Homogeneous porous sandstones are target horizons for geothermal and hydrocarbon reservoirs due to their high permeabilities. However, their sedimentary and structural heterogeneity may alter flow properties. We analyzed the permeability anisotropies and associated diagenesis of sedimentary and structural fabrics in eolian Rotliegend sandstones and highlight the importance of diagenetic alterations within sedimentary fabric. Results are compared with weakly lithified marine sands, demonstrating the importance of cross-bedding and bounding surfaces on flow.
While deformation bands are known to reduce reservoir permeability by one to three orders of magnitude, the differing degrees of cementation within cross-bedding inhibit flow more effectively. Cross-bedding is best cemented in coarse-grained and uncoated beds. The impact of erosion-derived bounding surfaces is quantified in two Rotliegend outcrops in Germany and the UK.
There is no general correlation recognizable between the different sampling sites whereas porosity and permeability are clearly linked at each site. This can be assigned to mineralogical composition and paragenetic sequence. ... mehrIn the data sets 2nd order bounding surfaces have led to a porosity loss of up to 5 %. Low vertical permeabilities (Ø 0.63 mD) decrease by on average 79.4 – 95.2 %, compared to vertical permeabilities across the undisturbed cross-bedded sediments. Horizontal permeabilities of 110 mD evidence the heterogeneous character of the sandstones. Third order surfaces instead show minor increases in porosity of approximately 1 % but without any unambiguous trend. Vertical permeabilities (Ø 130 mD) are 7.7 – 26.9 % higher across such bounding surfaces than in normal cross-beds.
The heterogeneity of the bounding surface permeabilities is ascribed to short-term erosion succeeded by differing depositional and diagenetic alterations. At 2nd order surfaces these are transported clay clasts originating from lacustrine areas and eodiagenetic calcite reducing porosity and/or permeability. Such surfaces may inhibit flow if they form a lithological contrast, but generally they appear to have a minor impact due to their lateral heterogeneity. Our studied third order bounding surfaces impose no negative effect on permeability. Overall, differently cemented cross-bedding has the highest impact on flow and deformation bands may reduce flow, but their lateral continuity needs to be taken into account.