The correlation of lower Paleozoic marine source rocks with reservoired oils by biomarkers is complex because of the uniform early Phanerozoic biomass (bacteria and algae) and the lack of land plant and animal input. Accordingly, the main source rocks for the most prolific oil province in the Baltic Basin are still a matter of debate.

The 10 source rocks and 15 oil samples from 5 northern European countries bordering the Baltic Sea Basin were analyzed by gas chromatography (GC) with flame ionization detector, GC–mass spectrometry (GCMS), and GCMS/MS to detect acyclic isoprenoids and aliphatic, aromatic, and nitrogen, sulfur, and oxygen biomarkers. Chemometric tools were applied to screen for meaningful source- and age-related biomarkers and to highlight genetics. Extended tricyclic terpane ratios, C₂₄ tetracyclic terpane/C₂₆ tricyclic terpane ratios, and relative C₂₉ sterane concentrations are considered the most promising biomarkers in differentiating Llandovery shales from Cambrian to Ordovician Alum Shale and for correlation with expelled oil. The uranium irradiation–related C₂₆–C₂₈ triaromatic-steroid concentrations provide possible distinguishing criteria for the source potential of the different Alum Shale units. Enhanced oil maturation by volcanic intrusion is highlighted by sterane biomarkers and polycyclic aromatic hydrocarbons.

The Alum Shale is here considered the main source rock for oil accumulations in lower Paleozoic reservoirs of the Baltic Basin. Oil seepage occurring in Ordovician limestone was mainly generated by the Middle Cambrian Alum Shale, and Middle Cambrian sandstone reservoirs were mainly sourced by Upper Cambrian and Lower Ordovician Alum Shale with higher maturity. Considerations about the assessment of migration distance are based on carbazole concentrations and C₂₉ sterane isomerization.

Advanced studies to unravel detailed lower Paleozoic oil–source-rock correlations are based on meaningful biomarkers, offer approaches to significantly reduce the exploration risk in this area, and could be applied to similar early Paleozoic petroleum systems in other basins.