The Baltic Syneclise is one of the Paleozoic basins along the western margin of the East European Craton. Commercial amounts of hydrocarbons have been found onshore and offshore in the Middle Cambrian sandstones and Upper Ordovician limestone reefs. The Middle-Upper Cambrian and Tremadocian bituminous shales have been identified as good quality effective source rocks. The existence of good quality source rocks in the Ordovician and Silurian profiles provides an impetus for conventional and unconventional hydrocarbon explorations in this region. Geochemical analyses revealed that the rocks of the Middle–Late Ordovician and Early Silurian horizons exhibit overall good to very good source rock quality. Within the Ordovician strata, the Sasino and Prabuty formations exhibit the highest amounts of organic carbon with the median total organic carbon (TOC) values of 1.96 and 1.23 wt. %, respectively. The Pasłęk and Pelplin formations in the lowest parts of the Lower Silurian stand out clearly from other formations with the median TOC values of 0.91 and 1.15 wt. % and median total hydrocarbon content (S₁+S₂) of 2.46 and 1.54 mg HC/g rock, respectively. The analyzed successions are dominated by immature/early mature, algal (oil-prone) type II, and mixed II/I kerogen deposited in a marine paleoenvironment with anoxic conditions at the bottom and oxic conditions in the photic zone. Immature organic matter prevails in the eastern and central parts of the study area (Gołdap–Kętrzyn–Olsztyn area), and the western part (Darżlubie–Hel–Gdańsk area) has mature kerogen.

In this study, we combined the geochemical characteristics based on Rock-Eval pyrolysis and gas chromatography – mass spectrometry (GC–MS) results with the facies analysis. These surveys were conducted within grey to black claystones/mudstones intervals of the Podil’tsi and Kokhanivka formations, related to the Pliensbachian and Toarcian–Bathonian ages, respectively. The geochemical results revealed that the Podil’tsi Formation contains mixed marine/terrigenous, early-mature to mature organic matter. The deposition of this formation took place in dysoxic redox conditions of a sulphate-poor marine palaeoenvironment, with oxygen scarcity within the photic zone, as documented by green- and brown-pigmented Chlorobiaceae. Oleanane is present within the samples from the Podil’tsi Formation, which is uncommon within Lower Jurassic sedimentary rocks. The Kokhanivka Formation contains mostly early-mature, terrestrial organic matter, deposited in suboxic conditions of a sulphate-poor, fluvial–deltaic palaeoenvironment. The absence of aliphatic diterpenoids within the Middle Jurassic strata points to the low significance of conifers in the sediment supply area at this time. All of the Podil’tsi and most of the Kokhanivka formations are characterised by poor hydrocarbon potential. Only the middle part of the Kokhanivka Formation, built by brown, organic-rich claystones, shows fair-to-good hydrocarbon potential. Based on our results, a chemostratigraphic correlation of the Toarcian–Bathonian strata from the Carpathian Foredeep with the same strata from the neighbouring Polish Basin was performed. The juxtaposition of the geochemical and facies results suggests that the interval of brown organic-rich claystones, from the middle part of the Kokhanivka Formation can be related to the Middle–Upper Aalenian Age.