Ordovician and Silurian Formations of the Baltic Syneclise (NE Poland): An Organic Geochemistry Approach

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.