Petroleum occurring in lower Paleozoic rocks is known to be present in southern Scandinavia, northern Poland, and the Baltic states. Oil has been produced from lower Paleozoic reservoirs in Sweden; northern Poland; and the Baltic countries Lithuania, Latvia, and the Russian exclave area of Kaliningrad. The sources for this petroleum are marine, organic-rich muds deposited in the Cambrian, Ordovician, and Silurian. This article concerns geochemical analysis of oils extracted from sandstones and carbonates from the Norwegian Oslo Graben rift and locations in Sweden and describes, in addition, insoluble bitumens collected from lower Paleozoic rocks in the Oslo Graben, locations in Sweden, and from upper Paleozoic rocks in a Norwegian North Sea well. The oils in this study have several geochemical characteristics shared with oils from the Baltic states and northern Poland, and the maturities of the oils are, in general, low. The occurrences of bitumen and migrated petroleum in the Oslo Graben lead us to believe that petroleum also has been generated and expelled in the related offshore Skagerrak Graben, indicating that a Paleozoic petroleum system operated in the Skagerrak Graben. This potential petroleum system has not suffered the degree of uplift, erosion, and destruction of reservoirs experienced by the onshore Oslo Graben, making preservation of any petroleum accumulations in the Skagerrak Graben more plausible. Although speculative, these considerations should interest anyone involved in petroleum exploration in the Skagerrak and the Norwegian-Danish Basin, not the least because of the proximity of Skagerrak and major energy markets in Europe. Jon H. Pedersen received his Cand. Scient. degree and his Ph.D in organic geochemistry and geology at the University of Oslo, Norway. Since 2006, Pedersen has been working in Statoil ASA as an exploration geologist and petroleum system analyst. His interests are, in particular, Paleozoic–aged source rocks and petroleum. Dag A. Karlsen is an associate professor in petroleum system analysis at the University of Oslo, Norway. Karlsen has conducted research in close cooperation with all oil companies active on the Norwegian shelf and supervises master's degree and Ph.D students. Karlsen continues to specialize in methods for examining the products generated in natura from coals and other source rocks. Nils Spjeldnæs died in March 2006. Spjeldnæs started his career at the University of Uppsala, Sweden. He became professor at the University of Århus, Denmark, in 1965, before joining the University of Oslo as a professor in paleontology from 1984. Spjeldnæs was also member of the Norwegian Academy of Science and honoree doctor at the University of Athens, Greece. Kristian Backer-Owe manages the Organic Geochemistry Laboratory at the Department of Geosciences, University of Oslo. For the past 15 years, he has worked on organic geochemistry applied to petroleum exploration. Kristian holds a B.S. degree in biochemistry and B.S. and M.S. degrees in geology from the University of Oslo. Jan Erik Lie has a master's degree in geophysics from the University of Bergen, Norway (1986), and a Ph.D in tectonics from the University of Oslo. Lie has worked with seismic processing, geophysical operations, and exploration in Norsk Hydro, Saga Petroleum, and RWE Dea Norway AS. He joined EMGS Americas in 2006, heading up their G&G activities in Houston. Harald Brunstad received his Cand. Scient. degree in sedimentology and petroleum geology from the University of Bergen, Norway. Since 1989, Brunstad has worked in the Norwegian Geological Survey, Norsk Hydro, and RWE Dea Norway AS. He has petroleum exploration experience from the North Sea and west Africa and is currently working in Lundin Norway AS.