DGMK/ÖGEW Frühjahrstagung

Celle

25.04.-26.04.2019

Schmidt C, Busch B, Hilgers C

Schmidt C

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. Values 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.