Diagenesis and reservoir quality of the Lower and Middle Buntsandstein(Lower Triassic), SW Germany

Abstract

This research work deals with the diagenesis and reservoir quality of the Lower Triassic Buntsandstein in south-western Germany. The Buntsandstein in the study area is a continental succession with mostly fluvial and subordinately lacustrine and aeolian sediments. The most promising reservoir units are the Lower and Middle Buntsandstein, consisting mostly of stacked fluvial channel sandstones, interlayered by only thin mudstone beds. The Upper Buntsandstein is shalying upward and is thus less favourable as reservoir. The mineralogy of the Buntsandstein indicates intense weathering. Plagioclase is completely absent, because it was already partly dissolved in the hinterland and finally replaced by kaolinite shortly after deposition. Dissolution vugs after plagioclase were apparently lost due to compaction. K-feldspar dissolution occurred after framework-supporting quartz cementation. This cement stratigraphic position is indicated by less compaction-affected vugs or “ghosts” after K-feldspar. In most cases, K-feldspar is replaced by kaolinite and minor illite. Neither plagioclase nor K-feldspar dissolution have therefore a significantly positive effect on reservoir qualities. Regionally different quartz cementation west and east of the URG was observed: The quartz cement content on the western flank hardly reaches 10%, while it partly exceeds 30% on the eastern flank. Consequently porosity and permeability significantly vary: on the western flank porosities commonly exceed 15 to 20% while they are usually below 15 to 10% on the eastern flank. Quartz dissolution at grain contacts (“pressure solution”) and particularly clay mineral transformations (smectite-illite, kaolinite-illite) were identified as probable silica sources. A probable reason for the regional differences in quartz cementation was revealed by the fluid inclusions (FIs) trapped in the quartz cement. Homogenization temperatures (Th) in the FIs of a few hundred °C at an estimated maximum burial depth less than 1500 m proofs a hydrothermal origin of the cementing fluids. Increased Th correlate well with the above mentioned amount of regional quartz cementation: On the western flank Th are usually below 250 °C, whereas Th reach maximum values of >350 °C exclusively on the eastern flank (Odenwald). Apart from the differing fluid temperatures the quartz cementation exhibited a significant sensitivity to clay mineral content. Particularly grain coating clay minerals (mostly illite) could restrain the precipitation of authigenic quartz on detrital grains. Clay mineral coatings are generally more abundant with decreasing grain size. Quartz cement content therefore generally also decreases with decreasing grain size, within the limits set by the strong differences in fluid temperatures. Iron-rich and barite-filled fracture mineralizations of younger age indicate highly saturated formation waters in the subsurface of the URG, which can cause technical problems particularly during geothermal production (scalings, corrosion). The Buntsandstein has the potential for a favourable geothermal as well as hydrocarbon reservoir (producing oil field Romerberg). However, regional diagenesis and its impact on PoroPerm properties of the matrix and rock-rigidity are essential components of reservoir properties.