Carboxylic acids of marine evaporitic oils from Sergipe-Alagoas Basin, Brazil

A. suite of 10 different marine evaporitic oil samples from Sergipe-Alagoas Basin, Brazil was studied for its biomarker content, in particular its aci dic constituents. The oils showed different molecular distributions and rel ative abundances of n-alkanoic, isoprenoid and hopanoic acids. The observed differences were assigned to the incorporation of immature organic matter in the oils and fractionation along the migration pathway. The diagenetic p recursor functionality (alcohol/ether or acid) was proposed based on the co mparison of the relative abundances of the neutral and acidic biomarkers (h opanoids, isoprenoids, alkyl-steranes, monoaromatic alkyl-steroids). In the acidic fraction, 3 series of steroid-alkanoic acids and monoaromatic stero id-alkanoic acids (steroid-methanoic, ethanoic and propanoic acids and mono aromatic steroid-methanoic, ethanoic and propanoic acids) were detected, wh ile in the neutral fraction only 2 series of each corresponding class could be observed (methyl and ethyl-steranes and monoaromatic methyl and ethyl-s teroids). These carbon shifts suggest that decarboxylation is an important process in the formation of the alkyrsteranes and monoaromatic alkyl-steroi ds, and we infer that carboxylic acids are the diagenetic precursors of the se classes of compounds. When alcohol or ether are the diagenetic precursor s (isoprenoids and hopanoids), no significant differences in the molecular distributions between neutral and acidic fractions were observed. (C) 2000 Elsevier Science Ltd. All rights reserved.