A geochemical and petrographical characterization of organic matter in the Jurassic Mikulov Marls from the Czech Republic

• The organic matter in the Mikulov Marls belong to the kerogen type II.
• The dominant maceral group in the Mikulov Marls are macerals from liptinite group.
• The beginning of the oil window is at 3 491 m based on hopane isomerization.

In this study a Rock-Eval pyrolysis, a petrographic composition of organic matter, vitrinite reflectance and gas chromatography were undertaken on borehole cores from the Mikulov Marls in order to define the organic matter type and determine the thermal maturity. The analyzed samples covered a depth interval of between 2300 m and 4500 m. The studied sediments were dark-colored, moderately laminated shales throughout which appear uniform upon visual examination. The geochemical analyses revealed that the source rock potential of the Mikulov Marls is from fair to good. According to the Rock-Eval pyrolysis, the organic matter in the samples was classified as kerogen type II–III. This classification is not supported by the results from the other used methods. The evaluation of the organic macerals demonstrated the dominant role of liptinite with prevailing liptodetrinite accompanied by a variable content of alginite, bituminite, and sporadic sporinite and resinite. The alginite consists mainly of lamalginite, derived from colonial planktonic or benthonic algae. Based on these results, the Mikulov Marls represent the kerogen type II. These results are also supported with a strong n-alkane odd-carbon-number predominance at n-C15, n-C17 and n-C19 range on the gas chromatography.