Porosity Preservation in Deeply Buried (8500 m) Jurassic Calcareous Mudstones of the Vienna Basin (Austria)

Significant porosity of more than 5% was found at depths of more than 8500 m in poorly sorted, calcareous mudstones of the Malmian Mikulov Formation. These overpressured rocks are the main hydrocarbon source rock in the Vienna Basin. Core samples from present-day depths of 8150 m to 8550 m were analysed using X-ray diffraction and scanning electron microscopy. The mudstones are composed of calcite, clay minerals and quartz, with minor amounts of ankerite, albite and pyrite. SEM imaging reveals various pore types, including phyllosilicate framework pores, carbonate framework pores, carbonate dissolution pores, and organic matter pores ranging from 10 nm to 2 μm. The preservation of porosity is attributed to several factors, including their dominant calcareous composition, poor sorting, early carbonate cementation, and overpressuring due to rapid burial by Alpine Nappes. These findings provide valuable insights into the diagenetic processes and porosity evolution in deeply buried mudstones.