The impact of diagenesis on the reservoir properties of Mesozoic sedimentary rocks of the Łódź and Miechów troughs, central Poland

Mesozoic siliciclastic and carbonate strata, analysed in 28 boreholes located in the Łódź and Miechów troughs, Poland, comprise Triassic and Jurassic rocks accumulated under terrestrial and marine conditions, and Cretaceous deposits of marine origin. Siliciclastic rocks (sandstones, siltstones, mudstones and claystones, with subordinate conglomerates) are the main lithofacies in the Lower and Upper Triassic, Lower and Middle Jurassic, and in the Lower Cretaceous. Carbonate rocks dominate in the Middle Triassic and Upper Jurassic. The Upper Cretaceous succession is represented by siliceous-carbonate deposits. These sedimentary rocks were studied in terms of sedimentary basin analysis and in particular as regards their reservoir characteristics, with detailed petrographic and porosity studies of these rocks and special emphasis on the sandstones and limestones. The focus was on the effects of diagenetic processes and their influence on reservoir properties. The most important diagenetic processes that affected the reservoir properties of the rocks were compaction, cementation and dissolution, and also replacement in carbonate rocks. Compaction and cementation have reduced the porosity of the sandstones by an average of ~44% and 29%, respectively. The effects of dissolution, mainly of feldspar grains and bioclasts, contributed to the development of secondary porosity in the rocks. Replacement processes – dolomitization of limestones – also increased the porosity of the rocks. Sandstones with good reservoir properties (porosity >15%, permeability >100 mD) are found in the Lower Triassic (Buntsandstein), Lower Jurassic and Lower Cretaceous. Among the carbonates, the Middle Triassic (Muschelkalk) deposits show the best reservoir properties, but most of the porous (>10%) carbonate and carbonate-marly rocks are impermeable. The Miechów Trough, by comparison with the Łódź Trough, includes better reservoir properties.