Hydrocarbons, mostly found as solid pyrobitumen, are known from more than 30 localities in southeast Norway. They occur as inclusions in a wide range of "reservoir rocks" spanning from Permo-Carboniferous breccias to veins (vein quartz and calcite veins) in Precambrian granites, gneisses and amphibolites, and especially in primary and secondary pores in Cambrian, Ordovician and Silurian sedimentary rocks. In some places, vesicles in Carboniferous rhomb porphyry lava (RP1), gabbro and basalt (B1) are partly cemented with pyrobitumen, together with calcite, quartz and various accessory minerals.

The pyrobitumen occurs as a jet-black, brittle, amorphous, non-fluorescent substance with a specific gravity of about 1.3 g/cm³ and a conchoidal fracture. Earlier analyses have shown great similarities with regard to the stable isotopes d¹³C in Alum Shale (Middle Cambrian to Lower Ordovician) and pyrobitumen, indicating that Alum Shale was the most important source rock. Petrographic investigations combined with stable isotope analyses (d¹³C and d¹⁸O) of the cement containing pyrobitumen indicate two phases of hydrocarbon migration. The first phase probably took place in Upper Silurian to Lower Devonian time, when the Alum Shale entered the oil window. These hydrocarbons are mostly found as pyrobitumen in primary voids and calcite cemented veins in Cambro-Silurian sedimentary deposits. The second phase is probably of Late Carboniferous/Permian age and was due to the increased heat flow during the formation of the Oslo Rift. These hydrocarbons are found in tectonically disturbed Precambrian basement rocks, veins in Lower Palaeozoic sedimentary rocks and as infills in amygdales in the two oldest extrusives (RP1 and B1) known from the Oslo Graben.

The first migration phase was probably mainly directed towards higher stratigraphic levels. The second migration phase is suggested to have been much more complex including lateral, vertical and even downwards migration to basement. The occurrence of pyrobitumen in basement rocks outside the Oslo Graben, where no Palaeozoic rocks have been preserved, indicates that a thick cover of Palaeozoic sediments and lavas was originally also present outside the Oslo Rift.