Modelling of reservoir quality in quartz-rich sandstones of the Lower Cretaceous Bentheim sandstones, Lower Saxony Basin, NW Germany

Abstract: The Lower Cretaceous Bentheim sandstones of the Lower Saxony Basin in Northwest Germany (one of the main onshore oil fields of Western Europe) are mature quartz arenites in terms of texture and mineralogy. This study shows that the mineralogical maturity to a large degree is the result of diagenetic processes. Hence, the present day detrital composition and texture is not what they were at the time of deposition. The sandstones are highly porous and permeable and show presence of quartz cement as overgrowths which cement the sandstone, partly dissolved detrital feldspar grains, oversized pores caused by complete feldspar dissolution and authigenic kaolinite-dickite booklets. The porosity and permeability have been measured in the laboratory. The results were combined with the observations made by conventional optical microscopy and quantification of detrital and diagenetic components-textures by point counting, cathodoluminescense petrography, and back scattered electron imaging. These techniques have been undertaken in order to assess the impact of the presence of quartz overgrowths and feldspar dissolution on porosity and permeability. XRD analyses were done for identifying the mineralogy of the main detrital and diagenetic components, in particular for detrital and authigenic clay minerals. Quartz cement in the form of syntaxial overgrowths on detrital quartz grains played an important role in determining the petrophysical properties in the Bentheim sandstones. This kind of cement usually causes harmful reductions in porosity and permeability. However, in the Bentheim sandstones quartz cementation possibly had a positive impact in terms of porosity preservation. Cement precipitated in limited amounts and probably helped the reservoir framework to withstand the destructive effect of overburden pressure and consequent mechanical compaction. The study shows that detrital quartz grains were important, and that possibly the different types of quartz grains had different susceptibility in terms of acting as host grains for precipitation of authigenic quartz. The presence of detrital feldspar and its composition was another major parameter during diagenesis and consequent porosity-permeability modification. Dissolution of feldspars may have been an important internal (local) source of silica and aluminium for kaolinite-dickite authigenesis and possibly also for quartz cementation, and created oversized secondary pores.