Abstract <p>The impact of mafic magmatism on hydrocarbon systems remains debated, particularly in Precambrian settings. The results of combined field and petrological studies, carbon isotope analysis, and Raman spectroscopy for mafic sills intruded into carbon-bearing sedimentary successions in the Paleoproterozoic Onega Basin (NW Russia) provide new insights into mechanisms of carbonaceous matter (CM) migration and deposition. Greenschist facies metamorphism has negligible impact on large‑scale CM redistribution, as evidenced by the occurrence of CM‑bearing and CM‑free sills at contacts with carbon‑rich sediments under similar metamorphic conditions. Raman data indicate CM precipitation at ~400°C suggesting a hydrocarbon fluid migration during sill cooling. The carbon isotopic composition in the CM-bearing dolerites (δ¹³C = –26...–29‰) is the same as in host sediments indicating them as the main source of CM. The thin zones of the dolerites enriched in CM at the upper contact of the sills are a result of near-contact transfer during magma emplacement. The concentration of CM in the interstices in dolerites and enhance of CM content from medium-grained dolerites to gabbro-pegmatites in the central parts of the sills suggest the low miscibility of CM‑rich fluids and their affinity to residual melts. Notably, the low permeability of sills limits fluid flow mostly to cooling joint fractures at roofs and base of the sills therefore sills acted as barriers rather than pathways for hydrocarbon fluid migration.</p>