Bleaching phenomena and their link to compaction in fluvial reservoir formations – Lower Triassic, Central Germany

In order to describe rock properties like rock texture and mineralogy and lower uncertainties regarding reservoir quality, analog studies are carried out. They can provide a µm- to 100m-scale equivalent of the targeted lithology and an overview of small- to large-scale heterogeneities.
Lower Triassic red beds in Germany frequently exhibit lamina-bound or pervasive bleaching, both in reservoirs and outcrops. In this study bleaching will be analyzed with respect to its primary cause and effect on reservoir quality. The outcrop is an analog for fluvial reservoir sandstones in Central Germany. The outcrop is composed of cross- and horizontally bedded channel sequences with fine- to very fine-grained sandstones. The petrographic assessment yields larger grain sizes and more intergranular porosity for bleached areas compared to unbleached ones (Δ grain size = 27 µm, Δ point-count porosity = 6 %). The mineral assemblage in bleached zones exhibits higher amounts of burial-related cements like quartz and illite but also early or uplift-related feldspar. Plugs from pervasively bleached blocks calibrate the effect of such diagenetic alterations in laminae on porosity and permeability. ... mehrIn addition to that, the influence of differential compaction onto the occurrence of bleaching is evaluated. Unbleached, more fine-grained laminae primarily contain more ductile detrital components and undergo stronger compaction, reducing porosity and permeability and subsequent bleaching. Thus, it is suggested that lamination characterized by larger grain size and differences in detrital mineralogy primarily facilitate fluid flow and only enable bleaching.