Six marine sediments from the seafloor of the Gulf of Mexico were studied in terms of the abundance, composition and distribution of organic matter as well as biomarker chemistry. The results show that there are great spatial variations in terms of organic matter abundance and hydrocarbon composition among these samples. The S-7 and S-9 samples show the characteristics of modern organic matter that has been modified by biodegradation, while the S-8, S-10 and S-11 samples are clearly contaminated with petroleum hydrocarbons from oil seepage beneath the seafloor. The lack of n-alkanes in samples S-8 and S-11 is indicative of severe biodegradation. Only sample S-1 contains methanogenic archaea-sourced lipid biomarkers, in particular 2,6,10,15,19-pentamethylicosenes containing 1-5 unsaturated double bonds. These compounds are distributed between n-C 22 and n-C 24 in the aliphatic fraction and have very depleted 13 C values ranging from )86.7& to )115.5&, compared to the d 13 C values of adjacent n-alkanes ranging from )28.4& to )34.6&. The occurrence and carbon isotopic compositions of these compounds suggest that the S-1 sample site is likely to be associated with a gas venting system or a gas hydrate setting.