Unconventional hydrocarbon resources of the Bjelovar Subdepression (Pannonian Basin System) in Croatia: an overview

The Croatian part of the Pannonian Basin System includes several Miocene chronostratigraphic (sub)stages mostly characterized by weak permeable elastic sediments. They are often also mature source rocks at depths of more than 2500 m, from Late Badenian to Early Pannonian ages, represented by marls and calcitic marls, and kerogene Types II and III. The other types of weakly permeable sediments are tight sandstone mostly of Badenian age. Those two lithotypes are potential unconventional reservoirs described in the Bjelovar Subdepression, regarding their age, geological evolution, lithology, porosity and permeability. Domination of kerogene Type III and low total organic carbon defined marls as gas-bearing source rocks. Both marls and tight sandstones mostly have porosity less than 10 % and permeability less than 10(-3) mu m(2). It is about 10-100 times lesser permeability than in conventional reservoirs. Weakly permeable zones are highly stochastically distributed and fluid flows are relatively short (several meters), which could be enhanced only by the using hydraulic or other fracturing techniques.