D/H isotope ratios of kerogen, bitumen, oil, and water in hydrous pyrolysis of source rocks containing kerogen types I, II, IIS, and III
Immature source rock chips containing different types of kerogen (I,II,IIS,III) were artificially matured in isotopically distinct waters by hydrous pyrolysis and by pyrolysis in supercritical water. Converging isotopic trends of inorganic (water) and organic (kerogen, bitumen, oil) hydrogen with increasing time and temperature document that water-derived hydrogen is added to or exchanged with organic hydrogen, or both, during chemical reactions that take place during thermal maturation. Isotopic mass-balance calculations show that, depending on temperature (310--381 C), time (12--144h), and source rock type, between ca. 45 and 79% of carbon-bound hydrogen in kerogen is derived from water. Estimates for bitumen and oil range slightly lower, with oil-hydrogen being least affected by water-derived hydrogen. Comparative hydrous pyrolyses of immature source rocks at 330 C for 72h show that hydrogen in kerogen, bitumen, and expelled oil/wax ranks from most to least isotopically influenced by water-derived hydrogen in the order IIS {gt} II {approximately} III {gt} I. Pyrolysis of source rock containing type II kerogen in supercritical water at 381 C for 12 h yields isotopic results that are similar to those from hydrous pyrolysis at 250 C for 72 h or 330 C for 133 h. Bulk hydrogen in kerogen contains several percent of isotopically labile hydrogen that exchanges fast and reversibly with hydrogen in water vapor at 115 C. The isotopic equilibration of labile hydrogen in kerogen with isotopic standard water vapors significantly reduces the analytical uncertainty of D/H ratios when compared with simple D/H determination of bulk hydrogen in kerogen. If extrapolation of their results from hydrous pyrolysis is permitted to natural thermal maturation at lower temperatures, the authors suggest that organic D/H ratios of fossil fuels in contact with formation water are typically altered during chemical reactions, but that D/H ratios of generated hydrocarbons are subsequently little or not affected by exchange with water hydrogen at typical reservoir conditions over geologic time. It will be difficult to utilize D/H ratios of thermally mature bulk or fractions or organic matter to quantitatively reconstruct isotopic aspects of paleoclimate and paleoenvironment. Hope resides in compound-specific D/H ratio of thermally stable, extractable biomarkers (molecular fossils) that are less susceptible to hydrogen exchange with water-derived hydrogen.
- Research Organization:
- Indiana Univ., Bloomington, IN (US)
- OSTI ID:
- 20006265
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 63, Issue 22; Other Information: PBD: Nov 1999; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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