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thickness, vertical and lateral variation of lithology, occurrence of faults,formation water chemistry, porosity and permeability are the primarygeological factors to evaluate in assessments of the geothermal prospectivityof an area and specific geothermal prospects. By combining 1) thegeographic distribution of the formations with known reservoirs, 2) theirmapped burial depths, and 3) estimates of the cumulative thickness of thereservoir sandstones a prospectivity map is constructed (Fig. 1). The mapshows where the identified reservoirs are expected to occur at depths of800–3000 m with a thickness exceeding 25 m and provides a first-orderindication of the regional geothermal potential.Burial depth and thickness of the reservoirsThe potential reservoirs show large variations in burial depths which can bemapped in areas with seismic and well data of sufficient density and quality(Fig. 2). In areas with few and poor data or areas with intense deformation interms of faulting and complicated salt structures, the maps are less reliable.Lateral variations of lithology also impose difficulties as they may complicatethe continuity of the seismic reflectors to be mapped. The mapping is carriedout in seismic time (two way travel time) and uncertainties regarding theconversion of travel time to depths cause some uncertainty regardingreservoir depths. In some cases it is not possible to resolve both the baseand top of a reservoir-bearing formation by seismic data, which typically havea vertical resolution of 20–40 m. The reservoir thickness may then beconstructed by combining seismic and well data, which may add uncertaintyto the estimate.Lateral and vertical continuityContinuity of the reservoir sandstones is mandatory for successfulgeothermal plants. The lateral and vertical continuity is primarily determinedby the depositional environments that formed the sandstones. For instance,the sandstones in the Bunter Sandstone Formation were primarily formed byephemeral braided rivers and eolian dunes in a warm semi arid climate,whereas intervening mudstones forming hydraulic barriers were formed inplaya lakes. On the other hand, the sandstones in Gassum Formation weremainly formed in a humid climate by extensive shoreface progradationinterrupted by flooding events forming marine mudstones that constitutevertical barriers (Nielsen 2003). These interpretations of the depositionalenvironments are based well data (well-logs, cores and cuttings) providinginformation on grain size, sorting, grain shapes, sedimentary structures,fossil content and mineralogy. Large faults with vertical displacementexceeding 100 m are common in many areas destroying the continuity, andare fairly easy to identify although their precise location may be uncertain(Fig. 2). Smaller faults with vertical displacements of less than the thicknessof the reservoir units may also reduce the lateral continuity but are moredifficult to reveal as the displacement may be smaller or close to theresolution of the seismic data.Risø International Energy Conference 2011 Proceedings Page 1315