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Middle and Upper Miocene source rock facies of Eastern part of Sava and Drava Depressions, Pannonian Basin

Main stratigraphic interval of the source rock occurrence in the Croatian part of the Pannonian Basin relates to the Middle Miocene and lower part of the Upper Miocene. These source rocks are determined in hydrocarbon wells and in the outcrops. Three locations with significant occurrence of source rocks on outcrops have been documented (Figure 1a). First, and most known is related to the active quarry in the vicinity of Našice described in Zečević et al. (2010). These belong to Badenian and Sarmatian succession with favorable TOC content up to 13.55 and HI values of 607 mgHC/gTOC determining type II kerogen. Second significant outcrops with source rocks can be located on Dilj gora (Figure 1 b) where three different facies belonging to different stratigraphic intervals can be observed (Cvetković et al. 2019). The oldest interval is dated in NN5 zone or Early Badenian. Maceral and Rock Eval analysis shows kerogen composition of mixed Type II/III with TOC ranging from 1.7 to 3.6% and HI values from 470 to 510 mgHC/gTOC. This interval is seven meters thick and possibly represents a significant source rock for oil and gas generation in deeper parts of the basins. Second interval is composed of two relatively thin intervals (80 cm each) in which source rocks have been redeposited as a debris breccia. Source rocks show excellent quality (8.0-10.8 % TOC, HI 547-604 mgHC/gTOC). Based on the nanoplankton and fossil analysis, the clasts can be associated with Sarmatian age as well as underlying and overlying marls which were deposited in shallow marine environment. Third interval is also a debris breccia interval of 2.5 meters thickness in which clasts can be larger than 1 m. Source rock clasts are Sarmatian while the overlying marls have biostratigraphically determined as Early Pannonian age. Third location with significant thickness of source rocks is located near Voćin (Figure 1c). Several intervals of laminated source rocks can be found within the Badenian-Sarmatian succession. Total thickness of these dark organic rich marls is over 10 meters while the preliminary TOC analysis shows concentrations of 3-5 %. The results of the study show important information for the re-evaluation of the hydrocarbon potential of the basin infill of both Sava and Drava Depressions, considering the structural and stratigraphic setting of the source rock intervals within the basin.

Source rocks, Badenian, Sarmatian, Lower Pannonian, Drava Depression, Sava Depression, Pannonian Basin

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