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Mineral Redox Buffers and The Stability of Organic Compounds Under Hydrothermal Conditions

Published online by Cambridge University Press:  10 February 2011

J. S. Seewald
J. S. Seewald*
Affiliation:
Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, jseewald@whoi.edu
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Abstract

Organic compounds play an integral role in numerous geochemical process in subsurface environments. To evaluate factors that regulate the stability of ethane, ethene, propane, and propene in hydrothermal systems a series of experiments were conducted at 300 to 325°C and 350 bars. The experiments contained the mineral assemblages pyrite-pyrrhotite-magnetite, hematite-magnetite-pyrite, and hematite-magnetite to buffer fO2, aH2(aq) and aH2S(aq) at geologically reasonable values.

Results of the experiments suggest that under appropriate physical and chemical conditions, metastable redox dependent thermodynamic equilibrium involving liquid water and inorganic iron-bearing mineral assemblages may regulate the relative abundance of short chain alkanes and their corresponding alkenes. In addition, alkenes represent an important intermediary in the conversion of n-alkanes to methane and oxidized species such as carbon dioxide, ketones alcohols, and organic acids.

The rates of redox dependent organic reactions during the experiments were strongly influenced by the presence of sulfur. Under relatively oxidizing conditions greater catalytic activity due to the presence of dissolved sulfur species was observed. Fluid speciation calculations suggest that oxidized aquous sulfur compounds represent the catalytically active species.

These results suggest that redox conditions and the presence or absence of dissolved sulfur species in natural sedimentary environments may strongly influence the stability of hydrocarbons. Accordingly, models used to predict the stability of oil and the formations of natural gas need to account for chemical processes that involve both organic and inorganic sedimentary components.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 432: Symposium S – Aqueous Chemistry and Geochemistry of Oxides , 1996 , 317
Copyright
Copyright © Materials Research Society 1997

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