Untersuchungen zur Beeinflussung der pyrolytischen Gasbildung aus Kerogen durch Muttergesteinsmineralien

The influence of source rock minerals on gas generation during kerogen pyrolysis was investigated using Posidonia shale samples (Lias ) from north-western Germany representing different maturity levels. The type II kerogen was isolated from its mineral matrix in order to define gas generation unaffected by minerals as point of reference. Microbiological pyrite leaching with $\textit{Thiobacillus Ferrooxidans}$ was applied in the procedure of kerogen isolation in addition to the common acid treatment. After 10 weeks about 80 % of pyrite was dissolved. The final kerogen concentrates contained 72 to 76 wt.-% total organic carbon (TOC). The experimental set-tip for quantitative analysis of the gaseous pyrolysis products hydrogen and light hydrocarbons (C$_{1}$-C$_{4}$) consisted of a tubular furnace and two gas chromatographs. According to the method o open-system non-isothermal pyrolysis the samples were heated to 950°C with a constant heating rate (0.1°C/min) in an argon atmosphere. The gas evolution profiles obtained were seperated into "primary" (T $\le$ 550°C) and "secondary pyrolysis" (T > 550°C) due to cracking reactions of some of the gaseous compounds above 550°C. Kinetic evaluation of a few selected gas evolution profiles for individual hydrocarbons revealed a strong dependence of the kinetic parameters on the mathematical model applied. Using a discrete distribution of activation energies E$_{A}$ values of E$_{A}$ and pre-exponential factors k$_{o}$ could be determined that were in agreemen with physico-chemical as well as with organic-geochemical concepts, but for several reasons these values only could be regarded as merely formal parameters for the mathematical description of the measured profiles. During pyrolysis of mixtures of a kerogen concentrate with individual minerals, quartz and illite showed almost no effect on gas formation. Calcite enhanced the amount of H$_{2}$ evolved but reducedthe hydrocarbon yields by 10 to 40 %. With increasing kerogen maturity the differences grew larger as compared to the kerogen concentrates. Addition of montmorillonite mostly affected the yields of iso-butane and iso-butene, which were increased by up to 360 % due to reactions via carbenium ions. However, this influence diminished with increasing kerogen maturity. The yield reduction of all pyrolysis gases caused by pyrite became more evident with mature and overmature kerogen. This loss may be explained by reactions of kerogen with the individual minerals or between primarily formed gaseous products. The same reasons may applyto the reduced H$_{2}$ formation in the presence of anhydrite, which only had little affection on the formation of hydrocarbons. As a rule the influence of the minerals was most obvious for hydrogen and methane generation. With increasing hydrocarbon chain length and kerogen maturity usually the effects diminished. This was evident not only with respect to the product yields but also in view of the different evolution profile shapes and the shift of the T$_{max}$-values. Pyrolysis of a kerogen concentrate in a water vapour atmosphere caused reactions similar to those in coal gasification. Based on these results true catalytic effects of the investigated minerals appear to be excluded under the experimental conditions. In addition the transferability of the experimental results to natural systems seems to be questionable, especially taking into account considerable differences between temperatures in laboratory and nature