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Organic matter occurring in Lower Palaeozoic rocks of variable lithology was investigated for its origin, depositional environment and thermal maturity. Samples from nine boreholes located in the Polish part of the Peribaltic Syneclise were collected for performing Rock-Eval Analysis, palynological investigation for Thermal Alteration Index (TAI), solvent extraction, preparative layer chromatography (PLC) and gas chromatography-mass spectrometry (GC-MS) of aliphatic and aromatic compound fractions. It was found that kerogen belongs to either type I or commonly, to type II. It derives from variable, planktonic and planktonic-bacterial primary organic matter deposited in dysoxic environments in the eastern part of the Syneclise and in anoxic/suboxic conditions in the western and central parts. Organic matter is in the thermal evolution stage from late diagenesis to advanced metagenesis. Correlation of biomarker parameters, TAI and Rock-Eval results show that the thermal maturity of organic matter falls into three regional groups, increasing from the north-east to the south-west along the crystalline basement of the Eastern European Platform. The organic matter contained in Ordovician and Cambrian sediments of the western part of the Syneclise is overmature, reaching metagenesis. This is most probably related to the activity of the Teisseyre-Tornquist Zone, where the Baltica and Eastern Avalonia paleocontinents collided during the Late Ordovician. It is also reflected in the profiles by elevated values of aromatic hydrocarbon and sulphur compound parameters, which show the highest levels of thermal maturity for organic matter present in the sediments of the Ordovician and Lower Silurian. Local influence of igneous intrusions on organic matter is also visible in changes in thermal maturity values at the direct contact with diabase dykes.

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