Diagenesis of Rotliegend sandstone reservoirs (offshore Netherlands): The origin and impact of dolomite cements

The Upper Slochteren Sandstones of the Rotliegend Formation (ROSLU) were studied in offshore Netherlands to evaluate the impact of diagenesis, and more particularly of carbonate diagenesis, on sandstone reservoirs. An integrated study applying petrography, geochemistry (SEM-EDS, XRD, C & O stable isotopes, Sr isotopes), and fluid inclusion measurements, was undertaken. Early phases consisting of silica-bearing minerals (authigenic clays, quartz overgrowths), associated with mechanical compaction plus dissolution of labile grains and/or feldspars, are rare. These are succeeded by a sequence of carbonate cements including non-ferroan dolomite cement, ferroan dolomite cement, and finally siderite. Dolomite cements are associated with fracturing and bleaching of the sandstone. The latest phases consist of another set of authigenic clays, quartz overgrowths, plus sulphate cements. Carbonate pore-filling cements dominate the authigenic phases, representing >50% of the total, of which 80% is dolomite and 20% siderite. Although mechanical compaction is the dominant factor influencing porosity reduction, dolomite cements also have a negative impact on porosity and permeability properties, which do not only depend on sedimentary characteristics. Non-ferroan dolomite precipitated at approximately 100 °C, from highly saline brines migrating downward from the Zechstein. Conversely during the Mid Cimmerian Unconformity (MCU), faulting and fracturing allowed the upward migration of CO2-bearing, acidic and reducing, hotter fluids from the Carboniferous, which includes hydrocarbon source rocks. The mixing of the upward and downward migrating fluids resulted in the precipitation of ferroan dolomites. These results indicate an increased risk of encountering pore-filling dolomite cements close to fault/fractured zones in that area, where carbonate cements may not have been properly assessed. Moreover, this study emphasizes again the importance of a combined analytical approach to gain a deeper understanding of carbonate diagenesis, which should be considered with the same caution as clay diagenesis within clastic-hosted reservoirs.