TY - JOUR
T1 - δ13C of low-molecular-weight organic acids generated by the hydrous pyrolysis of oil-prone source rocks
AU - Dias, Robert F.
AU - Freeman, Katherine H.
AU - Lewan, Michael D.
AU - Franks, Stephen G.
N1 - Funding Information:
These hydrous pyrolysis experiments represent the work of several researchers, of which this isotopic study is but one part. Hydrous pyrolysis of the Green River source rocks was performed by Tim Ruble in conjunction with the U.S. Geological Survey as part of his doctoral research (Ruble, 1996) . Hydrous pyrolysis of the Ghareb limestones was performed with Eli Tannenbaum (Kimron Oil and Minerals, Israel) and Hazem Ramini (Jordanian Geological Survey, Jordan) as part of a collaborative basin evaluation project with the U.S. Geological Survey. Hydrous pyrolysis of the New Albany shale was performed for a basin evaluation project headed by Dr. Michael Lewan (Lewan et al., 1995) . We thank Tanja Barth, Martin Schoell, and an anonymous reviewer for their thoughtful, constructive reviews. Funding for this work was provided by the ACS-PRF, the Atlantic-Richfield Corporation, and the U.S. Geological Survey.
PY - 2002/8
Y1 - 2002/8
N2 - Low-molecular-weight (LMW) aqueous organic acids were generated from six oil-prone source rocks under hydrous-pyrolysis conditions. Differences in total organic carbon-normalized acid generation are a function of the initial thermal maturity of the source rock and the oxygen content of the kerogen (OI). Carbon-isotope analyses were used to identify potential generation mechanisms and other chemical reactions that might influence the occurrence of LMW organic acids. The generated LMW acids display increasing 13C content as a function of decreasing molecular weight and increasing thermal maturity. The magnitudes of observed isotope fractionations are source-rock dependent. These data are consistent with δ13C values of organic acids presented in a field study of the San Joaquin Basin and likely reflect the contributions from alkyl-carbons and carboxyl-carbons with distinct δ13C values. The data do not support any particular organic acid generation mechanism. The isotopic trends observed as a function of molecular weight, thermal maturity, and rock type are not supported by either generation mechanisms or destructive decarboxylation. It is therefore proposed that organic acids experience isotopic fractionation during generation consistent with a primary kinetic isotope effect and subsequently undergo an exchange reaction between the carboxyl carbon and dissolved inorganic carbon that significantly influences the carbon isotope composition observed for the entire molecule. Although generation and decarboxylation may influence the δ13C values of organic acids, in the hydrous pyrolysis system described, the nondestructive, pH-dependent exchange of carboxyl carbon with inorganic carbon appears to be the most important reaction mechanism controlling the δ13C values of the organic acids.
AB - Low-molecular-weight (LMW) aqueous organic acids were generated from six oil-prone source rocks under hydrous-pyrolysis conditions. Differences in total organic carbon-normalized acid generation are a function of the initial thermal maturity of the source rock and the oxygen content of the kerogen (OI). Carbon-isotope analyses were used to identify potential generation mechanisms and other chemical reactions that might influence the occurrence of LMW organic acids. The generated LMW acids display increasing 13C content as a function of decreasing molecular weight and increasing thermal maturity. The magnitudes of observed isotope fractionations are source-rock dependent. These data are consistent with δ13C values of organic acids presented in a field study of the San Joaquin Basin and likely reflect the contributions from alkyl-carbons and carboxyl-carbons with distinct δ13C values. The data do not support any particular organic acid generation mechanism. The isotopic trends observed as a function of molecular weight, thermal maturity, and rock type are not supported by either generation mechanisms or destructive decarboxylation. It is therefore proposed that organic acids experience isotopic fractionation during generation consistent with a primary kinetic isotope effect and subsequently undergo an exchange reaction between the carboxyl carbon and dissolved inorganic carbon that significantly influences the carbon isotope composition observed for the entire molecule. Although generation and decarboxylation may influence the δ13C values of organic acids, in the hydrous pyrolysis system described, the nondestructive, pH-dependent exchange of carboxyl carbon with inorganic carbon appears to be the most important reaction mechanism controlling the δ13C values of the organic acids.
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U2 - 10.1016/S0016-7037(02)00871-2
DO - 10.1016/S0016-7037(02)00871-2
M3 - Article
AN - SCOPUS:0036687280
SN - 0016-7037
VL - 66
SP - 2755
EP - 2769
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 15
ER -