Lower Jurassic in the central part of the Polish Basin - Geochemical and petrological approach

Abstract

Lower Jurassic grey/black horizons composed of mudstones and claystones can be found in the Polish Basin area. These horizons are interbedded in thick sandstone packets, which to date have been mostly recognised as good geothermal reservoirs. In this paper, we examined the hydrocarbon potential of the Lower Jurassic claystones and mudstones from the central part of the Polish Basin. To properly evaluate these, we described in detail the organic matter's origin and maturity, as well as the palaeoenvironmental conditions. The collected core samples were examined by geochemical (Rock-Eval pyrolysis and GC-MS) and petrological (macerals content, vitrinite, and fusinite reflectance values) analyses. The obtained results revealed that the Lower Jurassic strata have poor hydrocarbon potential and contain mostly early-mature, terrigenous organic matter. Deposition took place under dysoxic conditions of the epicontinental sea, with the serious influence of deltaic/estuarine systems. Some samples contained mature, marine organic matter dominated mostly by bacterial input. One Sinemurian/Hettangian sample contained a high contribution of algal organic matter, which can be linked with the diatoms or diatom-related organisms as C25HBI, 24-norcholestane and 4-desmethyl-23,24-dimethyl steranes were detected. This might represent some of the oldest geochemical evidence of diatoms or diatom-related organisms. A wide spectrum of plant-derived biomarkers was marked in most of the samples, which allowed us to find traces of conifers, bryophytes, and fungi. Evidence of palaeo-wildfires was also noted. Newly introduced Pyrolytic Input Ratio (PIR) and Fire Maceral Index (FMI) were also examined. The use of PIR in samples with higher marine organic matter input indicated that PIR is not suitable for these samples, as the obtained results were overestimated. The PIR and FMI results obtained from the Lower Jurassic samples dominated by terrigenous organic matter suggested that both indicators gave proper results, as well as those samples with lesser paleo-wildfire product input.