Aridisols in the Southern Permian Basin of Lithuania: a key to understanding clay cement distribution

The change from fossil to geothermal energy supply includes the necessity to re-evaluate gas reservoirs in the Southern Permian Basin of NW Europe, as higher permeability reservoirs are needed for the exploitation of geothermal energy than natural gas. A key reservoir risk in Permian sandstones in the Southern Permian Basin is the presence of clay cement. The distribution of the clay cement is not properly understood. The negative effect can be attributed to complex grain coatings formed by tangential clay directly around the detrital grains followed by authigenic clay rim cement, both being genetically related. This study presents new results on the origin of the tangential and the authigenic clay. The Southern Permian Basin in Lithuania was a marginal area of the basin with low accommodation rates and extensive aridisol development. Here, sandstones of the Perloja Formation contain abundant clayey matrix in the form of grain and void coatings. The limited burial depth of a few hundreds of metres ensured that the original internal fabric and mineralogy of the clay coatings remained well preserved. Thin section microscopy revealed that the clay coatings are cutans that formed by post-depositional mechanical infiltration (illuviation) of suspensions of clay minerals, iron hydroxides and some clay to silt-sized quartz. Other pedogenic features are the development of nodular calcrete, rootlets, and of infiltrated clay. Aridisols were recurrently eroded by fluvial or aeolian activity causing wide-scale dispersion of siliciclastic grains with more or less complete clay cutans and also carbonate (calcrete) clasts in certain stratigraphic intervals.