Geochemical and petrological characteristics of the Middle Jurassic organic-rich siliciclastic sediments from the central part of the Polish Basin

Abstract

Organic-rich claystones and mudstones can be very good source rocks for hydrocarbons. Middle Jurassic organic-rich siliciclastic horizons are known from the Polish Basin area, however, there are no hydrocarbon accumulations associated with these horizons. The main scientific goal of this paper is to propose why these horizons did not produce significant hydrocarbons quantities. To deal with it, we characterised organic matter origin, maturity level, and hydrocarbon potential of Middle Jurassic strata. Palaeoenvironmental conditions of the Middle Jurassic Polish Basin were also described. The samples were examined by geochemical (Rock-Eval and GC–MS) and petrological (macerals content, vitrinite, and fusinite reflectance values) analyses. The analyses revealed that the Middle Jurassic strata at the study area, in general, contain mixed terrigenous/aquatic organic matter with a significant content of pyrogenic origin polycyclic aromatic hydrocarbons (PAHs). Inertinite and liptinite group macerals dominate in maceral composition. This suggests that analysed samples contain a mixture of type IV and II kerogens. Most of the analysed samples contain immature or early-mature organic matter. In general, the maturity level is higher in the southern part of the study area, where higher subsidence of Polish Through allowed to reach greater burial depths. Sediments were deposited in anoxic basement conditions of the epicontinental sea with the serious influence of deltaic/estuarine systems. The water and air transport are responsible for significant input of terrigenous organic matter, mostly reworked in frequent palaeo-wildfire events. The high content of charred organic matter is the main reason why despite high total organic carbon (TOC) values, the hydrocarbon potential of analysed horizons is poor/fair. Fire maceral index (FMI) was proposed and juxtaposed with pyrolytic input ratio (PIR) to propose that PIR is sensitive mostly to higher-temperature palaeo-wildfires products.