F15-A: a Triassic gas field on the eastern limit of the Dutch Central Graben

Abstract

The F15-A gas field lies at the eastern margin of the Dutch Central Graben, the reservoirs being found within the overpressured Triassic Main Buntsandstein Formation. The discovery well, F15-4, was drilled in 1985 and was subsequently followed by the two appraisal wells, F15-5 and F15-7. Structurally speaking, the F15 area is located in an intermediate structural compartment between the Central Graben, to the west and the Schill Grund High, to the east. The structure consists of a NNE–SSW-trending turtle back anticline, plunging to the south, with gently dipping flanks to the east and west. Northwards, closure is provided by a NW–SE-oriented erosional trough which cuts deeply into the Lower Triassic and is filled by Upper Triassic/Jurassic sediments. The primary reservoir is the Volpriehausen Sandstone Member, which is very fine to coarse grained and irregularly cemented by dolomite, anhydrite and halite. Four lithofacies have been recognized: dunal, interdunal, ephemeral fluviatile (distal wadi) and playa-lake related. These facies show a coarsening-upward profile, caused by the successive depositional environments becoming increasingly arid as they grade from playa lake to dunes. A seismic amplitude study was carried out, based on the links between porosity and seismic response at the reservoir level, to identify the areas with the best potential reservoir development. The field was originally identified and discovered on the basis of two-dimensional seismic data; however, a three-dimensional survey was subsequently shot and has increased the confidence in the structural interpretation. Based on this, one horizontal production well will be drilled in order to put the field on stream in June 1993.