The Silurian of southwestern margin of the East European Platform (Ukraine, Moldova and Romania) : lithofacies and palaeoenvironments

Silurian strata, stretching along the western margin of the East European Platform from the Baltic to the Black Sea, represent a potential target for both conventional and unconventional hydrocarbon exploration. Distribution of the black shale facies, prospective for shale gas, and the reef facies, prospective for oil, has been studied in respect of palaeoenvironments. The Silurian sequence has been investigated in the territory of Ukraine (Volyn-Podillyan Plate, Dobrogean Foredeep) and correlated with the data on Moldova and Romania (Moldovian Platform). The occurrence of Silurian strata, their thickness, and petrographic and lithological characteristics allowed reconstructing the distribution of open-shelf, reef and lagoonal facies. The reef facies migrated during the Wenlock–Middle Pridoli, shifting towards the open sea and back towards the shore, and therefore has been termed a migrating reef facies. Correspondingly, the boundary between the open-shelf and reef facies was shifting. The facies distribution was controlled by the transgressive-regressive cycles, which caused the fluctuations of the shelf water depth in different time intervals of the Silurian. The shelf water depth of about 100 m, where the top of the oxygen-minimum layer impinged on the sea bottom, was the boundary between the open-shelf facies, represented by organic-rich sediments, and the reef buildups.