Habitat and hydrocarbon potential of the Kimmeridgian strata in the central part of the Polish Lowlands

The quantity, genetic type and maturity of organic matter dispersed in the Kimmeridgian strata of the central part of the Polish Lowlands were determined on the basis of results of Rock-Eval, stable carbon isotope composition of bitumen, their fractions and kerogen, biomarker distribution in saturate and aromatic hydrocarbon fraction, elemental composition of kerogen, vitrinite reflectance and maceral composition analyses of 225 rock samples collected from 32 boreholes. The study was conducted separately for Lower and Upper Kimmeridgian rocks in the Szczecin-Miechów and Kościerzyna-Puławy synclinoriums and Mid-Polish Anticlinorium. The best source rocks with TOC up to 6.8 wt.%, were found in the vicinity of Gostynin in the Mid-Polish Anticlinorium. Generally, the Upper Kimmeridgian strata are fair and good potential source-rocks whereas the Lower Kimmeridgian strata have much lower hydrocarbon potential. Gas-prone, terrigenous Type-III kerogen predominates in the Lower Kimmeridgian strata. The contribution of oil-prone, marine Type-II kerogen increases in the Upper Kimmeridgian rocks. In the whole profile, only low-sulphur kerogen was recorded, although the situation, when high-sulphur Type-IIS kerogen was mixed with re-worked, non-generative Type-IV kerogen supplied to the sedimentary basin with rocks from eroded land, cannot be excluded. Sedimentary conditions of organic material were variable, usually anoxic and suboxic with domination of siliclastic material in mineral matrix. The maturity of the dispersed organic matter refers mostly to the final phase of the microbial process, or to the initial phase of the low-temperature thermogenic process (oil window). The most mature rocks, corresponding up to 0.75% in the vitrinite reflectance scale, were recognised in the deepest buried parts of the basin (axial part of the Mogilno-Łódź Segment of the Szczecin-Miechów Synclinorium). The most prospecting source rocks were recognised in this area.