THERMAL GENERATION OF CARBON-DIOXIDE AND ORGANIC-ACIDS FROM DIFFERENT SOURCE ROCKS

Carbon dioxide and organic acids are claimed to play an important role in sandstone diagenesis. In the present work hydrous pyrolysis was used for a qualitative comparison of the relative potential of different source rocks to generate organic acids and carbon dioxide. In addition, the carbon isotopic composition of the generated carbon dioxide was determined. Experiments with three immature source rocks are described; a Miocene brown coal from Germany, a coal from the Middle Jurassic Ness Formation in the northern North Sea and a rich marine type II source from the Late Jurassic Kimmeridge Clay Formation onshore U.K. Both coal samples are typically humic. A stepwise heating program from 150 to 365 degrees C was used. Closed Pyrex tubes containing immature coal (%R(m) = 0.37) were heated in order to measure vitrinite reflectance evolution. The experimental results were related to vitrinite reflectance, as a maturity reference parameter, and to a kinetic model using kinetic parameters for carbon dioxide generation (Burnham A. K. and Sweeney J. J. (1989) Geochim. Cosmochim. Acta 53, 2649-2657). Differences in the yield and carbon isotopic composition were obtained between the marine type II source rock and the coals. Organic acid generation converted between 1 and 3% of the organic carbon. Carbon dioxide was the major oxygen containing component, with molar yields 5-10 times that of organic acids. The experimental yields of organic acids and carbon dioxide were compared to a first order kinetic model for carbon dioxide generation, and a reasonable correspondence in the transformation ratio curves was observed.