Previous work based on chemical evidence and information from the hydrocarbon fractions of Marlim and Albacora, giant oil fields of Campos Basin (offshore Brazil), in connection with other geochemical parameters, led to conclusions on the oil accumulation, migration and biodegradation. The acidic components, as carboxylic acids of Marlim and Albacora were never investigated before and this work revealed a significant correlation between both fractions (neutral and acidic) therefore opening new, horizons for the use of these naphthenic acids as additional geochemical parameters to enrich and enlarge previous geological conclusions on Marlim and Albacora. A naphthenic acid of 16 carbon atoms was detected and based on its mass spectrum and that of its corresponding deuterated hydrocarbon an 8-carboxy-9-ethyl-drimane structure was proposed.186851860