The basal Permian Rotliegendes redbeds of the North Sea area, in the Northwest Europe Permo-Triassic basin, contain some of the world's largest gas reserves. Groningen field, The Netherlands, discovered in 1959, has proved plus probable gas reserves of 66.7 Tcf (1,904 × 10⁹ m³). Smaller Netherlands and northwestern Germany fields contain proved plus probable gas reserves of 8.4 Tcf (240 × 10⁹ m³); North Sea gas fields have 22.5 Tcf (700 × 10⁹ m³).

The Northwest Europe Permo-Triassic basin is north of the Variscan mountain chain, which was folded during Pennsylvanian time and which extends from the southern British Isles to central Germany. North of the Variscan mountain chain is a block-faulted foreland on which the Mid-North Sea and Fyn-Grinsted highs formed.

The WNW-ESE-trending Northwest Europe Permo-Triassic basin is between these highs on the north and the Variscan chain on the south. A second Permo-Triassic basin, whose boundaries are not defined, may lie north of the Mid-North Sea and Fyn-Grinsted highs.

The sediments of the basal sedimentary unit of the Permo-Triassic basin, the Rotliegendes, were derived mainly from the Variscan mountain chain. As this mountain chain rose and was eroded, increasingly arid conditions predominated. As a result, the Rotliegendes is primarily a desert deposit, though marginal fluviatile and local evaporitic sediments are present. When the arid climatic depositional conditions and desert origin of the subsurface Rotliegendes were recognized, it became imperative to develop criteria for the recognition of different desert-arid sedimentary facies, so that an exploratory program could be developed. Modern desert studies were made, and three major sedimentary facies were recognized: (1) wadi (intermittently flowing streams) gravel and sand which border highl nds; (2) eolian sand, derived from deflation of wadi deposits and outcrops, forms barchan dunes (probably formed from intermediate-velocity winds) and linear (seif) dunes, parallel with the dominant wind direction (probably formed from high-velocity winds); and (3) sabkha deposits of both the inland (wadi or oasis types) and coastal varieties, including both layered and interstitial evaporitic deposits.

All three facies have been recognized in the Rotliegendes.

Generally, the wadi facies are basal or peripheral within the basin. Most are broad alluvial fans along the northern flank of the Variscan mountain chain. The eolian facies is widespread--some as thick as 660 ft (200 m). The lack of linear dunes suggests intermediate wind-velocity values. Directional data indicate a west-to-east wind flow, as in the modern horse-latitude deserts. Sabkha facies also are widespread, particularly in the upper and central parts of the basin. These include interior-desert sabkha deposits, where wadi waters reached the surface, as well as coastal sabkha deposits, the precursors of the later Permian Zechstein sea.

Although the environmental conditions which produced the Rotliegendes persisted locally, these environmental conditions eventually were replaced by the rapid transgression of the Zechstein sea when the basal Kupferschiefer (Copper shale) and associated marine evaporite deposition began.

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