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

A suite of 10 di€erent marine evaporitic oil samples from Sergipe-Alagoas Basin, Brazil was studied for its biomarker content, in particular its acidic constituents. The oils showed di€erent molecular distributions and relative abundances of n-alkanoic, isoprenoid and hopanoic acids. The observed di€erences were assigned to the incorporation of immature organic matter in the oils and fractionation along the migration pathway. The diagenetic precursor functionality (alcohol/ether or acid) was proposed based on the comparison of the relative abundances of the neutral and acidic biomarkers (hopanoids, isoprenoids, alkyl-steranes, monoaromatic alkyl-steroids). In the acidic fraction, 3 series of steroid-alkanoic acids and monoaromatic steroid-alkanoic acids (steroid-methanoic, ethanoic and propanoic acids and monoaromatic steroid-methanoic, ethanoic and propanoic acids) were detected, while in the neutral fraction only 2 series of each corresponding class could be observed (methyl and ethyl-steranes and monoaromatic methyl and ethylsteroids). These carbon shifts suggest that decarboxylation is an important process in the formation of the alkylsteranes and monoaromatic alkyl-steroids, and we infer that carboxylic acids are the diagenetic precursors of these classes of compounds. When alcohol or ether are the diagenetic precursors (isoprenoids and hopanoids), no signi®cant di€erences in the molecular distributions between neutral and acidic fractions were observed. # 2000 Elsevier Science Ltd. All rights reserved.