Regional assessment of the ultra-deep geothermal (UDG) reservoir quality and potential of the Dinantian Zeeland Formation, the Netherlands

Abstract

Abstract Dinantian carbonates in the NW European Carboniferous Basin (NWECB) in the Netherlands form a target for ultra-deep geothermal (UDG) exploration. The NWECB was a deep water area with shallow water attached carbonate systems in the southwestern Netherlands and isolated carbonate platforms nucleated on horsts in the northern part of the Netherlands. Carbonate platforms in the SW Netherlands contain intra-Dinantian, sub-Namurian and sub-Cretaceous karst systems with irregular cavities, enlarged fracture systems and collapse breccias with relict porosity. Isolated carbonate platforms in the northern Netherlands were not affected by sub-Namurian or sub-Cretaceous karst but contain petrophysical anomalies interpreted as intra-Dinantian karst or fracture systems. The matrix reservoir quality of Dinantian limestones is generally poor but has been improved by a phase of regional dolomitization that affects the lower parts of both isolated and attached carbonate platforms. Dolomitization took place during burial and was associated either with late Carboniferous to early Permian burial or a thermal event during the middle to late Jurassic. Additional reservoir quality is present in a system of hydrofractures in late Tournaisian ramp carbonates in the SW Netherlands that post-date dolomitization; the fracture system may have formed during late Jurassic basin inversion. Present heat flow indicates that two isolated carbonate platforms in the northern Netherlands are buried to sufficient depth to be considered as potential UDG exploration targets. Attached carbonate systems in the SW Netherlands are too shallow to have been heated to the temperature required for industrial applications or for generating electricity from geothermal energy.