EVOLUTION OF ALKANES AND CARBOXYLIC-ACIDS IN ANCIENT SEDIMENTS FROM THE MARACAIBO BASIN

Aliphatic hydrocarbons and carboxylic acids were analysed quantitatively in rock samples of different degrees of maturity from the Icotea borehole (Eocene, Misoa Formation, Maracaibo Basin, Venezuela), in order to compare their geochemical evolution in ancient sediments. Previously a double generation process for the carboxylic acids has been reported, in which "immature" distributions of acids were generated at the highest maturities. It was suggested that the generation of the organic acids was dependent on the nature of the binding in the kerogen, and on steric factors. By comparison, the alkanes show different behaviour. Whilst a similar generation profile is observed for hopanes when compared to hopanoic acids, no inversion of isomeric maturation parameters was observed in this case. Hopanes may be generated first by a release of hydrocarbons from the kerogen at low levels of thermal maturity (R(o) = 0.46), and secondly by cracking at higher temperatures, the second generation being less significant than the first. Higher plant-derived triterpanes behave similarly. Pristane, phytane and n-alkanes are generated at relatively high levels of thermal maturity (R(o) > 0.7). The generation profiles for the n-alkanes are complicated by expulsion and cracking phenomena, however these components are generated after the major release of n-acids.