Geochemical methods have been applied in exploration that hydrocarbon gases migrate, essentially in a being employed, perhaps to an extent greater than ever.

All geochemical techniques are based on the assumption that hydrocarbon gases migrate, essentially in a vertical direction, from oil and gas accumulations to the surface of the earth. This assumption is supported by the observation that the saturated hydrocarbons that are present in near-surface soil air, or adsorbed on the soil itself, can be related to buried deposits.

Data have been published which appear to discredit hydrocarbon geochemical techniques by attempting to show that saturated hydrocarbons heavier than methane occur in the soil from sources other than petroleum. In 1963, Smith and Ellis reported the presence of unsaturated hydrocarbons and saturated hydrocarbons, ranging from propane through the pentanes, in grasses and roots, and suggested that vegetation was the source of soil hydrocarbon anomalies.

Studies have been made on grasses and roots which show that, aside from methane, only unsaturated hydrocarbons in relatively large amounts are present in or produced by vegetation. However, soils in the vicinity

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of oil or gas fields contain significant quantities of saturated hydrocarbons ranging from methane through the pentanes.

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