Geothermal reservoir quality of Upper Triassic - Lower Jurassic paralic sandstones in Copenhagen

Low-enthalpy geothermal energy is exploited from deep sandstones in Copenhagen in eastern Denmark, and the possibility of exploit the shallower sandstones of the Gassum Formation and the Karlebo Member is considered here. Petrography, porosity, permeability and provenance data from core and cutting samples on Zealand are compared in order to investigate the sediment composition and the reservoir properties across the area. Sandstones from central Zealand are on average coarser grained and better sorted than sandstones from the Copenhagen area, and this trend is found both within the Gassum Formation and the Karlebo Member. The sandstones from these units have a comparable and very mature mineralogy where quartz is the dominant component. Zircon age dating reveals that this is most likely caused by reworking of older sediments from the Ringkøbing-Fyn High. Good reservoir quality is presumably present in the vast majority of the sandstones in the Copenhagen area. Differences in maximum burial depth and associated diagenesis must be taken into account when estimating the reservoir properties of sandstones in the Copenhagen area. The high mineralogical maturity documents that these sandstone reservoirs would be optimal for geothermal exploitation since the water will be reinjected into almost inert sandstones.