engleski

Regional geological settings and hydrocarbon potential of Bjelovar sag (subdepression), R. Croatia

Bjelovar sag (subdepression) covers approximately 2900 km2. This area is explored by more than 500 wells, dozens of seismic reflection profiles, gravimetric and magnetometric measurements. Twelve oil and/or gas fields are located within the sag. A majority of the wells (96 %) is located on these fields ; others are regional (4 %). Badenian, Sarmatian, Pannonian and Lower Pontian sedimets are described as the most interesting intervals regarding reservoir and source characteristics. Badenian coarse-grained sediments of the Mosti member, accompanied with the top part of basement have generally the best reservoir properties, but all favorable structures in range of migration are already discovered. Poljana and Pepelana sandstones are regional reservoir units, with several smaller favourable structures in northern, central and eastern parts of the sag. The problem is the very changeable lithological content, even on small distances. Structural and tectonic interpretation was based on several regional EL-markers for the Drava depression. Two fault systems, transversal (N/NE-S/SW) and diagonal (W/NW-E/SE), are described. Domination of normal faulting in the Badenian-Lower Pannonian period, and reverse faulting in the Upper Pannonian-Quaternary period is described. Fault zones are described as permeable. Structural interpretation was the base for detection of possible new plays in the Mosti, Poljana, and Pepelana units. Geochemical data pointed out pelitic sediments of Upper Badenian as locally potential source rocks. The described kerogene is mostly of transitional type II/III. However, type III was described by Velika Ciglena and Grubišno Polje, and type I/II by Galovac-Pavljani. Upper Badenian sediments, characterized with "oil-window" Ro and TTI values, cover about 18.5 % of the sag, but organic matter content and unfavourable terrestrial type are often limiting factors. This is why the main hydrocarbon sources are supposed in deeper parts of the Drava depression. Assumed migration pathways up to 15 km reached the Bilogora zone and the northern part of the sag. Several new plays (zones) were set in the Mosti, Poljana and Pepelana units. Hydrocarbon accumulation probability is calculated. The Šandrovac zone in Pepelana, and Cremušina zone in the Poljana unit are selected as the most favorable for additional exploration/evaluation as new prospect.

the Bjelovar sag; stratigraphy; tectonics; thermal maturity; probability; units; zones

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