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Geological Society, London, Special Publications; 1994; v. 78; p. 261-274;
DOI: 10.1144/GSL.SP.1994.078.01.18
© 1994 Geological Society of London

Fluid Evolution: Migration and Precipitation of Hydrocarbons and Metals

Organic matter alteration and fluid migration in hydrothermal systems

Bernd R.T. Simoneit

Petroleum and Environmental Geochemistry Group, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA

Hydrothermal systems associated with oceanic spreading centres are now recognized as relatively common phenomena, and the organic chemistry occurring there is geologically rapid and novel. Especially in the marine hydrothermal systems at water depths >1.5 km, this chemistry proceeds at high temperatures (up to >400° C) and high pressures (>150 bar) in an aqueous open flow medium. Continental systems may also be of interest, as for example failed or dormant rifts and regions around piercement volcanoes. Organic matter alteration to petroleum hydrocarbons by reductive reactions occurs in hydrothermal systems over a wide temperature window (c. 60 to >400° C), under high pressure, and in a brief geological period of time (years to hundreds of years). Thus, the products are considered to be in a metastable equilibrium state during their brief formation and residence times at high temperatures. These conditions are conducive to organic chemistry which yields concurrent products by reduction (due to mineral buffering), oxidation (high thermal stress) and synthesis. Therefore, this brief time interval coupled with the wide range of hydrothermal petroleum compositions indicates that organic matter maturation and petroleum generation, expulsion and migration occur as an overlapping continuous process during hydrothermal activity. The behavior of organic matter (CH4-C40+) in near to supercritical water, in many cases with associated high concentrations of CO2 and methane, needs to be further characterized in order to understand the implications of this novel phenomenon in geological and geochemical processes.





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