Print this pageKerogen Maturation and the delta{13}C of Organic Acids in Oil-Associated Waters
DIAS, ROBERT F., KATHERINE H. FREEMAN, MICHAEL D. LEWAN, STEPHEN G. FRANKS
Because organic acids can enhance the dissolution of minerals, investigators have argued that acids enhance migration through the formation of secondary porosity, especially if they are liberated from source rocks into surrounding lithologies prior to the main phase of oil generation. The importance of acids in the generation of secondary porosity is disputed and, although pyrolysis data suggest that acids are released by cleavage of low-energy, non-covalent bonds during early kerogen maturation, the precise mechanism(s) and timing of organic acid generation and migration relative to the whole oil is uncertain.
Preliminary analyses of organic acids generated under catagenetic conditions (via hydrous pyrolysis of a New Albany Shale sample) demonstrate that acids become enriched in {13}C with increasing reaction temperature. These data also reveal isotopic enrichment with decreasing numbers of carbon atoms in each aliphatic acid. Possible mechanisms that can lead to the isotopic enrichment with increasing reaction temperature observed in the laboratory studies include: a) sequential release and subsequent mixing of isotopically distinct pools of non-covalently held acids; or b) kinetic isotope effect(s) associated with pyrolitic cleavage of organic acid precursors. Isotopic enrichment with decreased number of carbon atoms per molecule suggests an isotopically enriched carboxyl group, and can result from: c) carboxylation of organic materials (i.e., reactions that add inorganic carbon to organic compounds); or d) kinetic isotope effect(s) associated with oxidation of aliphatic carbon to form carboxyl groups. We will present results of isotopic analyses of individual organic acids released by hydrous pyrolysis of different kerogen types in order to evaluate mechanisms of acid generation during catagenesis.