Helium – Relationships to other reservoir gases and implications for exploration: the New Mexico example

Helium is the second most abundant element in the universe after hydrogen but is relatively rare on earth. Helium occurs as two stable isotopes, 3 He and 4 He. 4 He is the dominant isotope in crustal gases and is a radiogenic decay product of uranium and thorium mainly in granitic basement rocks. 3 He is dominantly primordial and primarily originates from the earth’s mantle. 3 He may also be formed by radiogenic decay of 6 Li which may be found in argillaceous sediments deposited in evaporitic settings. Although He occurs in most natural gases, it almost always occurs in extremely low, subeconomic concentrations, less than 0.1%. It is rare in concentrations more than 1%. A very few small reservoirs have gases with more than 7% helium. Other gases that constitute the dominant components of helium-bearing natural gases are hydrocarbons (HC’s), carbon dioxide (CO 2 ), and nitrogen (N 2 ). The highest He concentrations occur where the dominant gas is N 2 but most He has historically been produced as a byproduct where the dominant gases are HC’s. HC’s are generated from petroleum source rocks. Their presence in a reservoir is dependent upon the presence of a mature source rock in the basin and a migration path between the source rock and the reservoir. Large accumulations of CO 2 in the southwestern U.S. resulted from the degassing of rising Tertiary magmas and subsequent migration of the gases into reservoirs. N 2 appears to originate mostly from degassing of the mantle but may also be formed by the thermal maturation of coals and subsequent the degradation of ammonia in pore waters. The presence of economic concentrations of He in reservoir gases is dependent not only on an adequate source of 4 He generated from granitic basement rocks but also on accommodating flux rates of HC’s, CO 2 and N 2 . These gases differ in their origins, places of generation and rates of generation. Economic concentrations of He occur where the reservoir is incompletely filled with either HC’s or CO 2 . These reservoirs contain elevated concentrations of N 2 in addition to the elevated concentrations of He. Exploratory drilling for He on Chupadera Mesa in the late 1990’s and early 2000’s encountered He-rich gases in Lower Permian and underlying Pennsylvanian clastic strata. Isotopic analyses suggest that 94% of Chupadera Mesa He originated from radiogenic decay in crustal rocks while 6% is derived from the mantle or possibly evaporitic Permian shales. Marked differences in the CO 2 concentrations in different strata indicate that some strata acted as carrier beds for CO 2 while N 2 -rich and CO 2 -poor reservoirs were isolated from CO 2 sources. Identification of CO 2 carrier beds is therefore pertinent to exploration in regions with substantial Tertiary or Quaternary volcanic activity.