Petroleum systems in the South Atlantic margins

Results obtained from the detailed geochemical analysis of more than 150 crude oils were used to : (1) identify a number of different petroleum systems present in the South Atlantic margins; and (2) establish geochemical criteria useful in distinguishing lacustrine-derived oils from marine-derived oils. The multiplicity and type of effective source rocks were determined by using the geochemical characteristics of the crude oils to establish the number of compositionally distinct oil families, and by inferring palaeoenvironmental conditions of source rock deposition and possible age. Using multivariate statistical techniques and bulk geochemical parameters, isotopic compositions and biomarker distributions, a number of general petroleum systems were identified, differing from each other in terms of source palaeoenvironment and age. The oils are separated into two broad groups: lacustrine and marine. Further detailed examination of each broad group, sometimes using more specific geochemical criteria, typically resulted in the establishment of sub-groups of oils according to specific source environment, such as lacustrine fresh v. saline conditions. Areas where oils of mixed provenance occur are also identified. The geochemical criteria used in discriminating between lacustrine and marine oil types were determined based on geochemical differences that exist between known marine-sourced (mainly post-salt) oils from the Kwanza and Gabon (Port Gentil area) Basins and known lacustrine-sourced (pre-salt) oils from the Congo Basin.