Stable carbon-isotope geochemistry of low-molecular weight organic acids in oil-associated waters.

摘要

The stable carbon-isotope geochemistry of low-molecular weight organic acids is addressed in this study. Organic acids were generated in the laboratory from six oil-prone source rocks under hydrous pyrolysis conditions that simulated the oil-generation process. Acid concentrations and carbon-isotope compositions were determined in order to identify and characterize the relative importance of specific generation and post-generation reactions. Reaction mechanisms proposed in order to account for observed isotopic trends were tested using model compounds and site-specific 13C-labeled organic acids. Organic acids were found to exchange carboxyl carbon with inorganic carbon, leading to intramolecular carbon-isotope variability that depends on the delta 13C value of the inorganic carbon, the delta13C value of the alkyl carbon, the pH of solution and the reaction temperature.; Two new analytical methods were developed for the stable carbon-isotope analyses of aqueous organic acids. Solid-phase microextraction is a solventless extraction method that was used in this study with conventional continuous-flow isotope-ratio methodology to permit accurate and precise delta13 C analyses of dissolved organic compounds with minimal isotopic fractionation. Intramolecular carbon-isotope variation was demonstrated by using a new modification of the conventional irmGCMS technology. The combustion oven was replaced with a pyrolysis unit that, under continuous-flow conditions, caused the carboxyl carbon to cleave from individual organic acids and allowed for its delta 13C determination.; Carbon-isotope analyses of organic acids produced from the hydrous pyrolysis of source rocks showed that organic acids, in general, became more isotopically enriched at higher reaction temperatures and with decreasing molecular weight. These trends were source rock dependent and were interpreted to result primarily by exchange between the carboxyl carbon and dissolved inorganic carbon. Carbon-isotope analyses of organic acids from model compound experiments showed that under catagenic conditions, organic acids exchange their carboxyl carbon with aqueous inorganic carbon in a pH-dependent reaction. The rate of exchange is equal to, or more rapid than decarboxylation. Site-specific 13C-labeled model compound experiments and intramolecular carbon-isotope determinations showed that organic acids thermally generated from source rocks contained isotopically distinct alkyl and carboxyl carbon.