Petrography of the Upper Miocene Sandstones from the North Croatian Basin: Understanding the Genesis of the Largest Reservoirs in the Southwestern Part of the Pannonian Basin System

This paper presents a petrographic study of the Upper Miocene sandstones from exploration wells in the Sava and Drava Depressions in the North Croatian Basin (SW of the Pannonian Basin System), Central Europe. These sandstones represent the most important reservoir rocks for oil and gas in Croatia. A total of 130 core samples from depths of more than 3000 m were examined. The sandstones generally have a feldspatho-litho-quartzose (fLQ) composition. The modal composition of samples from the Sava Depression is Q39.2-61.0F8.9-26.0L26.1-42.3, and Q40.6-63.5F6.6-23.3L20.9-42.3 for those from the Drava Depression. Lithic fragments are dominated by extrabasinal carbonates with subordinate metamorphic grains (mostly schists) and less frequent magmatics (granitoids). Garnet, tourmaline, apatite, rutile, epidote, clinozoisite, zoisite, titanite, zircon, staurolite, and opaque minerals form the heavy mineral association and imply an area of provenance dominated by metamorphic rocks. The tectonic setting of the sandstones corresponds to recycled orogen, i.e., a subduction complex or fold-thrust belt. There are no significant compositional differences between the sandstone samples from the two depressions, thus indicating a common main source area and similar diagenetic processes in the subsurface. The sandstones originated from eroded parts of the uplifted Alpine-Carpathian fold belt and are distinct from the Lower and Middle Miocene sandstones in the North Croatian Basin. Observed variations in the sandstones' composition with regards to geographic location and depth are associated with variations in the erosion of parent rocks, including subsequent modification of detrital sediment during transport, mixing, and deposition, as well as burial diagenesis. The Sava and Drava Depressions were part of Lake Pannon in the Late Miocene with no significant topographical obstacles between them for the inflow of the detritus. The study will therefore help with the correlation of the Upper Miocene reservoir rocks in various parts of the North Croatian Basin (as well as adjacent parts of the Pannonian Basin System) and thereby assist with future hydrocarbon exploration in this area.