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Abstract

Geochemical studies, together with paleogeographical and geological evidence, suggest that most of the organic-rich Neocomian to Aptian rift-stage succession in the Brazilian continental margin was deposited in lacustrine environments. It is possible to differentiate two lacustrine systems responsible for about 85% of Brazilian oil discovered to date—a relatively large, deep, fresh-water type of basin, ranging in age from early Neocomian to Aptian, and a shallow lower to upper Neocomian saline system.

Fresh-water systems are characterized by thick beds of dark-gray to black shale (TOC <6%). In the oils and source rocks abundant high molecular-weight n-alkanes, low S and V/Ni values, low <513C values, high Pr/Ph ratios, absence of dinosterane and C30 desmethyl steranes, and low concentrations of steranes and porphyrins characterize the fresh-water depositional environment.

Saline systems are composed of thick beds of calcareous black shales (TOC up to 9%). The oils and rocks, in addition to showing diagnostic evidence of a nonmarine environment, are characterized by features typical of deposition under saline conditions. These include higher V/Ni ratios, presence of β3-carotane, high concentration of C30 αβ-hopane, moderately abundant gammacerane, Ts/Tm < 1, and high d13C values.

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