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Geochemical Studies of Natural GeocheJtnieal Studies of Nai1:ural Gas Part 1. HYDROCARBONS IN WESTERN CANADIAN NATURAL GASES' By BRIAN HITCHONt ABSTRACT mechanisms inducing modification vapour. Of the common "gases" of the regional variations include available for examination within Natural gases in Western Canada undersaturation of the formation are found in sediments ranging in water with natural gas at present the accessable portions of the age from Cambrian to Tertiary. reservoir temperatures and pres­ Earth, hydrocarbons heavier than The hydrocarbons in these gases sures, and variations in the solubil­ propane will be in the form of va­ Downloaded from http://onepetro.org/jcpt/article-pdf/2/02/60/2165264/petsoc-63-02-05.pdf by guest on 30 September 2021 are described and evaluated in ities of the hydrocarbons due to pours, Depending upon the actual terms of their geochemistry, origin, changes in flow direction and move­ migration and accumulation. ment into regions with formation conditions pertaining, so also may be the acid "gases" hydrogen sul­ Hydrocarbons are the chief com­ waters of different salinites. ponents in the majority of the phide and carbon dioxide, and the nearly 3,000 analyses of natural INTRODUcrION hydrocarbons ethane and propane. gases from Western Canada. Most Throughout these papers both va­ gases contain more than 90 per HIS paper is the first of a ser­ pours and true gases will be desig­ cent hydrocarbons, but as little as T ies concerned with the geo­ 0.26 per cent is present in some. nated as gases for ease of descrip­ The preferred order of abundance chemistry of natural gas in West­ tion. of the individual hydrocarbons is ern Canada. It discusses the bulk methane :> ethane:> propane >­ composition of natural gas, with Gases are minerals of a unique" n-butane:> isobutane:> isopentane particular reference to the hydro­ type (:1). In nature they are form­ :> n-pentane :> hexanes :> hep­ tanes. A decreasing n-butane:iso­ carbon components. Other papers ed as the result of a variety of pro­ butane ratio is accompanied by an will cover the geochemistry of hy­ cesses, including the actIon of high approach to unity of the isopentane drogen sulphide, carbon dioxide, temperatures, chemical reactions, :n-pentane ratio. Study of the C4 nitrogen and helium. biochemical transformations and :md Cs isomers indicates that no reservoirs are in thermodynamic radioactivity. Some gases, inclUd­ In each paper an attempt has ing components of the atmosphere, ::quilibrium. Variations in the C-t been made to establish the origin 3.nd Cs isomer ratios are attributed are of primordial paragenesis, and :0 catalysis by the sediments. of the components being studied formed an integral portion of the and to note important relationships 'Methane probably originates due early Earth. Others are of more to direct synthesis by bacteria, al­ between the components. Maps recent origin. There is a large though it is also a by-product of have been used to illustrate varia­ number of individual components, biochemical reactions that are able tions in the content of the compon­ both true gases and vapours, com­ to generate hydrocarbons with from ents from one region of Western two to ten carbon atoms. These re­ prising terrestrial gases, although actions include low-temperature de­ Canada to another, within a series only a few of these are considered carboxylation and reductive deam­ of stratigraphic units ranging in sufficiently common to warrant dis­ ination of amino acids and decar­ age from Middle Devonian to Late cussion here. boxylation of fatty acids. The part Cretaceous. The geochemical in­ played by bacteria in these reactions is probably important, although the formation and the maps of regional The habitat of terrestrial gases mechanism is not known with any variations have been used to relate ranges from large accumulations degree of certainty. the composition of the natural gas­ such as the atmosphere and natur­ Natural gases with relatively low es to the environment of deposition al gas deposits, comprised essential­ contents of methane and high con­ of the sediments, to the present en­ ly of a few major components, tents of heavy hydrocarbons are vironment within the Western Can­ through gases dissolved in the sea found in sediments deposited in ada Sedimentary Basin and to the and in formation waters, to those basinal environments. In sediments deposited in shelf environments the present flow regime of the forma­ associated with hot springs, vol­ natural gases have higher con­ tion fluids. canoes, and present as minute in­ tents of methane and lower con­ clusions in minerals. This series of tents of the heavy hydrocarbons. It is hoped that the results of papers will be concerned chiefly For pre-Cretaceous strata in West­ this study will be of interest and ern Canada, the depositional basins with natural gas deposits, but approximately coincide with the benefit to those whose task is the where pertinent, attention will be present structural basin, and fluid exploration for natural gas. directed to other habitats of gases. flow is updip. For Cretaceous strata, sediments in the structural basin Definition of natural gas contain natural gases with relative­ Gases in nature ly low contents of methane except Examination of the literature where modification due to downdip The attenuated state of matter concerning the composition of crude fluid-flow has occurred. Possible above its critical temperature (Tc) oil and natural gas shows some is termed _a gas, and below it a discrepancies in the definitions used by various authors. Implicit in the :t:Research Geologist, Research definitions of Rice (2) and Howell Council of Alberta, Edmonton, Al­ "'Contribution No. 208, Research berta, Canada. Council of Alberta. (3) is the unnecessarily restrictive 60 Journal of Canadian Petroleum ':'\ assumption that natural gases con­ sulted in reports by McLennan et Gross composition of natural gas ,'.~, sist solely of gaseous hydrocarbons. al. (11), Elworthy (12)&(13), and ,-,j Examination of the analyses in­ " Therefore the term natural gas has Rosewarne and Offord (14)' Com~ ;.1 been redefined, paraphrasing the pilations of analyses were publish­ dicates that the major components "j defiirition of crude oil quoted by ed by Stansfield and Nicolls (15), are hydrocarbons, nitrogen, carbon ,1 Howell: and Rosewarne and Offord (16) & dioxide, and hydrogen sulphide These components make up over ...., ,,' (17), Although all these papers Crude oil-A liquid in its natural were consulted for comparative 99,5 per cent by volume of the maj­ state and environment, com­ data, recent developments in analy­ ority of gases. Only in very rare posed principally of hydrocar­ tical techniques have rendered most instances do other components such bons, usually with sulphur, nit­ of them of historic interest only. as helium and hydrogen total more rogen and oxygen as elements than one half of one per cent by or compounds or both, and More recent sources of published volume. A tetrahedral composition traces of other elements and analytical data used in this study diagram (Figure l)-with hydro­ c9mpounds. comprise the reports of the Fuel carbons, nitrogen, carbon dioxide Research Laboratories of the De­ and hydrogen sulphide as the end 'Natural gas-A gas in its natural members-thus essentially describes state and environment, com­ partment of Mines and Technical the compositions of all gases exam­ monly composed principally of Surveys (Offord and Goodspeed, ined. In more than 75 per cent·of hydrocarbons, usually with hy­ (18); Offord (19); Offord and Yates Downloaded from http://onepetro.org/jcpt/article-pdf/2/02/60/2165264/petsoc-63-02-05.pdf by guest on 30 September 2021 the gases the content of hydrocar­ drogen sulphide, nitrogen and (20); Offord et aI, (21»), the Al­ bons is more than 90 per cent by carbon dioxide, and traces of berta Oil and Gas Conservation volume. The non-hydrocarbon end other gases. Board (22), and the Saskatchewan Department of Mineral Resources member compositions are approach­ Implicit in these definitions is a (23), Special compilations of sel­ ed in only 0,3 per cent of the gases. genetic relationship between crude ected components in Alberta nat­ The block diagram in Figure i in­ oil and natural gas. This genetic ural gases have been prepared by dicates the composition of the non­ implication is important, because Shaw (24)&(25>. In addition to hydrocarbon components in over 90 all physical variations exist in na­ these published data, the acknowl­ per cent of the analyses examined. I, A major portion of the gases con­ ,,- .... :. ture, and under conditions of ex­ edgments at the end of this paper '.'., treme temperature and pressure, indicate the sources of other an­ tain less than :five per cent of each natural gas and crude oil become alyses used. component, with amounts over five physically indistinguishable (4, p, Per cent 3581. The above definition of nat­ Per cent Analyses ural gas applies throughout this Hydro. within carbons each series of papers and allows the in­ block elusion of those uncommon gases 50.3 composed essentially of hydrogen ::::~4~~~~~~1.95~g : 90 28.1 sulphide, nitrogen, carbon dioxide ____·90 .. "', or other gases. ~- 80 14.2 /"="' 80 GEOCHEMISTRY AND ORIGIN OF HYDROCARB NS 5,7 Low MOLECULAR WEIGHT '11> 60 ALIPHATIC HYDROCARBONS 1"';::::----,.",--_6 0 One of the most characteristic fractions of most crude oils, and '.- , commonly the chief components of __30 natural gases, are the low molecu­ lar weight aliphatic hydrocarbons, ranging from methane to hydrocar­ bons with up to ten carbon atoms. 0.3 Ethylene (4, p, 361) and cycIopen­ tane (5) have been found occasion­ ally in natural gases but as neither N, were recorded in any gases from '0 " e, of. Western Canada, they are not con~ ',' '0'l. 10 " , sidered here. 5 0 Bource of analytical data Nearly 3,000 analyses of natural 15 1.5 gases from Western Canada were examined in this study.
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