When it comes to well loss and efficiency, the occurrence of wellbore skin layers is one of the strongest influencing factors. Besides difficulties to remove the skin layer that is necessary during the drilling process, it is also not easily possible to determine if a skin layer is present in a well and whether or not it imposes a certain degree of well loss. With this work, three types of skin layers are presented (surface cake, deep-bed filtration, layered cake in the aquifer), that have been observed at dewatering wells excavated in open-pits of the Rhenish lignite mining district in western Germany. Disturbed and undisturbed samples were analyzed for their geochemical and mineralogical composition in order to better understand the formation of the skin layer types and their fate during well operation. Geochemical analysis revealed the skin layer to be mainly composed of quartz (˜ 40 wt-%), kaolinite/illite (˜ 30 wt-%), organic material (5-15 %) and secondary gypsum precipitates (up to 12.5 wt-%). Despite the high quartz contents, the granulometry yields high fractions of clay and silt (75-85 %). However, preferential flow paths, transecting the skin layer are created by micro-cracks and erosion-pathways which will cause a higher hydraulic conductivity than could be expected from the granulometry.