462 Samples from 12 wells (Severna 1, Nanevo R-1, Elizavetino R-1, Bogdanov East 1, Varna West A, Galata 1, Epsilon 1, Samotino More 1, Izgrev 1, Ropotamo 1, P-79, LA/IV-91-2) have been analyzed using a multi-proxy approach to determine the lateral and vertical variation of the source rock potential of Eocene to Miocene rocks in the western Black Sea offshore Bulgaria. The data was extended by unpublished data sets from another 5 wells within the study area. The Oligocene Ruslar Formation unconformably overlies the Eocene Avren Formation. The Ruslar Formation extends from the Moesian Platform in the north to the thrust sheets of the Balkanides in the south. It is split into a lower part including the Dynow Member (NP23) and an upper part. A thin layer rich in biogenic opal occurs in the southern part of the study area within the upper Ruslar Formation. Seismic data show that extensive erosion removed large parts of the Ruslar Formation in the area of the Kaliakra Canyon and in front of the Balkanides. A deep incision (Kaliakra Canyon) into the Oligocene Ruslar Formation and Eocene Avren Formation is filled by the Samotino Group. Diatom-rich rocks with high amounts of organic matter occur in the middle part of the Samotino Group. Whereas, a Middle Miocene age is proven for the upper part of the Samotino Group, the age of its lower and middle parts is poorly constrained (Late Oligocene -Middle Miocene?). Diatom-bearing sediments with a Middle Miocene age overlie the Ruslar Formation in the southern part of the study area and have been termed Izgrev Formation. Following the nomenclature of Peters (1986), the lower part of the Ruslar Formation has a good source rock generative potential for oil and gas. The upper part of the Ruslar Formation has a fair gas potential. Very good oil-prone source rocks occur in the middle part of the Samotino Group. In contrast, the Izgrev Formation holds a good potential for gas (and minor oil).With the exception of Permo-Carboniferous coal-bearing sediments in well Nanevo 1, all investigated rocks are immature. Organic Geochemistry data (biomarkers, carbon isotopy) show that the lower part of the Ruslar Formation and the middle part of the Samotino Group have been deposited in prevailing anoxic environments with free H2S in the water column. They also point out mixed sources of organic matter for both formations.