Wadi deposits of the Rotliegend revisited – key to unravel reservoir geometries in the Base Rotliegend of the Dutch-German border area

An old play has recently been extended across the border from Germany into the Netherlands, an area previously thought to be barren of reservoir sandstones. The combination of core evaluation with observations from modern analogues help to refine the depositional geometries and architecture of the basal Upper Rotliegend play. The study area is located in the Southern Permian Basin, comprising wadi and dune sandstones and a central desert lake. The dimensions of the desert sand facies belts in North Oman are similar to those in the Southern Permian Basin and, similar to the base Upper Rotliegend situation, a relief characterises the hinterland of the desert system while a large desert lake is missing. For this setting lake Eyre (Australia) is an example. Here, the mountain ranges from which the fluvial feeder systems originate are more than 500 km away and the landscape surrounding the lake is flat. Flood events in the Rotliegend are usually thought to be unconfined sheet-type floods, leaving behind sheet-like sediments. However, the Lower Slochteren Member was characterised by a relief, which confined rivers and thereby channel deposits could develop. Fluvial events in desert systems from Oman and near Lake Eyre rather resemble braided streams during waning flow stages, where a water sheet is only present during maximum discharge. This is consistent with fluvial deposits in the study area, which are characterised by a depositional sequence reflecting a change from high density currents to low density currents to standing water deposits. Indeed, desert river floods are typically short-lived events with strongly changing sediment/water ratios and current velocities leaving behind a characteristic depositional “flashflood sequence”, analogue to the classic Bouma sequence. It is mostly assumed that fluvial sands entering the Silverpit lake were deposited by a sediment plume. After drying out, the sediment is assumed to be reworked by wind, leaving few traces of the fluvial activity. In addition to overprinted sediments, coarsening-upward subaqueous bar type deposits are locally common in the study area. Also in Lake Eyre, subaqueous delta facies and channel systems representing pro-delta features occur, which remain visible during periods of drought. Evaporitic cements and a high groundwater table may help to preserve the pro-delta deposits, which can be good quality reservoir sands. These improvements of the depositional model help to unravel the depositional settings of the basal Upper Rotliegend sandstones and allow for exploration of sweet spots in yet underexplored areas.