, ~ ( j ? j f GeoehelDieaI Studies of Natural Gas I PART m. INERT GASES IN WESTERN CANADIAN NATURAL GASES* ~ By BRIAN HITCHONt -j (This is the last of a Series of three Parts presented by the author tor publication in the Journal. Parts I I, and II appeared, in the Summer and Fall issues respectively.) ABSTRAGr 1 INTRODUcrION GEOCHEMISTRY AND ORIGIN OF In natural gases, the stratigra­ NITROGEN l phic and geographic variations in J T HIS paper is the third aud final Downloaded from http://onepetro.org/JCPT/article-pdf/2/04/165/2165406/petsoc-63-04-03.pdf by guest on 30 September 2021 1 the contents of both nitrogen and part of a series concerned with Before considering this subject heIiwn are of geochemical interest, further it is very important to con­ although only helium is of com­ the geochemistry of natural gas in 1 mercial importance. Nitrogen may Western Canada. It is a discussion sider the units in which the analy­ l originate from a great variety of of the inert gases nitrogen and he­ tical data are reported. Ideally, sources, including air, either orig­ lium. The first paper (1) dealt knowledge of the mass of the in­ J inally trapped in the sediments or '~ introduced dissolved in percolating with the hydrocarbons and the sec­ dividual components of natural groWldwaters, the denitrification of ond (2) with the acid gases. Due gases in the sedimentary basin is nitrogenous compounds or the de­ to their generally unreactive na­ desirable. From the reports of cay of certain radioactive minerals. ture, chemically, compared to hy­ Buckley and his co-workers (5) it In contrast, there exist radioac­ drocarbons or the acid gases, both may be surmised that the bulk of tive sources for all natural helium, nitrogen and helium are common­ the material is methane, with much and thus the problem of the migra­ lesser amounts .of other components. tion and accumulation of helium ly classed as inert gases. It is con­ in rocks differs from that of the venient to discuss them together in The concentrations reported in this other components of natural gases. this series of papers. series of papers are in volume per Helium owes .its origin mainly to cent. Consideration of regional the radioactive decay of uranium variations of the partial pressure and thorium, and its accumulation Although they differ in their to the differential rates of upward modes of origin, migration and ac­ of some components in various stratigraphic units indicates that diffusion through the sedimentary cumulation, both nitrogen and he­ strata from the basement rocks. the isoconcentration patterns for The geochemical evidence gives lium are of interest geochemically. partial pressure are essentially sim­ credence to a mathematical model With respect to both inert gases, ilar to those for volume per cent in of the diffusion process, with spe­ the discussion will follow the se­ cial reference to helium. the same respective units and for quence established in the first paper the same components. lContents of nitrogen ranging of this series. from about 8 per cent to 85 per In view of the origin of the hy­ cent characterize natural gases in southern Alberta from strata rang­ Nitrogen is the chief component drocarbons cited in a previous'pa­ _:',0 .. ing in age from Upper Devonian to of the Earth's atmosphere, com­ per (ll, it is important to realize Lower Cretaceous. In strata of the prising 78 per cent by volwne. It that high concentrations of nitro­ ~< Middle Devonian Elk Point Group, gen (and also of acid gases and ;':, , the amount of nitrogen in natural is most probably mainly a product '. :..-­ gases increases to a maximum of of the early degassing of the crust, helium) in some natural gases may nearly 20 per cent as the deposi­ although some may be a remnant result more from the lack of gen­ tional edge in the Peace River re­ from an early primordial atmos­ eration of hydrocarbons than from gion is approached. ElseWhere, in the generation of large' quantities other stratigraphic units, concen­ phere (3, p. 441). Its extraction trations are generally less than 10 from natural gas is therefore of of nitrogen. In part, a lack of­ per cent. Concentrations of helium little commercial importance. He­ generation of hydrocarbons may ac­ generally over 0.1 per cent are lium is of great economic value, count for the difficulty experienced found predominantly in natural in detennining the'cause or region­ gases from the Peace River arch, and the demand in North America the Sweetgrass arch and the hinge is expected to increase from the al variations in the content of nit­ belt of the Alberta basin. Contents present rate of 600 million cubic rogen in natural gases in Western in the range of 1.0 to 1.9 per cent feet per year to over 2 billion cubic Canada. helium have been found in the conunercial gas occurrences in Up­ feet per year by 1980 (4). At tWs Nitrogen in natural gases may per Cambrian strata in southern time (1963), the United States Gov­ be generated in a variety of ways. Saskatchewan. "'.' ernment controls virtually all pro­ The high content of nitrogen in the duction and distribution of helium atmosphere and its inert character ! and consumes over 75 per cent of have been noted by many writers, 1 the heIiwn produced in the free *Oontribution No. 242, Research who have attributed its presence in Council of Alberta: world. The heliwn-rich natural natural gases to air originally trap­ gases discovered recently in Saskat­ ped in the sediments. The absence chewan are therefore of consider­ of oxygen in natural gas is believed .. ;: able importance, and the technical tResearch Geologist, Research to be due to its removal during de­ Council of Alberta, Edmonton, Al­ and economic aspects of helium composition of the associated or­ berta, Canada. production from these gases have ganic matter and its affinity for been evaluated (4). metals, especially iron. Technology, Winter, 1963, Calgary 165 -.', . Nitrogen may originate from the Hoering and Moore (15) showed hypothesis merits further stUdy, denitrification of nitrates and nit­ that wide variations exist between and provides yet another possible rites by bacteria, and Rybakova the Nl5/N14 ratio of natural gases source for nitrogen in natural gases. (6) has found denitrifying bacteria and the associated crude oils, and in crude oil. Kaveev (7) concluded that the natural gas was relatively that the nitrogen in natural gases enriched in the lighter isotope. GEOCHEMISTRY AND ORIGIN OF at Tatar, U.S.S.R., appeared to be They attributed this difference HELIUM a product of the decomposition of to an origin of the nitrogen There exist radioactive sources organic sediments. A relationship through decomposition of the nitro­ for all natural helium (8), and thus Inight be expected, therefore, be­ genous compounds in crude oil or the problem of the migration and tween the content of nitrogen in sedimentary material. As the ra­ accumulation of helium in rocks natural gas and the total nitrogen tio of nitrogen to argon in natural differs from that for the other com­ in the associated crude oil. Nitro­ gas is much higher than in the ponents of natural gases. gen and other inert gases are pres­ atmosphere, they thought it un­ The fact that helium is a prod­ ent dissolved in sea water, and a likely that the nitrogen represented uct of the decay of such natural portion of the nitrogen in natural the remains of air trapped in the radioactive elements as uranium gas may originate by its release sedimentary material that fonned and thorium led early workers to from trapped sea water. A rela­ the source of the natural gas. At Downloaded from http://onepetro.org/JCPT/article-pdf/2/04/165/2165406/petsoc-63-04-03.pdf by guest on 30 September 2021 postulate that its origin in natural Lwnship between the nitrogen con­ the same time they observed sys­ gases is due to the presence of such [(:n t of natural gas and the degree 1 tematic variations in the N 5/N14 elements in either the sedimentary th(~ of wave turbulence to which ratio across a natural gas field, and rocks or the associated fonnation sediments have been subjected may attributed these variations to frac­ fluids (11, 16, 17, 18, 19, 20), In­ therefore be anticipated. tionation of the nitrogen isotopes deed, FaIT and Rogers (21) and 'Moderate quantities of nitrogen during migration. As neither the Rogers (22) even went so far as are present in certain radioactive ratio of the argon isotopes in these to suggest that petroleum originat­ minerals (8). This nitrogen could natural gases, the ratio of nitrogen ed by the action of helium on me­ reach the pore spaces of serlimen­ to argon, the absolute content of thane. Attempts to demonstrate tary rocks by diffusion from radio­ nitrogen, nor the present direction a correlation between the content of active minerals contained therein of fluid movement were determ­ helium in natural gas and the ra­ or from minerals in the Precam­ ined, the possibility that these iso­ dioactivity of the associated forma­ brian basement. Many authors, in­ topic variations are due to differ­ tion fluids have been unsuccessful. cluding Levorsen (9) and Landes ences in the source of the nitrogen, This led Lepape (23) to suggest (10), have noted a general relation­ as well as to the mode and direc­ that helium originated from fran­ ship between the contents of helium tion of migration, cannot be ruled cium, Which he presumed was a and nitrogen in natural gases. out. radioactive element with a half­ Lind (11) even suggested that the In several stratigraphic units life so short that it has since prac­ radioactive decomposition, caused (Beaverhill Lake Group, Woodbend tically completely decayed to its by alpha-particles, of a nitrogen Group, Mannville Group, and Bow daughter products.