Abstract Based on the data concerning the type, quantity and thermal maturity of the organic matter, several petroleum source rock complexes have been distinguished in the Paleozoic succession of the Baltic Syneclise. Maturity of organic matter varies from immature in the NE part of the Baltic Syneclise to peak generation in the central part and postmature stage towards the Teisseyre–Tornquist Zone. The isolated local postmature areas are most likely related to the occurrence of sills. The oils of the Baltic region have moderate gravities (26–42°API) that decrease with present reservoir depth, reflecting the thermal maturity. Quantitative modeling of the oil and gas migration and entrapment was performed by the means of the 3D software SEMI. This confirmed that oil generation and migration was most intense during Late Silurian–Early Devonian time. Based on modeling, the major Cambrian traps accumulated oil expelled from different stratigraphic source rock units: the Cambrian, Ordovician and Silurian organic rich shales.