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Refinement of structure model of the Grubišno Polje area based on 3D seismic (Bjelovar subdepression, Northern Croatia)

The Grubišno Polje gas field is located in eastern part of the Bjelovar subdepression which is filled in by up to 2500 m thick succession of Neogene sediments where one major oil field (Šandrovac) and a number of small-sized HC accumulations were discovered. Local stratigraphic relations, as defined from the wells of the Grubišno Polje field have the following characteristics. Basement is composed of Palaeozoic schists, gneiss and other metamorphic rocks. It is covered by up to 50 m thick marly and clayey limestones and calcarenites mostly of Middle Miocene age. Majority of sediments belongs to the Upper Miocene sandstones, sandy marls and clays that are around 900 m thick. The youngest, Pliocene and Quaternary sediments - gravels, sands and clays with coal interbeds are approximately 230 m thick. Middle Miocene sediments were formed in shallow-marine conditions while Late Miocene is marked by deepening that resulted in sedimentation of turbidites and associated marine/lacustrine facies that gradually passed into deltaic ones and finally in terrestric sediments of Pliocene and Quaternary. In the area north of the field, where the most interesting details were interpreted based on 3D seismic, all the sediments (especially Middle Miocene) are much thicker and composition varies. Structural evolution of the southern part of Pannonian basin system that lies in Croatia has at least four deformational phases. The first one is characterised by extensional movements that began in the Oligocene/Early Miocene time. Most of the basins however were opened during the second, major extensional phase in the Early and Middle Miocene times. The subsidence was then achieved by NE-SW to ENE-WSW trending normal faults presumed to be localised into numerous pull-apart basins bounded by transcurrent NE-SW trending sinistral faults (Prelogović et al., 1995). In the third phase, during the Late Miocene, the whole area subsided and depositional basins were enlarged. The subsidence was compensated by deposition of thick post-rift sedimentary sequence. The fourth, Pliocene and Quaternary compressional neotectonic phase is marked by the generally N-S oriented maximal horizontal stress which reactivated a number of older faults, changing the sense of slip along the fault planes (Prelogović et al., 1998). The 90 km2 of area explored in detail is covered by 3D seismic. The following seismic horizons were interpreted with confidence Base Neogene (crystalline basement), Base Pannonian (top of syn-rift section) and the two horizons within the Upper Miocene post-rift section - Z' and Delta. The Delta, youngest horizon is used to illustrate recent structure relations. Time-slice close to this horizon is given in Fig. 1a. The most important feature is the NW-SE trending fault following the NE margin of 3D seismic volume (marked by arrows) that was observed by the discontinuity of seismic reflections. Time structure map of the Delta horizon (Fig. 1b) depicts the changeable character of slip on this fault due to curved shape of the fault plane. Because of observed differences in thickness of the syn-rift sediments on eastern and western side, the fault is interpreted to be an older, extensional one with subsided NE block. In the neotectonic phase, this fault was reactivated under roughly N-S oriented S_Hmax. With different orientation along the strike, from NNW-SSE to NW-SE, it now belongs to the zone of the Periadriatic-Drava NW-SE trending dextral wrench fault system and has a strike-slip section in the northern part of the area and transpressional section that probably extends eastward. Numerous smaller normal faults of the N-S to NNE-SSW orientation were formed in the area east of the structure contoured in the central part of Fig. 1b. These are interpreted as normal faults that result from the local stress field variation generated on the dextral wrench fault described above. Prelogović, E., Jamičić, D., Aljinović, B., Velić, J., Saftić, B. & Dragaš, M. (1995): Dinamika nastanka struktura južnog dijela Panonskog bazena. In: Vlahović et al. (Ed.), First Croatian Geological Congress, Opatija, Oct. 18-21, 1995, Proceedings 2, 481-486, Zagreb (in Croatian with an English abstract). Prelogović, E., Saftić, B., Kuk, V., Velić, J., Dragaš, M. & Lučić, D. (1998): Tectonic activity in the Croatian part of the Pannonian basin. Tectonophysics, 297, 283-293.

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