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Neogene Sedimentological and Geochemical Model of the Mura basin, NW Croatia, SW Margins of the Central Paratethys

The region of NW Croatia belongs to the SW edge of the Pannonian Basin System. This part of Croatia features two basins with different Early Miocene succession: North Croatian Basin and Hrvatsko Zagorje Basin (PAVELIĆ & KOVAČIĆ, 2018 and references therein). The earliest Neogene deposits of the Mura Depression are Egerian in age. They are deposited in marine, brackish and freshwater settings (paralic environment) inherited from Oligocene. Continuing through Eggenburgian, Ottnangian, Karpatian, Badenian and Sarmatian various different clastic and carbonate sediments in marine, transitional and continental environments are deposited. Disconnection of the Paratethys from the Tethys Ocean during the Sarmatin caused significant paleoecological changes. Late Sarmatian uplift was followed by Early Pannonian subsidence and flooding thus forming the Lake Pannon characterized by brackish and reduced salinity environments. Most common sediments in shallow and proximal (littoral) environments were thin- bedded clayey limestones and calcite-rich marls, while pelite sedimentation continued in the deeper settings. During the rest of the Pannonian s. l. (approx. 11 to 5.5 Ma) progradational system from the north initially noted in the Early Pannonian, shifted towards the southeast transporting huge amounts of material from the Alps and Western Carpathians. Alternating beds of sandstones, siltstones and silty marls were deposited continuously but the environment became dominantly freshwater. Due to progradation turbidite lobes, subaquatic fans and channel fills were overlaid by slope, shelf and deltaic sediments, eventually leaving big portions of the Mura Depression on dry land. The environment became shallower thus deltaic, shallow lake, swamp, marsh and fluvial settings prevailed. The Upper Miocene deposits are overlaid by Pliocene siliciclastics accumulated in small freshwater lakes, swamps and rivers. Source rocks and hydrocarbons of the Mura area are analytically defined and correlated. Generally, gases of the Mura area are wet, thermogenic and condensate related. Condensates have similar characteristics both in bulk, as well as in specific parameters, indicating thermal condensates that originate from dominantly terrestrial precursors. Analytical parameters reveal processes of evaporative fractionation, probably caused by structural deformation and low seal effectiveness. Oil accumulations are small, characterized by extremely high solid paraffin content. Silty marls and limy pelites of Eggenburgian age, at depths exceeding 3000 m, are the main sources of wet gas and condensate (BARIĆ et al., 1996). Oil-prone source rocks of Sarmatian age generated high wax crude oils. Organic facies and maturity of identified source rocks are in accordance with reservoir hydrocarbons, which further proves short distance migration, either through the system of faulted zones, or by lateral drainage into the uplifted structural forms. Condensates and oils have been altered by the evaporative fractionation process. In some locations the degree of hydrocarbon alteration is extremely significant, as the lack of effective seals makes dismigration and dispersion of light hydrocarbons possible. On the basis of multidisciplinary analyses of the cores and drill cuttings from numerous boreholes, available wireline logs and seismic diagrams, important data were collected in order to define biofacies, lithofacies and organic facies as a function of the depositional environment. The data/facies were the key elements for designing the sedimentation model scheme and defining the paleoenvironments with all associated facieses in specific time period, and finally, creating the Gross Depositional Environment Maps (GDE maps). GDE maps are not only facies charts with specified age, paleoenvironment and mechanism of sedimentation but also graphic representations of the supposed spreading of particular sediments in the area of interest. The observed area is relatively small, but due to its dynamic geological past, it is extremely heterogeneous in terms of the environment, facies and lithology. Therefore, large amounts of various data were collected and used synergistically in order to define sedimentation models and consequently geochemical models, allowing detailed determination of hydrocarbon potential of the entire Mura Depression.

Mura depression ; Facies ; GDE ; Source rock ; Hydrocarbon potential

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