Hydrocarbon habitat in the Zielin Late Permian isolated carbonate platform, western Poland

In the Zielin oilfield in the Polish part of the Southern Permian Basin, oil and gas are produced from an Upper Permian Main Dolomite (Ca2) isolated platform. The paleohigh developed on a volcanic cone formed in Early Rotliegend time. Based on microfacies analysis of Ca2 reservoir rocks from four wells (Zielin-1, -2, -3 and -3KBis), four microfacies types are distinguished: peloidal dolomitized wackestone, intraclastic dolomitized rudstone and floatstone, biogenic boundstone, and intraclastic-bioclastic dolomitized packstone. In view of the lack of source rocks on the Zielin paleohigh revealed by Rock–Eval analyses, this isolated carbonate platform is regarded as a reservoir for migrating hydrocarbons. Biomarker analyses of two crude oils and nine carbonate rocks from the Zielin isolated carbonate platform and five crude oils from surrounding wells of the nearest Gorzow, Cychry, and Namyślin platforms indicate an origin for the oils from oil–prone type II kerogen, sourced from organic matter deposited under anoxic/suboxic conditions in a carbonate environment. Compared to organic matter extracted from the rock samples, oils from the Zielin-2 and 3KBis wells are characterized by higher values of the pristane/phytane (Pr/Ph) and dibenzothiophene/phenanthrene (DBT/P) ratios, lower relative concentrations of polyalkylaromatics, as well as a higher maturity evaluated from the methylphenanthrene (MPI-1) and methyldibenzothiophene (MDR) indices. The molecular composition of sesquiterpanes, n-alkylbenzenes, and polyalkylnaphthalenes revealed a diverse specificity within the oils analyzed and allowed their separation into two groups. Oils Rozansko-4, Zielin-2, Zielin-3KBis, and Namyślin-1 represent group I, whereas Barnowko-3, Mostno-2 and Cychry-2 group II. From the gradient direction in the n-decane/naphthalene ratio and maturity for respective crude oils belonging to group I, it is concluded that the oils accumulated in the Zielin atoll migrated from NW to SE following subsidence and tilting of the platform in this part of the Main Dolomite basin.