Sedimentological and diagenetic controls of gas in Lower and Early Upper Carboniferous sediments, NE Germany

Abstract

Fine-grained, organic-rich Paleozoic sediments are proven source rocks for conventional gas systems in NW Europe and large, unconventional gas reservoirs are assumed in Lower to Early Upper Carboniferous black shales. However, despite numerous boreholes exploring for hydrocarbons, the shale gas potential remains uncertain. This paper comprises detailed information on the sedimentary structure and lithofacies associations along with spatial distribution, thickness, thermal maturity, organic and inorganic petrology as well as geochemistry and mineralogy of Lower to Early Upper Carboniferous black shales in NE Germany to unravel the evolution of shale gas-relevant properties. Sediments of the “Kohlenkalk” Formation (Carboniferous Limestone) in the Rügen Island area were deposited on an extensive deep-water carbonate shelf. Transgressive–regressive cycles led to the formation of an up to 2000m thick intercalation of mud- and limestones. High total organic carbon contents (2.4–7.0wt.%) of a mainly Type II+III kerogen have been deposited under dysoxic to anoxic conditions and attained favorable present-day thermal maturities (0.8–1.8% VRr) indicating a shale oil potential in the upper part of the succession and a shale gas potential in the deeper parts. Sediments of the “Altmark–North-Brandenburg Culm” (Synorogenic Flysch Formation) were deposited as a turbidity current-derived intercalation of a sand/silt–mud interbedded and black mudrock lithofacies in a deep marine siliciclastic slope to basinal setting. The ~1900m thick succession has TOC contents of 0.5–2wt.% and present-day thermal maturities of 2.3–3.2% VRr indicating a possible hybrid gas accumulation in areas where the succession is buried <5000m.