Characterisation of the unresolved complex mixture (UCM) in the gas chromatograms of biodegraded petroleums

Abstract

The total hydrocarbons fraction of a heavily biodegraded oil was examined to determine the structure of the unresolved complex mixture (UCM). Bulk analytical techniques included ¹H and ¹³C FT-NMR, FT-IR, UV, elemental analysis, EI-MS and CI-MS, TLC, various size fractionation techniques and chromic acid oxidation followed by GC-MS analysis of degredation products. The UCM was found to comprise mainly aliphatic structures. The proportion of carbon atoms that are aromatic lies in the range 8.6–18.5% and a value of c. 10% appears to correlate with all analytical data. Oxidation products suggest that aromatic components comprise chiefly alkyl substituted benzenoid structures. Terminal alkyl chains up to c. C₁₉ are also present in the UCM. Cycloalkyl systems form an important part of the UCM structure, with aliphatic units probably having, on average, at least one ring and possibly c. 2.5. Oxidation products suggest that cycloalkyl systems comprise mainly single ring units rather than fused polycyclic systems, although the latter may be degraded to products too volatile to be detected. While the UCM was readily degraded by chromic acid oxidation, observed products accounted for only 10% of the degraded UCM. Some isoprenoidal components were detected among the oxidation products, as were n-alkane-α,ω-dioic acids, the latter deriving either from further oxidation of n-alkanoic acids produced from terminal n-alkyl chains or from polymethylene bridges within the UCM (of up to C₁₇). Comparison of the oxidation products with those obtained elsewhere from kerogen suggests that the UCM forms part of the hydrocarbon material expelled from the kerogen matrix during petroleum generation processes, but that it is not usually observed in non-biodegraded oils at the sample concentrations employed when determining n-alkane distributions by GC analysis.