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Provenance and diagenesis of the Upper Miocene sandstones from the Pannonian Basin System

This work demonstrates the current status and research plan on provenance and diagenesis of the Upper Miocene sandstones from the Sava and Drava depressions in northern Croatia, SW part of the Pannonian Basin System. The sandstones represent the largest oil and gas reservoirs discovered in Croatia and are of significant interest to the industry. They appear as prograding forms of delta environments associated with turbidites that were developing in brackish and partly freshwater lake Pannon formed after the isolation of the Central Paratethys in the Late Miocene. Sandstone samples from selected exploration wells were analyzed primarily to determine their composition, indicating provenance and diagenesis - factors that influence the reservoir quality the most. Analyses were conducted using the polarizing light microscope Olympus BX51 and the scanning electron microscope JEOL JSM-6510 LV with energy dispersive spectrometry (Oxford INCA X-act). Some of the samples were impregnated with blue- dye epoxy resin to indicate types of porosity and were stained with Alizarin Red S to differentiate carbonate minerals. The sandstones are dominantly fine- and very fine- grained, well to moderately sorted, composed of angular to sub-rounded grains forming tangential, and to a lesser extent concavo- convex and sutured contacts. They are mature to submature, largely composed of quartz, rock fragments and feldspar. Quartz is monocrystalline and polycrystalline ; among rock fragments, sedimentary and metamorphic fragments dominate - mostly in the form of carbonates and schists ; feldspars include alkali feldspar and plagioclase. The amount of phyllosilicates (mica) and heavy minerals varies. Heavy mineral associations to the greatest extent include garnet, tourmaline, rutile, apatite, epidote, titanite, zircon and opaque minerals. Intergranular volume of the sandstones is either filled with carbonate cement (calcite and ankerite), fine-grained matrix (combination of carbonate minerals, quartz, mica and feldspar- plagioclase, formed as a result of alteration and dissolution of main grains) or clays (kaolinite, illite, chlorite and different types of interstratified clays). Processes of compaction under the pressure caused by burial depth, chemical processes of mineral precipitation, dissolution of unstable grains, the formation of pseudomatrix and authigenesis of clays were detected as diagenetic changes formed after the sandstones' deposition within the basin. However, primary intergranular porosity is the dominant type of porosity, followed by secondary intragranular porosity, and microporosity among fine-grained matrix and differently oriented clays. Petrographic analyses enabled classification of sandstones, determination of source rocks and their tectonic settings, as well as detection of diagenetic processes, that are of utmost importance for reservoir characterization. The Upper Miocene sandstones from the North Croatian Basin correspond to feldspatho-litho- quartzose type, and less often to litho- quartzose type. They belong to the tectonic setting of recycled orogen (subduction complex or fold-thrust belt). Rock fragments point towards continental- block, recycled carbonate- clastic and axial belt metamorphic complex provenance. Composition of heavy minerals mostly implies metamorphic source rocks. Composition of the sandstones and detected diagenetic processes present the most significant impact on petrophysical parametres of reservoirs, i.e. their porosity and permeability. The research plans to include additional heavy mineral analyses, geochemistry and single grain geochronology, XRD analyses, petrophysical measurements and statistical data analyses.

Provenance ; Diagenesis ; Upper Miocene ; Sandstones ; Pannonian Basin System

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