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Samples for detailed micropaleontological research were selected from two deep exploration wells (Istra more-3 and Istra more-4) in the Adriatic Sea. On the basis of index taxa and selected species of planktonic foraminifera standard planktonic foraminiferal zonation for Oligocene sediments after WADE et al. (2011) have been applied (Zone O1-O7). The investigated deposits are overlying the upper Eocene (E16) sediments, and are overlain by lower Miocene rocks (Zone M1). Distinct changes in composition of the planktonic foraminiferal assemblage have been observed near the Eocene/Oligocene boundary. In the latest Eocene, at the Eocene /Oligocene boundary, genera Cribrohantkenina, Globigerinatheka as well as most species of the genus Turborotalia (T. cerroezulensis, T cunialensis, T. cocoaensis) became extinct. Chiloguembelina cubensis appears just before the E/O boundary, and Cassigerinella chipolensis, Chiloguembelina sp., Dentoglobigerina tapuriensis, D. tripartita, D. venezuelana, Dentoglobigerina sp., Subbotina sp. appear above the boundary. Transition between Eocene and Oligocene is also characterized by the size reduction of the specimens of genus Pseudohastigerina as well as of species Chiloguembelina cubensis and Ch. ototara. The boundary between the lower and upper Oligocene is defined by the high common occurence (HCO) of Ch. cubensis, i.e. proportion of this species in planktonic foraminiferal community in the small size fraction (125-63 μm) is > 5%. The Oligocene/Miocene boundary is marked by the first appearance of species Paragloborotalia kugleri.

The upper Eocene and lower Oligocene sediments from the wells Istra more-3 and Istra more-4, located on the eastern edge of the Venetian Basin, consist of hemipelagic deposits of marls and silty marls. These sediments contain abundant and very well preserved planktonic foraminiferal assemblages. The standard planktonic foraminiferal zonation of the Palaeogene after Wade et al. (2011) has been applied. The Eocene–Oligocene boundary in the Adriatic Sea is clearly defined by the extinction of all hantkeninids, the Turborotalia cerroazulensis group, and the last globigerinathekid species Globigerinatheka tropicalis, indicating marked climatic deterioration and global cooling. Significant changes in the small size fractions (