Use of Hydrogen and Carbon Stable Isotopes Characterizing Oils from the Potiguar Basin (Onshore), Northeastern Brazil

Abstract

Hydrogen and carbon isotopic abundances were measured in representative samples of marine- evaporitic, lacustrine, and mixed oils from the onshore Potiguar basin. The isotopic data, together with chemical compositions and geological information, were used to investigate relationships among dDoil and d13Coil and source, thermal maturity, biodegradation, mixing, and the distance of secondary migration of these oils. The marine-evaporitic oils are depleted in deuterium (-135 to -114‰), and the lacustrine oils are enriched (-101 to -88‰). Values of dD for mixed oils are intermediate (-113 to -104‰) between those of marine-evaporitic and lacustrine oils, showing that dDoil is a reliable tool to identify oils that differ genetically. Values of dDoil were not directly related to thermal maturation, biodegradation, or distance of the secondary migration. Hydrogen isotopic compositions of the oils can be related mainly to the dDwater in the paleoenvironments in which the primary producers lived; therefore, dDoil values can provide rough estimates of dDwater in the depositional environments of the source rocks, and consequently may provide evidence about paleoclimatic conditions at the site. Marine-evaporitic oils are enriched in 13C (-26.5 to -25.7‰), and lacustrine oils are depleted (-33.6 to -30.2‰) in 13C. Relative contributions of marine-evaporitic and lacustrine sources to mixed oils thus can be calculated independently from carbon or hydrogen isotopic mixing models. When these calculations are done independently, the mixed oils separate into two families, and a previously unrecognized component possibly associated with environments that formed during the lacustrine-to-marine transition is revealed. The investigation thus demonstrates the superior resolving power of two-element mixing systematics.