THE STRATIGRAPHY OF OXFORDIAN-KIMMERIDGIAN (LATE JURASSIC) RESERVOIR SANDSTONES IN THE WITCH GROUND GRABEN, UNITED-KINGDOM NORTH-SEA

Oil-bearing Upper Jurassic Oxfordian-Kimmeridgian sandstones of the Sg iath and Piper formations are of major economic importance in the Witc h Ground Graben, United Kingdom North Sea. They form the reservoirs in 14 fields that originally contained 2 billion bbl of oil reserves, in cluding Scott field, which in 1993 will be the largest producing Unite d Kingdom North Sea oil field to come on stream in more than a decade. The Sgiath and Piper formations represent Late Jurassic transgressive and regressive phases that began with paralic deposition and culminat ed in a wave-dominated delta system. These phases preceded the major g raben rifting episode (late Kimmeridgian to early Ryazanian) and depos ition of the Kimmeridge Clay Formation, the principal source rock of t he Witch Ground Graben oil fields. A threefold subdivision of the midd le to upper Oxfordian Sgiath Formation is formally proposed, with Scot t field well 15/21a-15 as the designated reference well. The basal Ske ne Member consists of thinly interbedded paralic carbonaceous shales, coals, and sandstones. This is overlain by transgressive marine shales of the Saltire Member. The uppermost Oxfordian Scott Member consists of shallow marine sandstones that prograded to the southwest (a precur sor of the Kimmeridgian wave-dominated delta sequence of the Piper For mation, which prograded to the south and to the west). The contact of the Sgiath and Piper formations is a basinwide transgressive marine sh ale (I shale), which can act as an effective barrier to fluid communic ation between the Sgiath and Piper reservoir sandstones. The recogniti on of transgressive marine shales, or maximum flooding surfaces, is of great use in understanding regional geological history and in solving exploration and development problems.