The Slochteren No. 1 well discovered in 1959 what is now known as the Groningen gas field in the northern Netherlands. The field is on a culmination of the large, regional Northern Netherlands high which was formed during the late Kimmeric tectonic phase (Late Jurassic-Early Cretaceous). However, there is some evidence that the structure existed as a positive element during earlier periods; i.e., during Triassic and possibly even in late Carboniferous time.

The reservoir overlies unconformably the truncated and strongly faulted coal-bearing Pennsylvanian strata which are considered to form the main source of the Groningen gas. The reservoir consists of fluviatile and eolian sandstone and conglomerate of the Rotliegendes formation (Lower Permian), 300-600 ft thick. These coarse clastics are overlain by a few thousand feet of Permian Zechstein evaporites, notably rock salt and to a lesser extent anhydrite and dolomite, which constitute the very effective reservoir seal. Because of intensive salt movements, the thickness of the overlying Mesozoic and Cenozoic strata ranges from 3,000 ft to more than 6,500 ft.

The field covers an area of 180,000 acres and the reserves presently are estimated at 58 trillion cu ft. Present production potential is 2 Bcf of gas per day from 9 "clusters" of about 6 closely spaced wells each. The favorable reservoir properties of the sandstone allow, at least for the time being, drainage of the field from the structurally highest southern part.

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