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. ⁴He is derived primarily from alpha decay of ²³⁸U, ²³⁵U and ²³²Th in granitic basement rocks. Both isotopes appear to migrate into sedimentary crustal reservoirs through deep-seated faults and fracture systems, although ³He may also be introduced into the crust through rising magmas. 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 He. It is possible to differentiate between mantle-derived N₂ and sedimentary N₂ by cross plotting He vs N₂ contents of gases.

Evaluation of U and Th concentrations in basement rocks via the drilling of deep wells through thousands of feet of basement with U and Th concentrations can be assessed either through analysis of continuous core or drill cuttings or with spectral gamma-ray logs, but such deep wells are extremely rare and are expensive to obtain. The work summarized in this abstract investigates if the generative capability of the basement may be indirectly assessed. In natural gases, concentrations of mantle-derived N₂ increase linearly with He concentrations. The slope of the He vs. N₂ plots (the He:mantle N₂ ratio) varies across New Mexico. Areas with higher He:N₂ ratios indicate greater contents of He relative to mantle-derived N_2­.

In the commercially produced Abo (Lower Permian) gases in Chaves County, New Mexico, in the northwestern part of the Permian Basin at the Pecos Slope Abo gas pools, the slope of the He:N₂ plot is 0.076. The He and mantle N₂ have migrated into the Abo reservoirs via deep-seated SW-NE trending strike-slip faults. To the southeast, gases in Lower Pennsylvanian reservoirs have an He:N₂ ratio of 0.034. Further to the east in Lea County where He contents are much lower, the He:N₂ ratio is 0.014 in Lower Permian gases and 0.018 in Lower Pennsylvanian gases. 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.

The He: mantle N₂ ratios of reservoir gases that have been produced for their He content (Pennsylvanian and Mississippian reservoirs of western San Juan County and the Lower Permian Pecos Slope Abo reservoirs of western Chaves County) are significantly higher(0.072 to 0.083) than the He: mantle N₂ ratios of gases recovered from Siluro-Ordovician, Lower Pennsylvanian and Lower Permian reservoirs in eastern Chaves, Lea and eastern Eddy counties. The He: mantle N₂ ratios vary from area to area but less so among different strata within the same area. These variations indicate differences in the flux of He relative to mantle-derived N₂ in different areas. Larger He:N₂ ratios may indicate enhanced production of ⁴He in the basement or may indicate lower or suppressed migration of N₂ from the mantle. That the first is indicated is suggested by the higher He content of gases with higher He:N₂ ratios. This relationship may aid in the indirect assessment of the relative He-generation capacity of basement in different areas.