Organic Geochemistry and Origin of Bitumen Seeps in the Upper Permian (zechstein) Bituminous Anhydrite in a Cu–Ag Mine in Western Poland

Migrating hydrocarbons in the form of bitumen seeps can provide detailed information about the overall features of a petroleum system, source rock characteristics and maturity. Here we present the results of organic geochemical analyses of five bitumen seeps collected at the Polkowice–Sieroszowice Cu–Ag mine in western Poland at the eastern margin of the Southern Permian Basin. Bitumen seeps occur in fractures in bituminous anhydrite in the Lower (Werra) Anhydrite (A1d) and in veins cutting the Zechstein Limestone (Ca1) dolomite. The A1d bituminous anhydrite is one of several anhydrite types interpreted to have been deposited in supratidal sabkha and pan settings. Thermal maturity parameters (isoprenoid-based, hopane, sterane and aromatic hydrocarbon ratios) show that the bitumen was generated in the early to peak oil window. The analysed samples are characterised by high abundances of C35 homohopanes (homohopane index, HHI 0.3–0.5), an even-over-odd predominance of C16–32 n-alkanes, high gammacerane (0.2–0.4) index and low Pr/Ph (< 0.7) ratios. These parameters together indicate (anoxic) evaporitic depositional conditions for the related source rock. The presence of C27–29 4-desmethyl steranes (C29 > C27 > C28) and bicyclic sesquiterpanes together with a DBT/P ratio of < 0.5 imply the dominance of algal material and a clay-rich source rock, respectively. The presence of characteristic biomarkers for archaeal lipids (2,6,10,15,19-pentamethylicosane, squalane and C40 head-to-head biphytane) and C40 carotenoids (β- and γ-carotane, C21 and C30 alkylbenzenes) confirms that the bitumen was generated from the A1d bituminuous anhydrite. The molecular characteristics of the analysed bitumen samples rule out the possibility that the hydrocarbons have migrated from underlying Carboniferous source rocks or from Kupferschiefer mudrocks (higher Pr/Ph ratios, lack of gammacerane, secohopanes, secosteranes, and β- and γ-carotanes).