Diagenetic conditions controlling authigenic formation of anatase and in situ LA-ICP-MS U–Pb isotope dating of this phase are studied in Upper Rotliegend II sandstones from two wells in NW-Germany. Anatase grew after breakdown of detrital Ti-phases (e.g. ilmenite) although local scale Ti transport and mobilization from detrital clay-hematite coats or condensed hydrocarbons cannot be ruled out. The anatase-forming reaction marks the change from a regime of reducing conditions imposed by first hydrocarbon generation to oxidizing conditions, probably caused by influx of a fluid from evaporitic Zechstein rocks. This change in fluid influx is interpreted as response to enhanced normal faulting and halokinesis during accelerated burial in Triassic times. Isotope data of U and Pb indicate incorporation of crustal common Pb from Mesozoic pore fluids but precisely mark diagenetic growth of anatase at 224.3 + 5.1/− 5.6 Ma in a Tera-Wasserburg plot. Locally, U–Pb isotope signatures are consistent with either additional incorporation of U and Pb from detrital precursor phases or with uptake of uranogenic Pb from hydrocarbons generated prior to 224 Ma. Anatase is shown to be a valuable authigenic phase suitable for U–Pb chronometry of diagenetic events, which appears to be unaffected by protracted burial and temperatures exceeding temperatures of crystallization.

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.

Summary Reservoir quality in deeply buried sandstone reservoirs (>4000 m burial depth) is often controlled by clay minerals, as cements and replacements. However, the presence of clay minerals in sandstones can also improve reservoir properties if the minerals are present as grain coatings, inhibiting or reducing syntaxial quartz overgrowth cementation. If e.g. illite is present at grain contacts however, reservoir properties might again be deteriorated, due to an increase in chemical compaction (or pressure solution). This complex interaction of geochemical, depositional, and mechanical processes need to be understood and accounted for in reservoir quality modelling. In the presented case study from the Permian Rotliegendes in northern Germany, a complex reservoir setting is introduced, which is affected by a complex interaction of aforementioned processes. Especially illite grain coating textures affect the compaction and permeability development in the reservoir interval. Three different grain coating characteristics (i.e. tangential, radial, and short radial) can be optically distinguished. Tangential grain coatings, present at grain contacts, enhance chemical compaction, whereas both radial phases contribute less to chemical compaction. Reservoir quality benefits most from short radial grain coatings, whereas radial and tangential types do not show a definitive impact on petrophysical data.