engleski

Genesis of oil in the deepest Miocene source rocks in the North-West part of Sava depression

For the most oils in Sava depression a genetic similarity and correlation to organic facies of algal origin, enriched by bacterial biomass, with minor input of terrigenous organic matter has been established. Oils were expelled in early to mid katagenesis, from marly source rocks deposited in anoxic to suboxic environment from Middle Miocene (Badenian-Sarmatian – Prečec Formation) to Upper Miocene (Lower Panonnian – Prkos Formation). During 2014., in exploration well A, discovery of oil was found in Miocene calcareous sandstones of Prečec Formation (2659-2679 m), which differs from most of Sava oils by its bulk and specific parameters (comparison with oil from well B is given), especially by very high paraffin content (7.7-17%). Similar content of solidified paraffins (14.1-18.5%) was determined in oil from Miocene petromictic conglomerates of Prečec Formation (2866-2917 m) in well C, situated ~20 km NW from well A. Particularly interesting is the fact that the mature waxy well A oil is of marine, algal-bacterial origin, from source rocks deposited in restricted clastic environments with prevailing anoxic to suboxic conditions during deposition. Therefore, it makes it different from oil from well C, which originated from terrigenous, hydrogen enriched lipids, i.e. from regionally present mudstones/shales/marls enriched by mature terrigenous organic matter, deposited in suboxic to oxic environment. Detailed geochemical characterization applying stable isotope and biomarker parameters established a positive correlation between well A oil and clayey and sandy marls from Prečec Formation in interval 2650-2690 m. Interesting, based on lithofacies and biofacies analyses those are determined as Karpatian sediments deposited in shallow marine clastic environment on inner shelf. Considering that there is a beginning of marine transgression clearly determined in Lower Badenian, such determination still needs to be incorporated into regional depositional model for entire depression, with more proves of regional marine environment (of Paratethys) during Lower Miocene. Assessment of remaining hydrocarbon potential in Croatian part of Panonnian Basin System includes an exploration of possibility of thicker Miocene organic-rich sequences of Prečec Formation in the deeper parts of Sava Depression. They could be good oil or gas source rocks, i.e. unconventional reservoirs. Therefore the determination of age and areal extension of the marine environment is of large importance.

Sava Depression, waxy oil, correlation, marine environments

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