Petroleum generation in the Carboniferous strata of the Lublin Trough (Poland): an integrated geochemical and numerical modelling approach

In the Lublin Trough, Eastern Poland, we analysed Carboniferous claystone and mudstone rocks by Rock-Eval, bitumen composition, n-alkanes and methylphenathrenes composition and carbon isotopic composition to assess their potential for the generation of hydrocarbons. In the Lublin Trough, oil and gas-prone type II/III and III kerogens occur in Upper Visean strata (average TOCav. 1.8 wt.%), whereas mainly gas-prone type III kerogen occurs in Namurian (TOCav. 1.98 wt.%) and Westphalian (TOCav. 1.4 wt.%) strata. The Carboniferous sequence displays maturity values of 0.5–1.2%Ro (Rock-Eval Tmax 430–460 °C). Palaeo-geothermal gradients in the past were much higher than now and locally reached values over 40 °C/km during the maximum burial event. Generated hydrocarbons probably consisted of natural gas with limited amounts of oil. Over most of the area hydrocarbon generation commenced in the Late Carboniferous (mainly 25% but up to maximum 60% transformation ratio) and was substantially complete by the time of the Asturian inversion. In the NW part of the Lublin Trough, petroleum generation continued until the end of the Cretaceous, with transformation ratios never exceeding 11% at the base of the Upper Carboniferous. Mainly gas-prone kerogen types, and inadequate maturity, suggests the discovered petroleum accumulations are derived from Devonian and/or Lower Paleozoic strata rather than Carboniferous units. However, the quantitative significance of these source rocks needs further geochemical research.