Isotopic characterization of the Cretaceous lacustrine and marine-evaporitic sequences and related oils from the Potiguar Basin, northeastern Brazil

The Potiguar Basin is the easternmost Brazilian equatorial basin and covers an area of about 48,000 km{dollar}sp2{dollar}. It formed during the breakup of Gondwanaland in the early Cretaceous. The sedimentary pile includes three sequences: rift, characterized by lacustrine fresh water shales, turbidites, and progradational deltas; transitional, comprised of carbonates, shales, and marls interbedded with deltaic sandstones; and oceanic, represented by transgressive shales and shelf carbonabes and by progradational siliciclastic facies, shallow carbonates and turbidites.; In this work, bitumens and kerogens from immature sections of the marine-evaporitic (Aptian, Alagamar Fm.) and lacustrine (Neocomian, Pendencia Fm.) deposits have been examined. Hydrogen isotopic analyses of the saturated hydrocarbons were used to assess paleoenvironmental conditions in both stratigraphic intervals. The results indicate a direct relationship between {dollar}delta{dollar}D{dollar}sb{lcub}rm sat{rcub}{dollar} and {dollar}delta{dollar}D{dollar}sb{lcub}rm water{rcub}{dollar}. Accordingly, {dollar}delta{dollar}D{dollar}sb{lcub}rm sat{rcub}{dollar} can provide key paleoenvironmental information bearing strongly on paleoclimates. Carbon isotopic compositions of specific compounds suggest that the Pendencia paleolake apparently had a stable photic zone occupied by both algae and cyanobacteria. Bottom waters and/or sediments were anaerobic and recycling of CH{dollar}sb4{dollar} was effective. In contrast, the complex carbon isotopic pattern of the specific compounds from the Alagamar Fm. is compatible with a more dynamic evolution of the paleoenvironment. The relatively "flat" isotopic profiles for the n-alkanes are a distinctive feature suggesting marine influence, and the variable isotopic differences between compounds from the upper and lower water column suggest different intensities in water stratification.; Hydrogen and carbon isotopic analyses of the Potiguar oils indicate that the {dollar}delta{dollar}D{dollar}sb{lcub}rm oil{rcub}{dollar} and {dollar}deltasp{lcub}13{rcub}{dollar}C{dollar}sb{lcub}rm oil{rcub}{dollar} reflect most of the source inputs, with no significant isotope effects attributable to biodegradation, thermal maturity, and distance of secondary migration. Similar to {dollar}deltasp{lcub}13{rcub}{dollar}C{dollar}sb{lcub}rm oil{rcub}{dollar}, {dollar}delta{dollar}D{dollar}sb{lcub}rm oil{rcub}{dollar} can be used as a tool for geochemical correlations and, in addition, can provide information about paleoclimatic conditions in the depositional environments of source rocks.