Habitat and hydrocarbon potential of the Palaeozoic source rocks in the Kraków–Rzeszów area (SE Poland)

The quantity, genetic type and maturity of organic matter dispersed in Ordovician, Silurian, Devonian and Lower Carboniferous strata in the basement of the Carpathian Foredeep between Kraków and Rzeszów were determined based on the results of organic geochemical analyses of 600 rock samples collected from 44 wells. The best source rocks were found in the Silurian strata where the total organic carbon (TOC) content is up to 6.6 wt% and the median value equals ca. 1.5 wt%. The median values of the initial organic carbon contents in individual wells vary from 1.2 to 3.5 wt%. The Ordovician, Lower Devonian and clastic facies of the Lower Carboniferous strata can be considered as an additional source of hydrocarbons with the median TOC values of 0.27, 0.56 and 0.53 wt%, respectively. The Middle and Upper Devonian strata as well as the carbonate facies of the Lower Carboniferous strata have much lower quantities of organic carbon, although in these strata levels with elevated TOC contents were observed. In the Lower Palaeozoic and Lower Devonian strata, the oil-prone, low-sulphur Type II kerogen is present, whereas in the younger divisions presence of the gas-prone Type III kerogen is visible. In the Lower Carboniferous clastics gas-prone kerogen dominates. The Silurian and clastic facies of the Lower Carboniferous strata have been deposited in the normal marine conditions, whereas the Ordovician, Devonian and carbonate facies of the Lower Carboniferous strata usually experienced reducing conditions. The source rocks are mostly at the initial and middle phase of the low-temperature thermogenic processes. Locally, immature (in the Lower Carboniferous carbonates in the vicinity of Łąkta gas-condensate field) or late-mature (in the Middle and Upper Devonian strata in the area of Grobla–Pławowice oil field) source rocks were observed.