Geochemical assessment of petroleum migration and mixing in the Potiguar and Sergipe-Alagoas basins, Brazil

Geochemical modifications associated with petroleum migration remain a matter of debate. Depletion trends associated with petroleum expulsion from source rocks have been recognized for series of biomarker compounds, but the fractionations inferred as the result of secondary migration processes are controversial.;Source rocks of the Potiguar and Sergipe-Alagoas Basins, Brazil are located in offshore structural lows and the petroleum they have generated has migrated laterally to shallower reservoirs within thermally immature strata; a setting that provides ideal conditions for investigations of molecular aspects of petroleum migration.;In the Sergipe-Alagoas Basin, the effects of secondary migration were investigated in series of single-sourced marine hypersaline oils. No biomarker fractionation associated with either geochromatography or phase changes was observed, but migration through water saturated pores promoted the loss of more soluble compounds by water washing.;By contrast, mixed oils with contributions from both lacustrine freshwater and marine hypersaline sources of the Potiguar Basin show compositional variabilities which reflect a combination of factors related to source contributions, maturity and migration history. The more migrated oils contain greater lacustrine freshwater contributions, whereas the less migrated oils display more pronounced contributions from younger and shallower marine 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 their migration to fill the shallower reservoirs first. Also, examination of biomarker parameters that were previously proposed as indicators of migration reveals trends related to source and maturity constraints.;Carbon isotopic compositions of whole oils, fractions and specific compounds for the mixed oils of the Potiguar Basin were measured and evaluated. No trends for individual compounds were clearly defined, but integration of biomarker abundances and their isotopic values helps to explain variabilities in the isotopic composition of whole oils and fractions.;Identification of compositional trends in biomarker profiles for mixed oils in the Potiguar Basin and the absence of systematic variations in similar characteristics for single sourced oils of the Sergipe-Alagoas Basin demonstrate the value of biomarkers as source indicators, independent of migration processes.