Evolution of Microporosity in Mature Mudstones of the Outer Carpathians, Poland

The Polish Carpathian region has over 170 years of conventional hydrocarbon exploration history, with numerous discovered oil and gas fields. In recent years, increasing attention has been given to the potential for new, alternative hydrocarbon deposits. This study evaluates three types of mature mudstones from the Oligocene Menilite Beds, Krosno Beds and Grybów Beds, collected from boreholes in the Polish part of the Outer Carpathians, as potential unconventional source and reservoir rocks. X‐ray diffraction (XRD), optical and scanning electron microscopy (SEM), N 2 adsorption and mercury injection capillary pressure (MICP) techniques were used to evaluate the pore system, organic matter and mineral composition in each lithofacies. The findings reveal that the Menilite Beds mudstone is thermally mature, with Type II/III kerogen. In contrast, the mudstones of the Grybów and Krosno beds are highly thermally mature, corresponding to the dry gas window stage, with kerogen Types IV and mixed III/IV, respectively. All three formations display a fair total organic carbon (TOC) value (0.7%–1.1%) but have low hydrocarbon generation potential (hydrogen index [HI] < 85 mg HC/g TOC). Microscopic observations confirm the presence of both mineral and organic matter pores in all three mature mudstones. Samples are dominated by nanoporosity (average pore size <14 nm), with low effective porosity values across all samples, ranging from 1.51% to 3.59%. Permeability is very low (0.029–0.081 mD). Nitrogen adsorption isotherms are type IV for all mudstones, with H3/H4 hysteresis loops, indicating slit‐ and wedge‐shaped pores. The S BET N 2 values range from 1.352 to 7.277 m 2 /g, with the highest values for Krosno Beds samples. The mudstones and claystones of the Menilite, Krosno and Grybów beds are primarily affected by compaction, with only a minor influence from cementation and dissolution. The evaluated mudstones do not qualify as effective shale oil/gas source rock; despite good thermal maturity, their hydrocarbon generation potential at the analysed depths is very low. It could be hypothesised that, given the overall volume of nanopores and their interconnectivity, such a potential unconventional hydrocarbon system may occur at greater depths or in regions where hydrocarbon generation potential has been documented, even at shallower depths.