JOURNALOF GEOPHYSICALRESEARCH VOLUME 66, No. 1 JANUARY 1961 Helium, Argon, and Carbonin SomeNatural Gases R. E. ZARTMAN AND G. J. WASSERBURG The Cali/ornia Institute o• Technology AND J. H. REYNOLDS University o.f Cali/ornia Berkeley, California Abstract. Thirty-nine samplesof natural gasesrepresenting varied chemicalcompositions and geologicaloccurrences were analyzed for their helium, radiogenicargon, and atmosphericargon contents.The total range in the (He/A)r•d ratio was found to be 1.6 to 130 with most samples having values between 6 and 25. This range of values is essentiallyequal to the productionratio from the uranium, thorium, and potassiumin average igneousrocks and a wide variety of sedi- ments. This indicates that all of these natural gaseshave obtained their radiogenic gasesfrom rather averagerock types. This is true in spite of the fact that the gasesrange in helium content from 37 to 62,000 ppm. A theoretical discussionof the origin of helium and argon in natural gasesis given. It can be shownfrom the ratio of nitrogen to atmosphericargon that most of the nitrogen in these gases cannot comefrom the entrapment of air. From a considerationof the concentrationof atmospheric argon in natural gasesit is possibleto estimate the proportion of gaseousand aqueous phases assumingdiffusive equilibrium. The isotopiccomposition of the carbonin the methane of these gaseswas found to be very light. It was shownthat for coexistingCH 4-CO 2pairs the carbondioxide was always isotopicallyheavier. INTRODUCTION reservoirgreatly enrichedin U and/or Th, or the accumulation from a rather normal The purposeof this study was to investigate the relationship between the abundances of rock reservoir.Lastly, there existsthe possibility helium and argon in natural gasesof different that the helium representssome more primordial compositionsand environmentsand to examine gases trapped in the earth during its early the isotopic compositionof the carbon in these history and subsequentlypartly releasedinto gases.Since the discoveryof terrestrial helium stratigraphicand structural traps. These various in natural gasesby Cady and McFarland [1906], possibilitieshave been recognizedby some of a vigoroussearch has beenmade for helium-rich the earliest investigators in the field [Rogers, gases,and a considerablenumber of total gas 1921]. It has been found [Faul, Gott, Manger, analyses that include helium determinations Mytton, and Sakakura,1954; Sakakura, Lindberg, exist in the literature. These have been made and Faul, 1959] that severalhelium wells, i.e., using both volumetric and mass spectrometric wells containingover 0.5 per cent helium, are techniques[Rogers, 1921; Andersonand Hinson, radioactive owing to a high radon content and 1951; Boone,1958]. appear to be associatedwith some uraniferous The origin of high helium natural gases,some petroleum residues. Other high helium wells, of which have helium contents as great as however, are devoid of such radioactivity. In 10 per cent, has beenthe subjectof considerable addition, many well gaseshave a high radon speculation. The He4 contained in them is contentand only smallconcentrations of helium presumablythe productof the radioactivedecay [Satterlyand McLennan, 1918]. The composition of U •'35,U•'% and Th 2•'-,and their intermediate of helium-rich gaseshas been found to be quite daughter products. A fundamentalquestion variable,although in somegas fields a correlation ariseswhether these quantities of helium repre- has been suggestedbetween helium and nitrogen sent thc accumulatcd dccay products of a content. 277 278 ZARTMAN, WASSERBURG, AND REYNOLDS The isotopiccomposition of helium from gas estimates of the contribution of 'atmospheric wells has been investigatedby Aldrich and Ni• gases. [1948]who report valuesof He•/He4 -- 10-7. EXPERIMENTAL TECHNIQUES The results of these authors represent the only publisheddata on He• content in terrestrial We have studied a variety of natural gases gases.The origin of the He• in thesegases has coveringa wide compositionaland environmental been consideredby severalworkers. Hill [1941] range in order to attempt some understanding suggestedthat this isotopeof helium could be of their origin. In this study we have restricted producedin rocksby the reactionLi6(n, a)H8-• our effortsto well gasesand have not investigated He• -]- /•-. More recently, Morrison and Pine volcanic or hot spring emanations.Thirty-nine [1955]have discussedthe relative productionof samples were analyzed for their helium and I-Ie• and I-Ie• in rocks. As shown by Wetherill argon contents and for isotopic composition. [1953, 1954], the principal sourcesof neutrons Partial gas analysesfor other major constituents are the reaction O•s(a, n)Ne • and spontaneous were also made. The isotopiccomposition of the fissionof U TM.By consideringthe variousnuclear total gas carbon, methane carbon, and carbon reactionswhich competefor neutrons,Morrison dioxide carbon were determined. The results of and Pine concludethat the He•/He• ratio this investigation are presentedin Tables 1 and observedby Aldrich and Nier in natural gases 2, together with pertinent well data for each is most reasonablythe productof theseprocesses sample. The accompanyingtotal gas analyses in materials which contain neither uranium nor representdata suppliedus by the participating thorium, or both, in very great concentration. petroleumand natural gascompanies, partial gas Their argumentswould not, however,preclude analyses performed in our laboratory, and a the originof this heliumto be fromfinely divided combination of these two sources. With the uranium minerals disseminated in a rather exception of three samples, the H•S contents normal rock. This would permit uranium con- were less than 0.01 per cent. For samples 14, centrationsof up to a few tenths of 1 per cent. 34, and 35, the H•.S contents were 0.04, 0.09, The radioactive decay of K •ø gives rise to and 0.12 per cent, respectively. the possibilityof high argon-containingnatural With the exceptionof samples29, 30, 31, and gases.Since potassium is a principalelement in 32, the well gaseswere collectedusing standard most crustal rocks as comparedwith uranium high-pressure,stainless steel gas cylinders with and thorium, a study of the ratio of radiogenic valves on both ends. The cylinders were con- heliumto radiogenicargon, (He•/A'ø)•, gives nected to the gas sourceand purged of air by more direct information on the possiblesource passingwell gas through them under positive of helium gas wells.• pressurefor several minutes. The outlet valve Someresults using this approachwere reported was then closed,and the pressurein the cylinder by Wasserburg,Czamanske, Faul, and Hayden was allowed to reach a satisfactoryvalue (20- [1957] for some helium wells in the Texas 3000 psig), and then the valve to the sourcewas Panhandle. These workers showed that the closed. All of these sample vesselswere at a argonin four heliumwells was about 70 per cent pressureof considerablyover I atmospherewhen radiogenicand that the ratio (He4/A•9•.awas they wereused for analysis.Samples 29-32 from about 10. This ratio was well within the values the Texas Panhandlegas field were obtained by to be expectedfrom the present-dayproduction Henry Faul in 1954. They were collected in rates of He• and A •ø in normal igneousrocks. glasscylinders that had openingsat both ends. They therefore concludedthat these helium After allowingthe gasto passthrough the cylin- wellswere probably formed from the accumulated dersat slightpositive pressure for a few minutes, radiogenicgases found in rockswith a ratio of both ends of the vesselwere sealedoff. In every U/K typical of normaligneous rocks, and that instancethe helium, argon, and carbon analyses in no way could these gasesbe the result of were done on the samesample. accumulation from an enriched uranium reservoir. The gas cylinderswere joined onto a vacuum For thesesamples it wasshown that the A•/A •6 line (Fig. 1) with metal-to-glasscouplings and ratio was the sameas that found in atmospheric the systemwas evacuatedwith all the furnaces argon and it was thereforepossible to make heated and outgassedbefore each run. After the HELIUM, ARGON, AND CARBON 279 *<---To hicjh vacuum Manometer Cold trap • t•ASample tubeM•c•Leodcjaucjefurnace Toe ==•..•fur Fig. 1. Vacuumapparatus used for helium,argon, and carbondetermination and extraction. systemwas foundto be vacuumtight, somegas waschecked by runningsamples of knownhelium was releasedinto a part of the systemof known concentrationand by comparingthe results of volume and the pressuremeasured on a manom- duplicate analysesin which the combustiontime eter. For most samples a tracer of A ss was and the amountsof gas were varied by a factor introduced and the gases mixed. The sample of 4. The purity was checkedby observingthe size rangedfrom 2 to 100 cc STP. For total gas Tesla discharge color. No detectable loss of carbon analyses, all gas was then pumped helium by diffusionthrough the fused quartz directlyinto the combustionsystem. For methane combustiontubes was observed.According to carbonanalyses, liquid N, was placedon a cold the work of Norton [1953] on the diffusion of trap and the noncond•nsiblefraction was helium through fused quartz at various tem- continuously pumped into the combustion peratures,it was expectedthat less than I per systemuntil no gas phaseremained. The com- cent of the helium would be lost by diffusion bustion procedure used was similar to that during the combustionprocedure. Blank runs described