Carbonaceous residues occur in mudstones and turbidites of the Lower Paleozoic accretionary prism in Ireland and Scotland (Longford-Down, Southern Uplands). The accretionary prism was host to fluid migration during both the Lower Paleozoic and Carboniferous–Triassic, so the age of the carbonaceous fluids is uncertain. Raman spectroscopy data for the carbonaceous residues in the Lower Paleozoic rocks indicate palaeotemperatures similar to those measured in their host rocks, and much higher than those in the Carboniferous cover rocks or in residues in the cover rocks. These data show that residues in the prism rocks are of Lower Paleozoic age, and were not contributed by younger mineralizing fluids. The residues represent hydrocarbons generated from source rocks in the prism that were part of the Ordovician–Silurian deposits of the Iapetus Ocean.