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The unique phenomenon of hydrocarbon gas sorption on mineral and organic matter surfaces provides the basis of a novel aid for geochemical petroleum exploration. A refined vacuum sediment degassing procedure permits separate analysis of sorbed and free gas fractions. Hydrocarbon gases from both individual fractions can be genetically classified (e.g., diagenetic, thermogenic, catagenic) by their molecular and 13C and 2H isotopic compositions.
In contrast to the normal bulk or free gas fractions, where the composition is frequently influenced by alteration effects such as bacterial gas generation or oxidation, comparative analyses indicate the the sorbed gas fraction can retain its unaltered genetic signature. Exchange between the sorbed and free gas fractions is severely restricted by surface sorption energies and perhaps by structure water, which may assist in the partitioning of gas fractions with different genetic characters. Gas transport within the sorbed fraction is probably a surface-controlled "handshake" diffusion process that minimizes contact with the free gases.
Examples of this phenomenon are provided by Gulf Coast and California surface sediment cores, which display strong biogenic methane formation and oxidation effects in the free gas fraction. In contrast, the corresponding character of the sorbed gas fractions is clearly distinguished as thermogenic and originating in subsurface. Evidence of thermogenic hydrocarbons is obscured during routine bulk analysis by the biogenic free gas component.
This sorption phenomenon has good potential for the identification of subtle geochemical hydrocarbon anomalies previously masked through bulk analysis.
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