This study provides a comprehensive geochemical and mineralogical characterisation of the fine-grained, organic matter-rich sedimentary rocks of the Lower Palaeozoic in the central part of the Baltic Basin (Western Lithuania). The primary objective was to deduce environmental conditions for the Lower Palaeozoic succession which is very well preserved in the Baltic Basin, while the secondary goal was to obtain information on maturation and thermal history of the rocks. The sample set covers different stratigraphic units of the Ordovician- Lower Silurian succession and was analysed with a wide range of different methods including elemental analysis, organic petrography, Rock-Eval pyrolysis, molecular geochemical analysis of saturated and aromatic hydrocarbon fractions, inorganic geochemistry, mineralogy (XRF, XRD analysis) and magnetic susceptibility (MS) of rock samples. Their integration allowed a determination of the redox conditions, paleoproductivity and clastic input during sediment deposition. The fine-grained sediments are dominated by clay minerals and quartz and to a lesser extent carbonate, pyrite and feldspar. Microscopic analysis revealed abundant solid bitumen and zooclasts. Biomarker data indicate a marine origin for the precursor organic matter. Maturity parameters revealed the presence of a thermal maturity ‘anomaly’ in the southwestern part of Lithuania, where the late phase of oil generation and possibly beginning of condensate-wet gas generation with corresponding maximum paleotemperatures of 140–180 °C has been reached. Bottom-water conditions varied from oxic to anoxic/euxinic during deposition driven by interaction of climate states, water mass circulation and stratification, water geochemistry and sea bottom bathymetry. Four shaly horizons (at the Upper Ordovician Fjäcka and Mossen Fms., Silurian Dobele Fm., and near Llandovery-Wenlock boundary levels) were recognized as being formed in oxygen-depleted benthic settings as indicated by high TOC and TS contents, sedimentation features, high metal enrichment or degree of piritization. The deposition of highly organic matter-rich Silurian Dobele Fm. might have coincided with oceanic anoxia and Late Aeronian bioevent, accompanied by an upwelling regime with increased paleoproductivity. The interpreted coarsening upward trend at the Early Silurian Adavere Regional Stage terminates with fall in relative sea-level at the Llandovery-Wenlock boundary. The geochemical proxies near Llandovery/Wenlock boundary may reflect the perturbations in oceanic water geochemistry which in turn may have eventually led to Ireviken mass extinction event. • Maturity parameters revealed the presence of a thermal maturity ‘anomaly’. • Bottom-water conditions varied from oxic to anoxic/euxinic during deposition. • Organic matter is of marine algal origin with accompanying bacteria and zooclasts. • Silurian horizons may reflect anoxic events associated with mass extinctions.