Spatial distribution of micro- and nanoporosity in Oligocene Menilite and Cretaceous Lgota mudstones (Outer Carpathians): Organic porosity development as a key to understanding unconventional hydrocarbon reservoirs?

Abstract

The Oligocene Menilite Beds are considered the most important source rock for hydrocarbon accumulation in the Polish Carpathian region, whereas the Cretaceous Lgota Beds have been regarded as an additional potential source rock. Understanding their petrophysical and geochemical properties is essential for evaluating the hydrocarbon potential of these beds.This paper presents mineral and organic porosity characterization and focuses on factors responsible for the development of organic pores as a reflection of the depositional and diagenetic processes. Mudstones were evaluated as potential source and reservoir rocks, describing their diagenetic and thermal history and examining their effective porosity and permeability. The results show that the Lgota Beds mudstone in the Huczwice quarry is thermally mature (late oil/early gas window, Tmax 460–470 °C), containing type III kerogen and TOC between 0.68 wt% and 4.2 wt%, in contrast to the Menilite Beds mudstone (Monasterzec outcrop), which is thermally immature (Tmax<426 °C), containing type II kerogen and TOC content from 1.24 wt% to 8.7 wt%. The geochemical properties show that the Lgota mudstone is currently an ineffective source rock, whereas the Menilite mudstone can be a potential source rock. SEM-identified pores include mineral pores, organic pores and microfractures. Organic porosity is observable both in immature oil-prone type II kerogen and highly mature gas-prone type III kerogen. The amount of pores in organic matter increases with maturity, and no relation between TOC and organic porosity development has been observed. Palynofacies analysis showed that the Menilite and Lgota mudstones are dominated by amorphous organic matter, and that the Lgota mudstone also contains opaque woody material. MICP measurements indicate high (up to 15%) effective porosity values for the Menilite Beds and up to 8% for the Lgota Beds, with very low permeability values (<0.1 mD) in both cases. Isotherms obtained from nitrogen adsorption are type IV for the Lgota Beds and type II for the Menilite Beds, while the BET surface areas are around 13 m2/g and 3 m2/g, respectively. The Lgota Beds demonstrate advanced diagenetic processes such as compaction, cementation, dissolution, replacement, and transformation, which contributed to the significant reduction in porosity, while the Menilite Beds represent an early stage of burial with the prevailing impact of compaction and thus less destruction of original pores. Finally, the Menilite Beds from the Monasterzec outcrop do not demonstrate sufficient conditions for shale-oil/-gas source rock due to the lack of proper thermal maturity. Such criterion is fulfilled by the Lgota Beds in the Huczwice quarry, but due to very low hydrocarbon potential, the Lgota mudstone is an ineffective source rock. However, given the other petrophysical and geochemical properties, the analysed formations may constitute a basis for further research on the occurrence of unconventional reservoirs in the entire Carpathian region.