OIL OIL AND OIL SOURCE-ROCK CORRELATIONS IN THE CARPATHIAN FOREDEEP AND OVERTHRUST, SOUTH-EAST POLAND

The chemical composition of 35 crude oils and the extracts of potential source rock samples from south-east Poland were studied in detail by geochemical methods in order to understand their genetic relationships. Twenty-six oils, including three seep samples, were collected from Cretaceous to Oligocene reservoirs within the Carpathian Overthrust. Nine oils originated from Carboniferous to Miocene reservoirs within the Carpathian Foredeep. The rock samples studied were selected after Rock-Eval pyrolysis screening of a large suite of samples and consist of one Lower Cretaceous sample and five Oligocene Menilite shale samples (from three different outcrop locations all within the Carpathian Overthrust). Geochemical fingerprinting indicates at least two families of crude oils: the first, represented only by one oil, has no specific characteristics except for a light carbon isotope composition; the second family is characterized by the presence of 28,30-dinor-hopane, oleanane and a highly-branched isoprenoid C25 alkane. Within this latter family four subfamilies were recognized: (A) 19 oils having the aforementioned characteristics, (B) seven oils similar to subfamily A, but with a relatively high sulphur content, (C) one oil with high abundance of higher-plant derived triterpanes, and (D) seven oils with abundant 28,30-dinor-hopane. Although maturity influences the relative abundance of 28,30-dinor-hopane, it suggested that subfamilies A, B, and C had an initial low 28,30-dinor-hopane content, whereas subfamily D an initial high 28,30-dinor-hopane content. Only one oil of subfamily A could potentially be a high maturity counterpart of subfamily D. A Palaeozoic source rock is suggested for the first family, and the second family, including the majority of the oils from the Carpathian Foredeep, is most likely derived from the Oligocene Menilite Shales. An oil/source rock relationship with the Menilite Shale is supported by the presence (although not in all samples studied) of 28,30-dinor-hopane, oleanane or its precursors, and the highly-branched isoprenoid alkane. The strong facies variations observed within this shale gave rise to the different types of crude oils in the second family.