Triassic correlation and stratigraphy in the South Central Graben, UK North Sea

Triassic strata in the South Central Graben of the North Sea have yielded moderate to good recoveries of palynofloral assemblages and have sufficiently distinct petrophysical responses to enable the subdivision of the succession into between seven and eleven units. The Triassic succession of the Amoco well, 30/12b-3, is proposed as the type section for the Skagerrak Formation of the area. Despite lacking good biostratigraphic control, typical log character for the sequences is well displayed. The mainly Scythian (early Triassic) red mudstones of the Smith Bank Formation can locally be divided into three units, but generally only one is recognized. A new lithostratigraphic nomenclature for the middle to late Triassic Skagerrak Formation is proposed. Six members, with eight units, are recognized over UK Quadrants 29 and 30, and may possibly continue into the southern part of Quadrant 22 and into Norway Quadrant 7. The member names are partly derived from the Phillips fields in which they were first recognized. The basal Judy Sandstone Member is named after the Judy Field discovery well 30/7a-4A, and is overlain by the petrophysically distinct Julius Mudstone Member. The Joanne Sandstone Member is named after the Joanne Field well 30/7a-3, and is overlain by the Jonathan Mudstone Member, which can be further subdivided into upper, middle and lower units on log character. The uppermost members are the Josephine Sandstone Member, from the type well on the Josephine structure, 30/13-1 (Deegan & Scull 1977), which is overlain by the Joshua Mudstone Member. The upper members are rarely preserved, due to extensive early to middle Jurassic uplift and erosion.