GENERATION AND MATURATION OF CARBOXYLIC-ACIDS IN ANCIENT SEDIMENTS FROM THE MARACAIBO BASIN, VENEZUELA

Cyclic and acyclic carboxylic acids in various rock samples from the Misoa and La Luna formations, Maracaibo Basin, Western Venezuela, were analyzed in order to study their generation and maturation processes. For this purpose, rocks from various degrees of maturity (Misoa: %R(o) 0.35-0.77; La Luna: %R(o) 0.5-1.8), and different origins (Misoa: type III OM; La Luna: type II OM) were studied. Linear acids and tricyclic as well as pentacyclic (hopanoic) triterpenoic acids were analyzed quantitatively by gas chromatography-mass spectrometry. Qualitative data from the isomeric distribution of the hopanoic acids (HAs) showed the expected trend of increasing abundance of the thermodynamically more stable (17-alpha,21-beta) and (17-beta,21-alpha) isomers, with the simultaneous disappearance of the thermodynamically less stable (17-beta,21beta) isomers. However, at high maturity levels, the (17-beta,21-beta) isomers reappeared, to become the only HAs to be present. A similar trend of apparent maturity change was observed for the linear acids by studying the CPI of these compounds. A double generation process of carboxylic acids is suggested and confirmed by quantitative data. The latter showed that acids of different degrees of maturity are generated at various stages prior to the oil generation maximum. It is suggested that a major portion of the acids biosynthesized in recent environments are incorporated into the geopolymeric fraction during early diagenesis and are as such protected from further diagenetic degradation. It is also suggested that their maturation and generation is strongly dependent on their "speciation" within the geopolymer, and the structural changes in the geopolymeric matter during thermal maturation. The importance of these observations with respect to the application of carboxylic acids as biomarkers of oil migration is discussed.