Abstract This paper provides analysis of the published materials on the occurrences of the Dacian and Cimmerian molluscs in the Danube River valley as well as the results of Pliocene sediments study based on core material of the boreholes drilled at the Black Sea Shelf east of the Danube River Delta. In the early Pontian time, the Dacian Basin was a large sub-basin of Paratethys which, due to an abrupt drop in sea level, separated into the Euxinian, Dacian and Caspian basins. At the end of the Bosphorus time, the discharge of the Dacian Basin waters into the Euxinian Basin formed a wide valley from the Galati-Reni region to the east through the Galati gateway. During the Cimmerian transgression, a vast bay existed on the site of the modern Danube Delta, from which mutual migrations of the Dacian and Cimmerian molluscs took place along the runoff valley. The cessation of runoff occurred during the regressive phase of the Late Cimmerian. The rhythmically bedded thick strata originated during the existence of the runoff valley. These strata were identified as the Pridanubian Formation (Suite). The cryptogenic form of Tulotoma Tulotoma ( =Viviparus ) ovidii nasonis (Bogachev) is characteristic of the lower and middle parts of the suite. The presence of the Dacian and Cimmerian molluscs in this suite became the basis for the correlation of sediments of the Dacian and Cimmerian regional stages. The Duabian molluscs were registered in the Cimmerian deposits of the Transcaucasus (the Duabian layers), Priazovye and the Kerch–Taman region. The migration of these molluscs took place during the regressive phases due to the circular current in the Euxinian Basin similar to the one existing in the Black Sea today. The Pliocene formation contains marine and continental deposits of the Lower and Upper Pliocene, which are represented by the Pridanubian Formation (Lower and Upper), Cimmerian deposits (non-subdivided Lower and Middle Cimmerian), Lower Kujalnician deposits, Upper Poration deposits, complex of red-coloured palaeosols (the Upper Miocene–Lower Pliocene non-subdivided). The formation of the Pliocene sediments on the Black Sea Shelf, east of the Danube Delta, was controlled by the inter-basin connectivity of the Eastern Paratethys.

Abstract Oligocene and Lower Miocene deposits in the Paratethys are important source rocks, but reveal major stratigraphic and regional differences. As a consequence of the first Paratethys isolation, source rocks with very good oil potential accumulated during Early Oligocene time in the Central Paratethys. Coeval source rocks in the Eastern Paratethys are characterized by a lower source potential. With the exception of the Carpathian Basin and the eastern Kura Basin, the source potential of Upper Oligocene and Lower Miocene units is low. In general, this is also valid for rocks formed during the second (Kozakhurian) isolation of the Eastern Paratethys. However, upwelling along a shelf-break canyon caused deposition of prolific diatomaceous source rocks in the western Black Sea. Overall, Oligocene–Lower Miocene sediments in the Carpathian Basin (Menilite Formation) can generate up to 10 t HC m −2 . Its high petroleum potential is a consequence of the interplay of very high productivity of siliceous organisms and excellent preservation in a deep silled basin. In contrast, the petroleum potential of Oligocene–Lower Miocene (Maikopian) sediments in the Eastern Paratethys is surprisingly low (often <2 t HC m −2 ). It is, therefore, questionable whether these sediments are the only source rocks in the Eastern Paratethys.

Abstract Oligo-Miocene (‘Maikopian’) deposits are considered the main source rocks in the Black Sea area, although only a few source-rock data are available. Geochemical logs from nine wells are used together with age constraints provided by calcareous nannoplankton, well and seismic data to determine vertical and lateral changes of the source potential. Oligocene rocks overlie Eocene deposits with a major unconformity on the western Black Sea shelf in Bulgaria. A west–east-trending erosional structure (the Kaliakra canyon) developed during Lower Oligocene time and was filled with Oligo-Miocene deposits. Potential source rocks are present in different stratigraphic units, but the most prolific intervals accumulated during time intervals when the isolation of the Paratethys resulted in oxygen-depleted, brackish environments with high bioproductivity. These include Lower Solenovian rocks related to blooms of calcareous nannoplankton, which form an extensive layer outside the Kaliakra canyon. This unit hosts a good potential to generate oil and gas. Diatom-rich, very good oil-prone source rocks accumulated during a second isolation event in the Kozakhurian. Thick sections of these diatom-rich rocks occur within the canyon and are present in thin layers outside of it. High productivity of siliceous organisms is attributed to upwelling within the canyon. All studied units are thermally immature on the shelf.