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

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.Popular summary: Hydrocarbons, ground water and economic minerals occur in pores (voids) between sediment grains, which later may form more solid and less porous sedimentary rocks. Thus porosity is a term that describes the abundance of these pores and can be defined as the quantity of oil, gas, water and economic minerals a rock can hold and it is expressed as percentages (%). The other important parameters in any oil, gas or water reservoir rock (mostly sedimentary rocks such as sandstones and limestone) is permeability. Permeability is the ability of a rock to transmit oil, gas and water and can depends on the interconnectedness of the pores. The more interconnected pores will lead to more efficient production of oil in oil wells and water in ground water wells. Not all sedimentary rocks are characterised by sufficient porosity and permeability. Accordingly not all rocks can be considered as good reservoirs. Each well (especially oil wells) costs huge amounts of money. When oil companies invest they naturally want to minimise the risk of not striking oil. In other words the ratio of success in striking (finding) oil is directly dependent on the amount of porosity and permeability. Exploration geologists always try to predict porosity and permeability ahead of drilling, by examining adjacent exposed bedrock. They play a profound roll in estimating the reserves (the amount of oil or gas which can be produced) and production rates in suspected oil fields. Porosity and permeability can be accounted for and evaluated through the direct and indirect parameters which control them. One of the many factors controlling porosity and permeability is cementation, i.e., the process of forming new minerals inside the pores, which leads to diminishing porosity and permeability (negative effect). In many quartz grain rich sandstones, such as the Cretaceous Bentheim sandstones and the Jurassic North Sea reservoir sandstones, the cement mineral is quartz minerals which precipitate on already-existing quartz grains. Sandstones consist of different kinds of quartz grains such as monocrystalline quartz (which consists of one single crystal) and polycrystalline quartz (which consists of more than one crystals). Different quartz types have different susceptibilities for hosting quartz cements. Hence, if we know which type of detrital quartz grain is abundant in a certain reservoir rock, so we can presuppose the degree of cementation which in its turn the degree of porosity and permeability reduction. It is believed that monocrystalline quartz grains are more susceptible for quartz cements (secondary quartz) than polycrystalline quartz. This suggestion is tested in this study. In some cases the formation of quartz cement is strengthening the framework of the reservoir rocks and thus help the entire reservoir to withstand the destruction effect of mechanical compaction (mechanical compaction is resulted from the pressure of overlying bed rocks which compresses the pores, thus leading to less pore ratios within the rocks). In this project, it is concluded that the quartz cements were precipitated in limited amounts, supporting the reservoir sand grains against negative effects of mechanical compaction. It was proved in this study that some minerals dissolve and produce new pores and largely enhance the total amount of porosity. This project stresses that multitude factors control porosity and permeability, and that each reservoir is unique and must be studied and examined separately. Hence, conclusions should not necessarily hold for all reservoirs because of the multitude of geological factors influencing porosity and permeability