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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 56 (1972)

Issue: 5. (May)

First Page: 925

Last Page: 940

Title: Vegetation and Geochemical Prospecting for Petroleum

Author(s): Leo Horvitz (2)


The value of geochemical prospecting for petroleum has been challenged by Smith and Ellis, who reported that soil hydrocarbons, saturated as well as unsaturated, result from vegetation in the form of grass and roots. These investigators claimed that 1 g of this type of vegetation is sufficient to produce the same quantity of saturated hydrocarbons found in 200 g of soil, and imply that it is the source of hydrocarbon anomalies. Unfortunately, no quantitative analytical data were supplied by them and, therefore, the claim is not substantiated.

Investigations by the present writer show that less than one part per billion by weight of saturated hydrocarbons, in the range from ethane through pentane, is contributed by 1 g of grass or roots to 200 g of soil. This concentration is much lower than that normally found in the soil of background or barren areas and, therefore, is of no importance in geochemical exploration. Actually, few significant anomalies contain ethane through pentane values lower than 25 parts per billion by weight.

Small amounts of saturated hydrocarbons may be generated by heating grass or roots in a partial vacuum, either alone or in the presence of phosphoric acid, but in the presence of concentrated nitric acid the production of saturated hydrocarbons is so inhibited that less than one part per billion of ethane through pentane is contributed to 100 g of soil by as much as 5 g of this vegetation. Therefore, the presence of any of this group of saturated hydrocarbons in grass or roots, in the natural state, is doubtful.

As additional evidence to condemn geochemical exploration, Smith and Ellis cited their unsuccessful attempts to find anomalous hydrocarbon values over oil fields. These failures are explained easily by the fact that their soil gas extraction technique is incapable of removing the major part of the saturated hydrocarbons that are adsorbed on the soil, making it difficult to discern an anomaly. Moreover, the near-surface anomalies, originally produced by the fields they sampled, may have become weakened or even have disappeared because of changes in reservoir conditions that take place as petroleum is being produced.

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