ORGANIC-ACIDS FROM SOURCE-ROCK MATURATION - GENERATION POTENTIALS, TRANSPORT MECHANISMS AND RELEVANCE FOR MINERAL DIAGENESIS

Hydrous pyrolysis has been used to simulate maturation of source rocks with respect to the generation of organic acids and CO2. The results from pyrolysis experiments on eight different source rocks at this laboratory are given and compared with published data. The investigations show levels of generation for acetic acid equivalent to 0.2-1.2% of the TOC of the source rock and total acids at a level of 1-2% of the TOC. The generated CO2 is equivalent to 1-10% of the TOC. The relative amounts of CO2 and organic acids and the maturity dependence of the yield are a function of the source rock type. The amounts of acids generated are sufficient to give significant concentrations in the fluid phases of the source rock and, after migration, in adjacent rocks. Reaction and equilibration with minerals will occur at the first contact, which in most cases will be in or close to the source rock. The organic acids may initially be dissolved in the oil phase, but their high aqueous solubilities will make them diffuse rapidly out of the oil phase and into the surrounding water phase as soon as the oil has migrated into porous carrier beds. The migration of the petroleum phase is driven by buoyancy and there is no equivalent drive for water flow from source rock to reservoir. If the distance between source and reservoir is large, the organic acids found in the reservoir formation waters must therefore have been transported in the oil phase or generated in situ from emplaced oil or disseminated kerogen. The high water solubility of the acids will, however, limit the distances they can be transported in the oil phase. In situ generation in the reservoir must therefore be seriously considered.