FREE ALIPHATIC-ACIDS IN SULFUR-RICH LACUSTRINE SEDIMENTS - THEIR ORIGIN AND RELATION TO HYDROCARBONS

Free fatty and hopanoic acids, and hydrocarbons of five organic- and sulfur-rich lacustrine sediments from Nordlinger Ries black shales (southern Germany), have been studied by gas chromatography and computerized GC-MS. The results indicate that Nordlinger Ries sediments contain abnormally high, concentrations of those free acids (up to 57% of total extractable bitumen). Besides n-alkanoic, monomethyl, and isoprenoid acids common to any other immature sediments, high concentrations of C-16 and C-18 10-oxoalkanoic acids are unusual constituents of these sediments. The high concentrations of fatty acids in the bitumens are attributed to early diagenesis. The carbon distributions of the acids are consistent with an origin mainly from planktonic and microbial organisms. Only the acids in the sediments from well NR-30 located closer to the rim of the basin indicate a slightly elevated contribution from terrestrial sources. Distributions of n-alkanoic acids (C-11-C-22) in the samples from the center of the basin (well NR-10) show striking similarities to the n-alkane distributions, indicating that both series may, at least in part, be diagenetically related by decarboxylation of the acids. Isoprenoid and hopanoic acid distributions indicate common precursors but clearly show that decarboxylation is not a significant pathway to convert acids into hydrocarbons.