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Structural Diagenesis in an Upper Carboniferous Tight Gas Sandstones Reservoir Analog

Abstract

The effective exploration of unconventional hydrocarbon reservoirs such as tight gas sandstones is getting more important as conventional hydrocarbon reservoirs are becoming increasingly scarce. Gas from tight gas reservoirs has been successfully produced in the Lower Saxony Basin, Germany, for more than four decades but only contributes with a minor amount to the overall gas production. Unconventional reserves however are vast and could significantly support the supply with domestic gas in Central Europe over the next decades, if reservoir quality predictions as well as production technologies can be improved. We integrate quantitative data from a reservoir outcrop analogue and contribute to the understanding of the effect of structural diagenesis, which in turn may contribute to an enhancement of recovery factors of tight gas sandstone reservoirs in the region. We demonstrated that the Piesberg quarry near Osnabrueck, NW-Germany acts as a suitable reservoir outcrop analogue to Upper Carboniferous tight gas fields of the Lower Saxony Basin in terms of size, facies, structural inventory and diagenesis. This study focuses on the multi-scale reservoir heterogeneity exposed in the Piesberg quarry, comprising fluvial sandstone cycles of Pennsylvanian age. The main porosity is secondary and resulted mostly from carbonate leaching and limited dissolution of feldspar. Porosity variations are both stratigraphically and structurally controlled. Primary pore space was occluded by the development of a pseudomatrix resulting in low porosities (<10%) and very low permeabilities (<0.01 mD). Lateral and vertical variations of reservoir properties within depositional facies and stratigraphic cycles are well documented in high resolution wall panels displaying porosity and permeability distributions. Structurally controlled matrix porosities are increased up to five orders of magnitude (up to 25%) in fault corridors. Fractures and fault planes are quartz-cemented around faults, indicating localized mass transport and may be associated with the structural and diagenetic evolution of the Upper Carboniferous of the Piesberg area. Within this R&D project a predictive model for the carbonate cement distribution and related porosity-permeability variations in Upper Carboniferous sandstones will be established. Reservoir quality is structurally and stratigraphically controlled, which might lead to new well placements close to faults. This may change future tight gas exploration.