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Geometry and Origin of a Giant Asymmetrically Shaped Mound Complex, West of the Troll Field, Northern North Sea


Well and seismic-stratigraphic analysis of the Tertiary succession of the Northern North Sea provided insight to detailed investigation of an asymmetrically-shaped mounded structure located in the western part of the Troll field. The structure has an irregular NE-SW trend geometry and formed from deformation of a ∼ 25 x 15 x 0.3 km unit (112.5 km3). In plan view, it tapers bluntly in a N-S direction away from the centre and forms a cone-like geometry in cross section. The studied sequence was divided into five major units (U1-U5), bounded at the base and top by the base Cretaceous and base Pleistocene unconformities respectively. Unit 1 encompasses a complete Cretaceous to Eocene-aged sediments. Unit 2 consists of lower Oligocene sediments characterised by low-amplitude, discontinuous reflections clearly differentiated from overlying unit 3 by a strong, and high-amplitude continuous refection interpreted as Opal A-CT diagenetic boundary. Unit 3, upper Oligocene-base Miocene in age houses ∼80-90% of the mounded structure. Although this unit is pervasively deformed by polygonal faults, they are hardly present within the structure. Unit 3 is overlain by unit 4 and comprises of thick sandstones of the Utsira Formation than thins considerably above the structure. Unit 5 is Pliocene in age and consists of high-angled clinoforms. Presence of isolated and lateral amplitude anomaly directly observed above the mounded structure in this unit could be related to gas-charged sands. Internally, the structure has contrasting seismic facies character from its surrounding with a transparent background. Often, high-amplitude and discordant reflections are identified randomly and at the base of the mound. Lack of well penetration makes it difficult to constrain the lithology within the mud, but similar features are interpreted as remobilized/injected sands in other parts of the study area. Based on our findings, we propose a possible genetic link between the structure and an underlying relay-fault zone. This stems from direct occurrence and limit of the mound above the fault zone; that probably served as migration pathways for deep-seated fluids into overlying sand bodies. Our most preferred origin of this giant mound is related to jack-up in the overburden caused by forceful injection of these fluidized sands into thick mudstone unit and possibly enhanced by differential compaction.