The Oligocene Ruslar and the upper Eocene Avren formations in the western Black Sea area were studied to detect vertical and lateral changes in lithology and in source potential. Cuttings and core samples from four wells in the Kamchia Trough, offshore Bulgaria (LA-IV-3, Samotino More, Samotino Iztok, LA-IV-2) were analyzed. Six facies zones, from base to top, can be differentiated in the Ruslar Formation: calcareous shale (unit I), calcareous shale and marl (unit II), shale with marly layers (unit III), shale (unit IV), marl (unit V) and diatomaceous shale (unit VI). Seismic lines show that significant parts of the Ruslar Formation were eroded during the Late Oligocene. Therefore not all units are present in all wells. Although TOC contents generally range from 1 to 2 %, the source potential of most units is poor. A fair generative potential for gas and oil (nomenclature follows Peters, 1986) occurs in units II and VI of Samotino More and in units I and II in Samotino Iztok. Unit IV in LA-IV-3 and units I and V in LA-IV-2 have a good gas+oil potential. The organic matter is immature and the kerogen type ranges from II to III. The depositional environment of unit II (anoxic-dysoxic; reduced salinity) differs significantly from those of units III and IV (strictly euxinic; mesohaline). Whereas aquatic organic matter predominates unit II, the organic matter in units III and IV is dominated by terrestrial organic matter.