Source rock evaluation and fluid inclusion reconnaissance study of Carboniferous and Zechstein rocks in the northern margin of the Southern Permian Basin, onshore Denmark

Abstract

Hydrocarbon exploration onshore Denmark has been unsuccessful with no commercial discoveries. Several potential plays have been identified of which the Zechstein carbonate play relying on organic-rich Zechstein (Z2) carbonates as source rock is considered the most promising. Additional source rocks might be lower Carboniferous calcareous and carbonaceous units. This study aims at documenting Palaeozoic source rock(s) in the Upper Mississippian/Lower Pennsylvanian and Zechstein Z2 cored sections in wells drilled in the northern margin of the Southern Permian Basin in the Danish Baltic Sea area. The source rock potential, organic matter composition, and thermal maturity was examined using Rock-Eval pyrolysis, vitrinite reflectance measurements, and organic petrography of cuttings and core samples from two wells. In addition, a fluid inclusion reconnaissance study was conducted on samples from four wells. The examined Upper Mississippian/Lower Pennsylvanian marine calcareous mudstones are thermally immature to marginally mature with a generally low TOC content consisting of minor amounts of dispersed vitrinite and inertinite and scattered yellowish fluorescing mainly terrigenous liptinite macerals. The source rock potential is nil to poor. The analysed Zechstein Z2 section is also thermally immature and consists of resedimented carbonate mudstones deposited in a marine environment on a slope. The mudstones are organic-lean with identifiable tiny vitrinite particles and some telalginite. The source rock quality is poor with a potential for some gas generation. Thus, identification of rich source rocks failed, but the occurrence of solid bitumen in the Zechstein Z2 suggests the presence of restricted source rock levels in the mudstones or migration from deeper richer source rock sections. Further, hydrocarbon fluid inclusions are direct evidence for petroleum generation and migration. The dark appearance, high vapour/liquid ratio at room temperature and their homogenisation behaviour clearly indicate a mixed hydrocarbon composition with methane and higher hydrocarbons, like gas condensate. This demonstrates the presence of a working petroleum system in the northern margin of the Southern Permian Basin in southern Denmark.