Sediment Structures Affecting Flow in Eolian Rotliegend Sanstones

Eolian dune sandstones are recognized as homogenous reservoir units by both E&P and geothermal industry. Nevertheless, such assumptions neglect the effects of sedimentary structures within the rock mass. This study focusses on analog outcrops of Lower Permian reservoir sandstones in 1) the North German Basin and 2) the Vale of Eden (GB). We sampled vertical plugs within the sediment packages as well as across sediment structures limiting them. Differences in the petrophysical measurements are linked to petrographic results and the flow properties are compared to horizontal plugs of the same unit.
Structures sampled in the quarries are bounding surfaces, erosive boundaries within the sediment. Such surfaces limit sedimentary sets and cosets (equivalent to 3rd and 2nd order surfaces) and their impact on flow will subsequently be proven in a quantitative way.

The English and German samples are of the same age but differ significantly with regard to mineralogy, diagenetic features, porosity and permeability. When compared to bulk samples 3rd order reactivation surfaces from the English quarry partially display a porosity enhancement of <1 % and permeability increase of up to 80 mD (27 %) equivalent to one order of magnitude. ... mehrIn the German outcrop, 2nd order superposition surfaces were sampled that exhibit φ-losses of up to 5 %. Kv-values averaged only 0.6 mD but relative reduction amounted to 79 – 95 %. Still, values are inconsistent for several samples of a single surface. By contrast, superposition observed at the English site corresponds to a coherent layer of fines indicating a strong influence on otherwise permeable units. The average horizontal permeability of 5.2 % in the German quarry exceeds the vertical permeability by one order of magnitude.

Sedimentary deposition and diagenetic overprint are taken into account to gain an understanding of the differing properties and the occurence of such surfaces. The permeability increase around reactivated planes detached from any petrographic feature is ascribed to the rearrangement of the 3D granular assemblage but is rarely preserved at a dune’s crest. Superposition surfaces feature permeability-reducing calcite cements, clay clast alignment and more fines resulting from superimposed dunes migrating on the lee slope oblique to main transport and with prolonged exposure. Due to their lateral inconsistency in permeability, differential cementation remains the mechanism most effective in a conventional eolian reservoir.