²³⁸U, ²³⁵U and ²³²Th. The sources of ⁴He are found primarily in granitic basement rocks. The He migrates into sedimentary crustal reservoirs through deep-seated faults and fracture systems in the basement. N₂ in crustal gases is derived from the mantle and also from sedimentary sources: 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 within the basement. It is possible to differentiate between mantle-derived N₂ and sedimentary N₂ by cross plotting He vs N₂ concentrations of gases.

U and Th concentrations in basement rocks 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 expensive to drill. The work summarized in this paper investigates if the generative capability of the basement may be indirectly assessed through analysis of He and N₂ concentrations in gases. Concentrations of helium increase linearly with mantle-derived N₂ concentrations. The slope of He vs. N₂ plots (the He: mantle N₂ ratio) varies across New Mexico. Areas with higher He:N₂ ratios indicate greater concentrations of helium relative to mantle-derived N₂ .

He: mantle N₂ ratios vary substantially from area to area. The variation is considerably less among different stratigraphic intervals within a given area than between different areas. In the northwestern part of the Permian Basin in central Chaves County, the He:N₂ ratio for Lower Permian gases is 0.076; average helium concentration is 0.486%. 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 concentrations are 0.026% (Lower Permian), 0.027% (Lower Pennsylvanian), and 0.108% (Siluro-Ordovician). In northwestern New Mexico on the Four Corners Platform and in the San Juan Basin where helium has been commercially produced since the 1940’s, the He:N₂ ratio is 0.083 in Pennsylvanian reservoirs and 0.072 in Mississippian reservoirs; average helium concentrations are 3.23% (Pennsylvanian) and 4.21% (Mississippian). Higher helium concentrations in 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 substantially larger helium concentrations in the gases. This concept may aid in the indirect assessment of the helium source rock in a helium gas system.