Bitumen/gas shows in Mesozoic intervals of deep boreholes in central Poland: implications for hydrocarbon migration and preservation potential

Abstract

This study evaluates the significance and origin of small and relatively scattered Mesozoic hydrocarbon shows in the LódY Trough, Kujavian Swell and the P3ock Trough, which have long been neglected in detail. The depiction of hydrocarbon shows on newly prepared maps and diagrams shows their link to Permian-Mesozoic geological structures and the mineralization of formation waters. Many bitumen shows in cores are observed at the Jurassic/Cretaceous boundary; the most common hydrocarbon inflows occur in the Middle-Upper Jurassic reservoir levels sampled. The Triassic-Lower Jurassic hydrocarbon occurrences are distributed more randomly, probably because of a scarcity of boreholes piercing those intervals. The majority of the observed hydrocarbon shows are related to the Mesozoic cover of salt anticlines or fault zones reaching the pre-Mesozoic basement. The presence of hydrocarbons in Mesozoic rocks overlying salt domes and elevated areas of the East European Platform characterized by low thermal maturity is interpreted as a result of their migration. The tectonic involvement of the Mesozoic strata, related to halokinetic movements and the Late Cretaceous-Early Paleogene inversion of the mid-Polish Trough, along with the documented scarcity of efficient sealing horizons and halting of the process of kerogen transformation, have likely prevented the formation of hydrocarbon accumulations. There is, however, still insufficient data on organic matter content and hydrocarbon occurrences in Triassic deposits containing potentially effective caprocks