During the Devonian, large parts of Sweden were covered by sandstones, shales and limestones of Precambrian to upper Silurian Age. At the end of the Devonian (360 M. Yr B.P.) a meter struck, causing a large impact crater at Lake Siljan with a diameter of 50–60 km. Annular depression was formed which is still preserved today; this depression contains undisturbed as well as tilted Paleozoic rocks. Numerous sedimentary occurrences of asphalts and seep oils are reported from quarries in the sedimentary rocks, and some shallow boreholes contain a thin layer of free oil floatingon arterian groundwater. A representative set of Paleozoic shales and limestones found in the Siljanarea was analyzed with respect to organic richness, type and thermal maturity. Three intervals of organic-rich and oil-prone source rocks (2–10% TOC) were distinguished, and extracts of these rocks were geomically compared with the seep oils using stable carbon isotopes and terpenoid biomarkers. Even though these low maturity oils were altered by biodegradation, a correlation with Ordovician source rocks was established. All the analyzed organic-rich rocks are on the verge of oil generation as determined by 20S/20R 5 (H) C 29 sterane ratios. In addition, burial history indicates insufficient overburden available for oil generation. The seep oils, however, are more mature and within the oil window, yet below peak generation levels. Thus, it appears that the heat effect of meteor impact locally matured the source rock, allowing for geologically instantaneous hydrocarbon generation and expulsion, resulting in the formation of seep oils.