Extended abstract: UPPER PERMIAN HALIBUT CARBONATE FORMATION, WITCH GROUND GRABEN AREA, NORTH PERMIAN BASIN, UKCS: SEDIMENTOLOGY, CORRELATION AND RESERVOIR QUALITY

Abstract

In the Witch Ground Graben area in the centralnorthern UK North Sea, four deviated wells (referred to here as wells A-D) have intersected the Halibut Carbonate Formation. This formation, which ranges in thickness from ca. 350 ft to ca. 100 ft from west to east, was deposited locally as part of the Upper Permian succession in the Northern Permian Basin. It consists of the lower Argyll Carbonate Member and the upper Innes Carbonate Member separated by the Iris Anhydrite Member. The formation is broadly equivalent to the Z1 and Z2 cycles of the Zechstein Supergroup in the better-known Southern Permian Basin. A distinctive conglomerate caps the Argyll Carbonate Member and is also recognised within the Iris Anhydrite Member, and is a possible equivalent to the Bosies Bank Formation. Downhole data from the studied wells (which includes cores and open-hole logs) has identified six different lithologies in the Halibut Carbonate Formation: dolostones, limestones, anhydrite and conglomerates, together with subordinate sandstones and claystones. Twenty-seven core facies have been determined from these lithologies and have been grouped into five facies associations. Facies association 1 (FA1) consists predominantly of dolomudstones and algal laminated dolostones (Fig. 1A) deposited within tidal flats and adjacent muddy lagoons. Facies association 2 (FA2) comprises dolofloatstones, dolorudstones and grainy dolostone facies interpreted as higher-energy sandto gravel-grade sand shoals. Facies association 3 (FA3)