Compositional Variability of Mixed Oils as Measures of Sources and Migration Pathways: Case Studies from the Potiguar Basin, Brazil

Abstract

Regional studies of petroleum samples in cross sections from both offshore and onshore reservoirs reveal compositional differences attributable to varying extents of oil mixing with discernible landward trends. These discrepancies can be related to a combination of petroleum source, maturity, and migration history. The source rocks of these petroleum source, maturity, and migration history. The source rocks of these petroleums are classified in two groups on the basis of their geochemical and molecular characteristics: (1) Neocomian and Aptian lacustrine freshwater shales and (2) Aptian marine evaporitic shales and marls. Both families of source rocks are located in an offshore structural low. The geochemically contrasting oils derived from these source beds migrated updip through Aptian carrier beds to immature strata, constrained by a monoclinal structure. Evaluation of the distribution of compositional heterogeneities and investigation of migration effects on oil composition revealed trends related to the amount of mixing that reflect the timing of oil generation and migration and the direction of migration. The more migrated oils contain greater contributions from freshwater sources whereas the less migrated oils display increasing contributions from the shallower hypersaline source beds. Biomarker maturity parameters indicate that the most migrated oils are less mature, consistent with theirmore » early generation from the deeper lacustrine freshwater source beds and migration to first fill the shallower reservoirs.« less