Investigation of the carbon and nitrogen stable isotope distribution in oil shales using stepped combustion analysis

O'Donnell, Catherine T (1987). Investigation of the carbon and nitrogen stable isotope distribution in oil shales using stepped combustion analysis. MPhil thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00010161

Abstract

The research undertaken falls into two sections; (I) the use of the technique of stepped combustion to investigate the carbon release properties and the distribution of the carbon stable isotopes within kerogens; and (ii) an assessment of the reproducibility and efficiency of the stepped combustion technique and a preliminary study of the technique for the investigation of the nitrogen release and nitrogen isotope distribution within oil shales.

(i) A study has been made of the changes occurring in the carbon release and isotope profiles with increased maturation for a series of type II organic rich shales. This was complimented by the analysis of samples of type I kerogen which had been artificially matured using hydrous pyrolysis. Distinct changes in both the carbon release and isotope profiles were observed with increased natural maturity, implying a maturation process involving the cyclization and condensation of isotopically light aliphatic side chain material. Similar observations were made for the artificially matured samples. A series of shales which exhibit shifts in the ratio of the total organic carbon were analysed, in an attempt to determine whether or not this was a result of fluctuations in the contributions of isotopically distinct organic matter types. The analysis of one of the samples allowed two isotopically distinct organic components to be resolved; an isotopically heavy lower H/C ratio component and an isotopically lighter higher H/C ratio organic matter type. The fluctuations in the S13C ratio of the total organic carbon were able to be explained as a decrease in the S13C ratio of the higher H/C organic component with a possible contribution from the decrease in the amount of the lower H/C ratio component.

(ii) A series of experiments was carried out to assess the reproducibility of the stepped combustion technique for types I and II organic matter. The variations observed in the reproduced carbon release and isotope profiles were small in comparison with the variations used as criteria for interpretation of the data. Analyses made using varying combustion periods, indicated that for types I and II organic matter, 60 minute combustion periods are necessary in order to obtain comparable results with bulk combustions of the sample. This is thought to be an effect of low oxygen pressure in the combustion vessel. The preliminary evaluation of the stepped combustion technique for the study of the nitrogen release and isotope profiles indicates that low oxygen pressure effects were more apparent during analyses on the nitrogen extraction line. This effect was more obvious for samples containing large amounts of organic carbon. Observations of the behaviour of the nitrogen release and isotope profiles were noted for small samples and lead to the conclusions that the major fraction of nitrogen occurs in aromatic sites in the kerpgen structure which are affected by the condensation processes of kerogen maturation. A decrease in the S15N of the total organic matter may occur with Increased maturation.

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