Petroleum Migration and Mixing in the Potiguar Basin, Brazil (1)

Abstract

Regional studies of petroleum samples from both offshore and onshore reservoirs of the Potiguar basin, Brazil, reveal compositional differences, with discernible landward trends, attributable to oil mixing. These chemical discrepancies can be related to a combination of source rocks, maturity, and migration history. The source rocks are classified in two groups based on their geochemical and molecular characteristics: (1) Neocomian and Aptian lacustrine freshwater shales, and (2) Aptian marine evaporitic shales and marls. Both lie in an offshore structural low. The geochemically contrasting oils derived from these source beds migrated updip, constrained by a monoclinal structure, through Aptian carrier beds to thermally immature strata. Evaluation of compositional heterog neities in the oils and an investigation of migration effects on their compositions reveal trends related to the amount of mixing which, in turn, reflects the timing of both 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 more pronounced contributions from the younger and shallower hypersaline source beds. Biomarker maturity parameters indicate that the most migrated oils are less mature, consistent with their early generation from the deeper lacustrine freshwater source beds and migration to fill the shallower reservoirs first. The results suggest that drainage of the basin evolved from lateral to vertical, and enable prediction of the distribution of petrol um types within the basin.