The North-German Basin offer at depth of approximately 5000 m sufficiently high temperatures for efficient and economic heat extraction by geothermal wells. The permeability of the carboniferous strata at that depth is very low so that conductive pathways have to be created by hydraulic fracturing. For some planned projects we conduct the characterization of the carboniferous strata as exposed in "analogous outcrops". The geological setting for fracking is demanding, as the strata comprise strong sandstone, shale and weak coal seams. All necessary petrophysical properties will be collected by extended testing. The regional horst-and-graben system allows for mapping in outcrops and extrapolate the structural content towards the rock mass at depth.

1. Introduction

The North German Basin (NGB) is together with the Upper Rhine Valley and the southern Molasse basin the preferred area for deep geothermal wells in Germany Several commercial (Neustadt / Glewe) and research projects (GeneSys; Groβ Schönebeck) have been developed. The prognosis of temperatures at depth for Germany is shown in Figure 1. Some planned projects focus on the western part of the North Germany Basin and due to the fact that permeability of the target horizons at depth is very low fracking will be executed to create conductive pathways in the otherwise dry and hot rock. This type of heat mining is called Enhanced Geothermal System EGS which aims at producing electricity.

The red spots in Figure 1 suggest a temperature of around 180° C at a depth of 5000 m. This information seems to be valid since many oil and gas wells were drilled and still are operated in the NGB. We focus in this paper on the area near the border to the Netherlands, on the utmost western part of the NGB. Here several commercial projects are aiming for drilling deep wells and for executing multiple fracking from horizontal drillholes for obtaining high flow rates. This area is well known from exploration on hydrocarbon, 3D seismic lines have been executed and some deep wells have been drilled for exploitation.

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