Generation and Migration of Light Hydrocarbons

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Science  14 Dec 1984:
Vol. 226, Issue 4680, pp. 1265-1270
DOI: 10.1126/science.226.4680.1265


Light hydrocarbons (containing from 1 to 14 carbon atoms) are formed from disseminated organic matter in sediments at the parts-per-billion level by biological and low-temperature (< 50°C) chemical reactions and at the parts-permillion level by high-temperature (> 50°C) cracking reactions. The cooler reactions produce mainly branched hydrocarbons, whereas the hotter reactions yield more straight chains. Hydrocarbon generation zones in the subsurface can be recognized on the basis of hydrocarbon distribution patterns. Hydrocarbons with tertiary carbon atoms form at lower temperatures than those with quaternary carbons. Methane and ethane migrate vertically through fine-grained shales by diffusion and solution, whereas many of the C3+ hydrocarbons show little or no vertical migration. Concentrations of light hydrocarbons, including methane, in fine-grained source rocks decrease to low values in deep, high-temperature (>200°C) sediments. This decrease may be one reason why no economic accumulation of gas has been found to date deeper than 8.2 kilometers (27,000 feet).