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Origin and Maturation of organic Matter in New Zealands Coals

  • The New Zealand coal covering the complete maturity range from peat to high volatile bituminous, thus from early diagenetic to catagenesis coalification levels, has been studied in order to bring out new insights into molecular alterations, macromolecular structural evolution, elemental-compositional changes as function of maturation and to propose which processes cause these changes. As particular note from the previous observations that many immature coals from around the world often have rather high extraction yields. It is uncertain whether or not bitumen affecting on petroleum potential and structural evolution of coals. My purposes were therefore to find out the possible interaction between kerogen and bitumen during pyrolysis, and to elucidate the role of bitumen in defining petroleum potential and structural evolution of coals. Furthermore, it is assumed that low rank coals appear to be well suited for feeding the deep subsurface microbes. The products are released as either CO2 or CO that could be substrates for microbial activity. Thus, in this dissertation, I have calculated the loss of CO2 during diagenesis to give the quantitative feeding potential link to deep biosphere, using a mass balance model. To achieve these purposes, at the primary step, the facies variability as well as the molecular compositional changes within the coal band sequence in regard to distinguish the influences of organofacies and maturity need to be clear. Hence, the first aim was to gather information about depositional environment and insights into the plant communities that have contributed to New Zealand coals. Numerous organic-geochemical techniques were used to analyse the free lipids and macromolecular organic matter. Total organic carbon determination (TOC), bulk δ13Corg isotope analysis, the Rock-Eval pyrolysis, pyrolysis- gas chromatography and infrared spectrometry were performed on the original samples and the residue after solvent extraction. The crude lipid extract was separated into fractions that were then analysed by gas chromatography and gas chromatography-mass spectrometry. The obtained data shows that organic matter of New Zealand coals contains mainly terrestrial higher plant material, with a more or less constant background supply of bacterial biomass, deposited in oxidising environment. Angiosperms contributed as the main proportion of the organic matters. Gymnosperms, particularly the Podocarpaceae, Cupressaceae, Taxodiaceae, Pinanceae and Araucariaceae conifer species, still dominated during the Cretaceous. New Zealand coal is classified as mixed gas- and oil-prone. Hydrogen index values increase from 120 to 280 (mg/g TOC) with increasing maturity, which has been explained by the loss of oxygen during diagenesis. A Transformation Ratio of CO2 (TRCO2) has been formulated here in order to quantify the loss of CO2 for any given coal type. It obviously shows CO2 generation is one of the major features of diagenesis that might feed the deep biosphere. In case of study, about 10 to 105 mg CO2 per gram of total organic carbon have been released during maturation from peat to high volatile bituminous. This is equivalent to 0.23 to 2.4 millimoles CO2 per g TOC. For methanogenesis via CO2 reduction, between 0.92 and 9.6 millimoles hydrogen would be required for complete CO2 reduction during diagenesis. Future work must determine if this is feasible or not. The important role of bitumen in defining the petroleum generation potential was elucidated. The presence of hydrogen transfer agents in bitumen helps to stabilize free radicals hence prevents recombination/ repolymerization processes thus preserving the petroleum generating potential in original coals. Specially, second-order reactions between kerogen and bitumen occurred during pyrolysis that reduce the primary gas yield, but increase the potential secondary gas as well as oil yields. Therefore, it is proposed that pre-extraction of source rocks before pyrolysis, especially coals where extraction yields are particularly high, is not recommended. The comparative investigation with previously studied higher rank Carboniferous German coals showed an excellent fit for both pyrolysis and infrared spectrometry data, suggesting that the New Zealand coals can be considered as natural precursors of the German coals. The structural evolution of coals during maturation is firstly characterized by the enrichment of the aliphatic structures in low rank, peat to high volatile bituminous, then decreases with further maturation. This enrichment of aliphatic carbon content in low rank is accompanied by an increase in the average aliphatic chain length. A slightly enrichment of CH3 group is observed in maturity range 0.9- 2.1% vitrinite reflectance. Secondly, during coalification the content of protonated aromatic carbons increases until R0 ~ 1.6%, then decreases with further maturation.
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Author: Thi Anh Tiem Vu
Title Additional (German):Ursprung und Reifung organischen Materials in neuseelaendischen Kohlen
Advisor:Prof. Dr. Maria-Theresia Schafmeister, Prof. Dr. Ralf Littke, Prof. Dr. Brian Horsfield
Document Type:Doctoral Thesis
Date of Publication (online):2008/04/24
Granting Institution:Ernst-Moritz-Arndt-Universität, Mathematisch-Naturwissenschaftliche Fakultät (bis 31.05.2018)
Date of final exam:2008/01/31
Release Date:2008/04/24
Tag:coalification, deep biosphere, depositional environment, structural evolution
New Zealand coals, angiosperms, biomarkers, bitumen, diagenesis, gymnosperms, kerogen, organofacies, petroleum potential, pyrolysis
GND Keyword:Kohle
Faculties:Mathematisch-Naturwissenschaftliche Fakultät / Institut für Geographie und Geologie
DDC class:500 Naturwissenschaften und Mathematik / 550 Geowissenschaften, Geologie