This paper investigates reservoir quality development in tight Upper Carboniferous fluvial sandstones (Westphalian C/D) in the Lower Saxony Basin, NW Germany. The study integrates data from three outcrops (Piesberg, Woitzel and Hüggel) in the south of the basin with that from two wells (Wells A and B) located at gas fields approximately 50 km to the north. Petrographic and petrophysical data are related to the diagenetic evolution of the sandstones and the burial and structural history of the Lower Saxony Basin. The outcrop and subsurface data sets are compared in order to investigate the factors controlling reservoir quality evolution. Upper Carboniferous fluvial sandstones from the Woitzel and Hüggel outcrops and from Wells A and B have similar matrix permeabilities (0.01 to 10 mD), but matrix porosities vary between Well A (average 6%), Well B (average 10%), Woitzel (average 15%), and Hüggel (average 19%). Permeability reduction during burial is related to the formation of clay mineral cement, which was mainly controlled by variations in both the palaeo‐climate and in the sandstones’ depositional composition. Matrix porosity was controlled by local differences in burial history related to basin inversion tectonics. The greater amount of inversion‐related uplift at Well B (about 2.8 km) resulted in lower thermal exposure of the Westphalian sandstones at this location, which thus show higher matrix porosities than the sandstones at Well A which were uplifted by only about 1.2 km. Further increases in porosity in the outcrop sandstones may be related to the dissolution of carbonate cement during late‐stage uplift in near‐surface conditions. Upper Carboniferous fluvial sandstones from the Piesberg quarry show the poorest reservoir characteristics compared to the samples from the subsurface and the other outcrops, with matrix porosities averaging 6% and permeabilities <0.01 mD. Reservoir quality reduction was controlled by thermal anomalies associated with a large fault at the Piesberg quarry. By contrast, a few outliers in the sample data sets from Well B and the Piesberg quarry, which have permeabilities of more than 100 mD, show that faulting or natural fracturing may enhance reservoir quality within a particular area. Faults/fractures may act as potential migration pathways for leaching fluids, or may provide fracture‐permeability systems with production potential. Depositional setting, burial‐related diagenetic processes and structural characteristics in the Lower Saxony Basin need to be carefully evaluated in order to provide an improved understanding of the reservoir quality of the Upper Carboniferous sandstones.

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Understanding the spatial variability of reservoir properties in tight gas sandstones can significantly reduce uncertainties in reservoir characterization. This study focuses on two outcrops of Upper Carboniferous (Westphalian D) fluvial sandstones in the southern part of the Lower Saxony Basin, NW Germany, which are analogues for tight, gas‐producing reservoir rocks at fields in the north of the basin. Large‐scale differences in reservoir quality occur between the two outcrops which are separated by a distance of approximately 15 km and by 600 m of stratigraphy. Smaller‐scale heterogeneities in the form of channel geometries occur within individual fluvial cycles, as evidenced in a 30 m (high) by 150 m (long) porosity‐permeability profile at one of the outcrops studied. In the Woitzel quarry, lower Westphalian D deposits consist of fining‐upwards cycles of channel and bar sandstones with intercalated floodplain sediments and coal seams. In abandoned quarries at the Hüggel location, upper Westphalian D strata are composed of grey and red coloured sandstones which suggest deposition in a fluvial – alluvial plain environment under increasingly arid conditions. Reservoir quality is higher in the upper Westphalian D sandstones at Hüggel (permeability: up to 1 mD; mean porosity: 19%) than in the lower Westphalian D sandstones at Woitzel (permeability: <0.1 mD; mean porosity: 15%). Due to the high degree of compaction of the upper and lower Westphalian D deposits (intergranular volumes of 21.9% and 19.4% respectively) and the high proportions of pseudomatrix, porosity in these sandstones is mainly secondary with up to 80% microporosity. Reservoir quality is in general higher in the upper Westphalian D sandstones where kaolinite is present. In contrast, intense illitization has degraded the reservoir quality of the lower Westphalian D sandstones. A positive correlation between permeability and grain size is exhibited by the kaolinite‐bearing upper Westphalian D sandstones. No such correlation is evident in the illite‐bearing lower Westphalian D sandstones, unless samples with permeabilities of <0.01 mD are excluded from the correlation. However, such a low permeability cut‐off is not related to sorting, grain size or authigenic quartz cementation. At a small scale, permeability varies laterally by two orders of magnitude from 0.001 mD to 0.1 mD in a single, approximately 3 m thick tight sandstone bed over a distance of 150 m. Repeated, centimetre‐scaled fining‐upwards grading and bed‐internal erosion contribute to lateral variations in reservoir flow characteristics. Integrated studies of the depositional environment, diagenesis and sedimentary geometries of the deposited units are a key to the understanding of heterogeneities in tight fluvial sandstones and thus to the successful development of Upper Carboniferous reservoirs in the Lower Saxony Basin and elsewhere.

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