Triassic and Jurassic reservoir distribution influenced by Zechstein salt, in a normal fault system near the margins of the Utsira High, South Viking Graben, North Sea

Abstract

During the Triassic and Jurassic periods, structural and stratigraphic development in the margins of the Utsira High, in the South Viking Graben, resulted in the development of depressional mini-basins and the halokinesis of the Zechstein Salt Group. The subsequent fluvial, alluvial and delta plain deposits filled in this accommodation space created by the salt withdrawal. Continuous sedimentation and salt development in the Triassic resulted in a pod-like occurrence, while subsidence and dissolution of Zechstein salt in the Jurassic created further accommodation space, for coarse clastic sediments. The difficulties of having rift-related normal faults in combination with salt movement, has complicated the distribution of reservoir sandstones within the Triassic Skagerrak Formation and Jurassic Hugin and Draupne Formations. In this study, a multi-faceted approach including 3D seismic, well data and synthetic seismograms have been applied, in order to obtain surface distribution, thickness' and attribute maps for investigation of reservoir distribution. The situation of having poor seismic resolution, and few Permian penetrations(most of wells being dry), further complicates the work. Surface-, thickness and attribute maps have been generated to further understand the influence the evaporite-dominated Zechstein salt, has upon reservoir distribution within Triassic and Jurassic strata.Sandstones are seen to thicken towards the Sleipner Terrace and thin towards the Utsira High, with intermittent local pod development, that I perceived to complicate the reservoir distribution. Whether the mini-basins are isolated or connected is unknown, due to significant noise within the seismic data, little well control and uncertainties when interpreting the top of the salt pods.Lundin drilled well 16/4-7 in 2013; in which they encountered excellent reservoir properties in salt depressions at the margins of the Utsira High. Even though the well was dry, there might be potential hydrocarbon accumulations within reservoirs consisting of stacked channels/braidplains, which promotes interest for further exploration.