Depositional environment and source potential of Jurassic coal-bearing sediments (Gresten Formation, Höflein gas/condensate field, Austria)

Coal-bearing Jurassic sediments (Gresten Formation; Lower Quartzarenite Member) are discussed as source rocks for gas and minor oil in the basement of the Alpine–Carpathian frontal zone (e.g. Höflein gas/condensate field). Core material has therefore been analysed to characterize depositional environment and source potential of the Lower Quartzarenite Member (LQM). Geochemical data from the Höflein condensate are used to establish a source–condensate correlation.

The LQM was deposited in a flood basin with transitions to a delta-plain environment. Coal originated in frequently flooded mires and evolved within an oxygenated and acidic environment. It is inferred from geochemical data that organic matter from aquatic macrophytes and gymnosperms contributed to coal formation. Wildfires were abundant and oxidation of plant remains occurred frequently. This resulted in the formation of dull coal with very high inertinite contents. Bituminous shales were formed in deeper waters under dysoxic conditions. Apart from abundant algae and micro-organisms, it is concluded that there was an increased contribution of higher land plants relative to macrophytes to the biomass of the shales.

Despite high inertinite contents, coal within the LQM has a significant oil potential. Bituminous shales contain a Type III–II kerogen. According to pyrolysis–gas chromatography data, coal and shale generate a high wax paraffinic oil. The organic matter is immature to marginal mature (0.55% Rr). Bituminous shales are considered a potential source for the Höflein condensate. Coal may be the source for gas and minor oil in the Klement Field, but is not the source for the condensate. The equivalent vitrinite reflectance of the condensate is 0.8%, suggesting condensate generation at 4–4.5 km depth. The Gresten Formation reaches this depth near its depocentres, implying southward-directed migration of the Höflein condensate.