181 cutting- and three core samples from three wells (Yury Shimanov-A, Samotino (Melrose) 1, Bogdanov North) offshore Bulgaria were studied to quantify the source potential of Cenozoic rocks in the Bulgarian sector of the Black Sea and to detect vertical and lateral changes in their source potential. Published and unpublished data from four additional wells were used for the interpretation of the results. Seismic data show a major erosional channel offshore Bulgaria, which cuts into the underlying Oligocene Ruslar Formation and the Eocene Avren Formation. According to seismic interpretation and calcareous nannoplankton data, channel incision took place in Early or Middle Miocene time. The Miocene channel fill is termed Samotino Group to distinguish it from Miocene formations, which were deposited outside of the channel. The studied Eocene to Miocene succession is thermally immature and contains potential source rocks in several horizons. Following the nomenclature of Peters (1986) good to very good source rocks for oil and gas occur in the Ruslar Formation, the Samotino Group and Miocene units outside of the channel. Layer II of the Samotino Group is the best source rock interval for oil. This diatom-rich interval has been deposited within the Miocene channel in a brackish, anoxic environment. In well logs it is characterized by low gamma radiation and low density. The seismic facies is characterized by chaotic reflectors. Oil-prone source rocks also occur in the Lower Oligocene part of the Ruslar Formation. The lower part of the Ruslar Formation is composed of shales and overlying marls deposited in anoxic to dysoxic environments with varying salinity and free H2S in the water column. The Ruslar Formation is laterally extensive, but erosion by Miocene channels has to be considered. Specific biomarker data (abundance of steranes, presence of thiophenes and presence of diatom-related biomarkers) will allow distinguishing between oils generated in the Ruslar Formation and the Samotino Group. Deposition of source rocks was controlled by anoxic and dysoxic conditions, varying salinities, the varying contribution of autochthonous (algae, diatoms) and allochthonous (landplants) organic matter and the basin configuration (channel incision, upwelling).