Maturation related changes in the distribution of ester bound fatty acids andalcohols in a coal series from the New Zealand Coal Band covering diageneticto catagenetic coalification levels

摘要

A rank series of lignites and coals of low to moderate maturation levels (vitrinite reflectance (R_0): 0.27-0.8%) from the New Zealand Coal Band were investigated using alkaline ester cleavage experiments toreveal compositional changes of ester bound components (fatty acids and alcohols) during increasingmaturation. Ester bound alcohols are found to be present in highest amounts in the very immature lignitesamples (R_0: 0.27-0.29%), but show a rapid decrease during early diagenesis. Ester bound fatty acids alsoshow an initial exponential decrease during diagenesis, but,reveal an intermittent increase during earlycatagenesis before decreasing again during main catagenesis. This intermittent increase was related tothe short chain fatty acids. To obtain a maturity related signal and to eliminate facies related scatteringin the amounts of fatty acids in the coal samples, the carbon preference index of fatty acids (CPIFA) param-eter is introduced here. For the long chain fatty acids (C_(20)—C(32))originating from terrigenous plant debris,the CPI_(FA)decreases with increasing maturity, showing a strong maturation related signal. During diagen-esis, the same trend can be observed for the short chain fatty acids, but the intermittent increase in theamounts of short chain fatty acids is also accompanied by high CPI_(FA)values. This indicates less alteredorganic biomass at this advanced maturation level and is in contrast to the mature CPIFA signal of the longchain fatty acids of the same samples. One possible reason for this discrepancy could be extremely dif-ferent amounts of short and long chain fatty acids in the original source organic matter of these samples.However, another intriguing explanation could be the incorporation of immature bacterial biomass fromdeep microbial communities containing C_(16)andC_(18)fatty acids as main cell membrane components. Deepmicrobial life might be stimulated at this interval by the increasing release of thermally generated poten-tial substrates from the organic matrix during early catagenesis. In contrast to the fatty acids, the highamounts of alcohols in the immature lignite samples are also visible in the alkene distribution fromthe open system pyrolysis experiments of the organic matrix before and after saponification.