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Abstract: Structural Evolution of Central Graben Salt Diapir Fields, N. Sea

DAVISON, IAN, NEIL EVANS, and MARKUS SAFARICZ, Royal Holloway, University of London; IAN ALSOP, University of St. Andrews, UK.

Central Graben salt diapirs initiated in Triassic times as salt walls along extensional faults, and evolved into diapirs in the Jurassic without going through a prolonged pillow phase. Diapiric growth was by downbuilding with a subtle relief maintained through the Cretaceous to late Tertiary period. This relief controlled the deposition of Palaeocene turbidite sandstone reservoirs which thicken away from the diapir crest. Important unconformities are developed during the Middle Miocene with high-angle onlap reflectors above the unconformity surface which indicate high sea bed relief on the diapir. This is interpreted to be produced by Alpine compression, which is recognised in the Central Graben for the first time. Core studies indicate that most of the deformation in the Paleocene sandstones takes place by soft sediment slumping and bedding-parallel slip along interbedded shale horizons. On the diapir crests open fractures occur which increase fluid flow, but these diminish rapidly down the diapir flanks where they become cemented by calcite. Dissolution seams have developed with thicknesses up to 20 cm, and they produce multiple fluid migration barriers, which can hold back up to 5000 psi overpressure. Approximately 50% of the chalk has been dissolved away along the seams and some of this material has been removed from the system so that pressure solution may induce significant amounts of subsidence in the Central Graben.

All the diapir fields are leaking hydrocarbons into the overlying overburden, and these cause drilling problems due to overpressuring and fracturing. Offset platforms will minimise drilling problems and provide optimum production well trajectories.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah