Geochemical Studies of Natural Gas Part I. Hydrocarbons in Western Canadian Natural Gases
- Brian Hitchon (Research Council of Alberta)
- Document ID
- Petroleum Society of Canada
- Journal of Canadian Petroleum Technology
- Publication Date
- June 1963
- Document Type
- Journal Paper
- 60 - 76
- 1963. Petroleum Society of Canada
- 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 5.1.1 Exploration, Development, Structural Geology, 4.6 Natural Gas, 4.1.5 Processing Equipment, 5.2 Reservoir Fluid Dynamics, 4.1.2 Separation and Treating, 4.3.4 Scale, 5.6.2 Core Analysis, 5.6.5 Tracers, 5.4.6 Thermal Methods
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Natural gases in Western Canada are found in sediments ranging in age fromCambrian to Tertiary. The hydrocarbons in these gases are described andevaluated in terms of their geochemistry, origin, migration andaccumulation.
Hydrocarbons are the chief components in the majority of the nearly 3,000analyses of natural gases from Western Canada. Most gases contain more than 90per cent hydrocarbons, but as little as 0.26 per cent is present in some. Thepreferred order of abundance of the individual hydrocarbons is methane >ethane > propane > n- butane > isobutene >isopentane > n-pentane > hexanes > heptanes. A decreasingn-butane:isobutane ratio is accompanied by an approach to unity of theisopentane :n-pentane ratio. Study of the C4 andC5 isomers indicates that no reservoirs are in thermodynamicequilibrium. Variations in the C4 and C5 isomer ratiosare attributed to catalysis by the sediments.
Methane probably originates due to direct synthesis by bacteria, although itis also a by-product of biochemical reactions that are able to generatehydrocarbons with from two to ten carbon atoms. These reactions includelow-temperature decarboxylation and reductive deamination of amino acids anddecarboxylation of fatty acids. The part played by bacteria in these reactionsis probably important, although the mechanism is not known with any degree ofcertainty.
Natural gases with relatively low contents of methane and high contents ofheavy hydrocarbons are found in sediments deposited in basinal environments. Insediments deposited in shelf environments the natural gases have highercontents of methane and lower contents of the heavy hydrocarbons. Forpre-Cretaceous strata in Western Canada, the depositional basins approximatelycoincide with the present structural basin, and fluid flow is updip. ForCretaceous strata, sediments in the structural basin contain natural gases withrelatively low contents of methane except where modification due to downdipfluid-flow has occurred. Possible mechanisms inducing modification of theregional variations include undersaturation of the formation water with naturalgas at present reservoir temperatures and pressures, and variations in thesolubilities of the hydrocarbons due to changes in flow direction and movementinto regions with formation waters of different salinites.
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