Lateral Heterogeneity of Reservoir Properties in an Homogenous Lower Triassic Sandstone, Southern Germany

Characterizing a subsurface reservoir, the geologist is reliant on core material, which allows for only a small-scale view into the target area. Hence, analytically determined reservoir properties from core material can barely serve as proxy for a large 3D prospect. This analog study of a proposedly homogenous Lower Triassic sandstone in Southern Germany offers a means to fill gaps like sedimentary facies and clearly points out the variability along a lateral section concerning porosity φ and horizontal permeability kh. Successively, the measured plugs served for petrographic assessment.
The quarry Röttbach is located in Lower Franconia and consists of very fine- to fine-grained, dark red sandstones that are of fluvial origin. Macroscopic observations reveal a homogenous, massive sandstone with few localized features like indistinct, tangential cross beds as well as clusters of silty to clayey rip up-clasts. A striking feature are bleaching phenomena that may occur pervasive, lamina-bound or speckled.
Petrophysical measurements yield φ-values of 11-19 % (avg. 16 %) while kh ranges from 0.02 to 48 mD (avg. 9 mD). Samples originate from three horizontal profiles of different stratigraphic height within the outcrop. ... mehrValues for φ and kh show no correlation of the vertically aligned samples, whilst the northwestern part of the quarry is of generally lower permeability (<1.3 mD). Thin section analyses reveal a mainly subarkosic to lithic arenite composition. The detritus consists of on average 7 % sheet silicate-bearing lithofragments and abundant iron
oxides (avg. 12 %). Their overall content increases to the NW of the outcrop (22-29 %) while the mean grain size remains relatively constant. Partial bleaching is ubiquitous in plug samples, appears unsystematic and coincides with samples generally characterized by reduced φ- and kh-values, strikingly so in the otherwise more porous and permeable SE-part of the locality. On the contrary, detailed point counting of partially bleached and unbleached parts of the sample separately reveals that the bleached intervals show higher intergranular volumes than the unbleached parts. Grain sizes are comparatively large with more quartz cements (16-22 %, avg. 15 %) and slightly more diagenetic illite (6-18 %, avg. 8 %). From detrital clay mineral content, the vicinity of e.g. a flood plain can be inferred influencing reservoir quality although lithological description and mean grain size do not give any such indication. Here, only lamina-bound and pervasive bleaching improves porosity and permeability as it gives way to strong dissolution but also enhanced cementation. Quartz cements in this case appear to stabilize the mineralogical framework instead of reducing porosity and overall reservoir quality.