The composition,origin and fate of complex mixtures in the maltene fractions of hydrothermal petroleum assessed by comprehensive two-dimensional gas chromatography |
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| Institution: | 1. Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA;2. Department of Petroleum Geoscience, GNS Science, PO Box 30-368, Lower Hutt 5040, New Zealand;3. COGER, King Saud University, 11451 Riyadh, Saudi Arabia;4. Department of Chemistry, Oregon State University, Corvallis, OR 97331, USA;1. Institute for Geo-Resources and Environment, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8567, Japan;2. Technology and Research Center, Japan Oil, Gas and Metals National Corporation (JOGMEC), 1-2-2 Hamada, Mihama-ku, Chiba 261-0025, Japan;1. Université Paris Est, Institut de Chimie et des Matériaux Paris-Est (UMR7182), CNRS, UPEC, 2-8 rue Henri Dunant, F-94320 Thiais, France;2. Institut de Science des Matériaux de Mulhouse, (UMR 7361), CNRS, UHA, 15 rue Jean Starcky, F-68057 Mulhouse, France;1. Institute of Chemistry, Environmental Protection and Biotechnology, The Faculty of Mathematics and Natural Sciences, Jan D?ugosz University in Cz?stochowa, Armii Krajowej 13/15, Cz?stochowa 42-201, Poland;2. Department of Heteroorganic Chemistry, Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, ?ód? 90-363, Poland;1. Computational Biology and Bioinformatics, Graduate School of Science and Engineering, Kadir Has University, Istanbul, Turkey;2. Department of Biophysics, School of Medicine, Bahcesehir University, Istanbul, Turkey;3. Institute for Biocomplexity and Informatics, University of Calgary, Calgary, AB, Canada;4. Department of Bioinformatics and Genetics, Kadir Has University, Istanbul, Turkey;1. Department of Functional Materials Science, Saitama University, Saitama, 338-8570, Japan;2. Department of Materials Science and Engineering, Iwate University, Morioka, 020-8551, Japan;3. CERMAV-CNRS, UPR5301 & Grenoble Alpes University, 601 rue de la Chimie, BP53 38041 Grenoble, Cedex 9, France |
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| Abstract: | Sedimentary organic matter in hydrothermal systems can be altered by high temperature fluids to generate petroleum. The saturated and aromatic fractions of these hydrothermal oils are compositionally similar to conventional oil with the exception that they often contain higher concentrations of polycyclic aromatic hydrocarbons (PAH) as well as substantial mixtures of coeluting organic compounds that produce dramatically rising signal on the baseline of gas chromatograms termed unresolved complex mixtures (UCMs). Little is known about the compounds that compose UCMs and why or how they form. This is in part due to an inability to discriminate between in situ and migrated components that characterize the petroleum generated in hydrothermal systems. However, UCMs are also a product of the limitations imbedded in analytical separation techniques. With the advent of comprehensive two-dimensional gas chromatography (GC × GC), a revision of what should constitute molecular complexity needs to be considered. We address these problems by comparing the molecular compositions of the maltene fractions of three previously published hydrothermal petroleum samples using time of flight-mass spectrometry (GC × GC–ToF-MS) and 12 hydrothermal petroleum samples in cores from three locales using comprehensive two-dimensional gas chromatography with flame ionization detection (GC × GC–FID). The sediment cores were collected from Middle Valley, located off the axis of the Juan de Fuca Ridge, and the Escanaba Trough, along the Gorda Ridge, both in the NE Pacific Ocean, as well as from the Guaymas Basin in the Gulf of California. We define a UCM in GC × GC data to be a condition in which ?25% of the detected peaks within a chromatographic area coelute in either the first or second dimension. In turn, complex (CM) and simple mixtures (SM) are defined as having 5–24% and <5% coelution, respectively. All CM and UCMs were dominated by an array of configurational isomers, which becomes increasingly aromatic with higher molecular weight. We relate this to a multi-molecular complexity metric (MCM) by quantitatively comparing the difference in total peak variance and peak density for a GC × GC chromatogram. MCM values correlate with biomarker thermal maturity ratios for the Escanaba Trough and Guaymas Basin samples indicating that molecular complexity in these hydrothermal environments is in part a function of burial temperatures. Partial Least Squares (PLS) linear regression was applied to the total number of peak retention times as a proxy for the bulk molecular differences between each hydrothermal oil sample. Differences in the sample regressions correlate with the thermal maturity and the degree of PAH alkylation, indicating that this technique can be used to assess the degree of oxidative weathering due to dehydrogenation and hydrocarbon cracking. Subtracted chromatograms were then used to quantitatively track all of the individual molecular changes within the pyrolytic regime at Escanaba Trough. These subtracted chromatograms indicate that high molecular weight PAHs are highly mobile in hydrothermal fluids and may represent a phase partitioning that is occurring at greater depths. This phase condenses just below the seafloor to form an UCM in the near surface sediments. Saturated hydrocarbon biomarkers, such as hopanes, steranes and biphytanes are less mobile and more prone to being cracked and/or aromatized prior to migration toward the ocean floor. Together these techniques suggest that the molecular complexity of hydrothermal petroleum maximizes during the early stages of thermal maturation. The diversity of compounds forming these UCMs then decreases with increasing dehydrogenation, dealkylation and condensation reactions associated with elevated thermal stress and exposure to oxidants within the hydrothermal fluids. |
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