Influences of secondary oxidation and sulfide formation on several maturity parameters in Kupferschiefer

Vitrinite reflectance and methylphenanthrene indices are important maturity parameters for coal and other sedimentary organic matter. They have been widely used for determining the maturity of organic matter in Kupferschiefer. However, during the study of organic matter in Kupferschiefer, it has been found that vitrinite reflectance and methylphenanthrene indices were influenced by oxidizing fluids and sulfide formation. In order to study the influence degree on maturity parameters, three Kupferschiefer profiles from the Konrad, Polkowice and Sangerhausen mines were analysed by microscopic and geochemical methods. The results indicate that the Kupferschiefer in the studied areas was deposited in normal marine and euxinic environment according to organic facies parameters. The kerogen belongs to Type II. Under the influence of secondary oxidation, organic matter was altered from Type II to Type III. In the oxidized samples, the values of vitrinite reflectance are clearly higher and the values of Methylphenanthrene Indices are lower than those of non-oxidized samples because of the depletion of alkylated phenanthrenes. The actual maturity can be calculated by revision of measured vitrinite reflectance and calculated vitrinite reflectance in the oxidized Kupferschiefer samples. In high mineralized samples hydrogen donation for thermochemical sulfate reduction occurred in alkylated phenanthrenes and resulted in a decrease of methylphenanthrene index values during Kupferschiefer mineralization. The methylphenanthrene index values in these samples can be calculated empirically.