Use of hydrogen and carbon stable isotopes characterizing oils from the Potiguar basin (onshore), northeastern Brazil

Hydrogen and carbon isotopic abundances were measured in representative sam ples of marine-evaporitic, lacustrine, and mixed oils from the onshore Poti guar basin. The isotopic data, together with chemical compositions and geol ogical information, were used to investigate relationships among delta D-oi l and delta 13C(oil) and source, thermal maturity: biodegradation, mixing, and the distance of secondary migration of these oils. The marine-evaporiti c oils are depleted in deuterium (-135 to -114 parts per thousand), and the lacustrine oils are enriched (-101 to -88 parts per thousand). Values of 6 D for mixed oils are intermediate (-113 to -104 parts per thousand) between those of marine-evaporitic and lacustrine oils, showing that delta D-oil i s a reliable tool to identify oils that differ genetically. Values of delta D-oil 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 delta D-water in the paleoenvironments in which the primary producers lived; therefore, delta D-oil values can provi de rough estimates of delta D-water in the depositional environments of the source rocks, and consequently may provide evidence about paleoclimatic co nditions at the site. Marine-evaporitic oils are enriched in C-13 (-26.5 to -25.7 parts per thous and), and lacustrine oils are depleted (-33.6 to -30.2 parts per thousand) in C-13. 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, th e mixed oils separate into two families, and a previously unrecognized comp onent possibly associated with environments that formed during the lacustri ne-to-marine transition is revealed. The investigation thus demonstrates th e superior resolving power of two-element mixing systematics.