Abstract

Numerous field and subsurface studies have shown the sensitivity of gravity-flow deposits to seafloor topography. Understanding the driving mechanisms of paleo-seafloor evolution will provide insights to the depositional architecture of such successions. The Gudrun field of Block 15/3 in the Norwegian Vilje subbasin in the South Viking Graben (SVG) exploits such sandstone reservoirs in the Upper Jurassic Draupne Formation. Spatial definition of the reservoir is hampered by low or even absent impedance contrast. However, detailed reservoir characterization based on sedimentological, petrophysical, petrological, and outcrop analog data in conjunction with dipmeter data, helped to optimize the development and production of this field. In the study area, the middle Oxfordian to Tithonian Draupne Formation ranges in thickness from 350 m to more than 800 m (1100–2600 ft) and is divided into Draupne Formation 1 (oldest) to 4 (youngest). Targeted reservoirs are the gas-condensate-filled Draupne Formation 1 and the oil-filled Draupne Formation 3. Dipmeter data in conjunction with a 2-D structural reconstruction indicate that the depositional slope in the Vilje subbasin was primarily controlled by the flexural downbending of the Brae hanging wall, resulting in a drowning of the eastern SVG shelf and the retrogradation of the Oxfordian Draupne Formation 1. Coeval normal faulting along the Gudrun fault created hydraulic jumps in seafloor topography, leading to preferential deposition of Draupne Formation 1 in the Gudrun hanging wall. Draupne Formation 1 and 2 were deposited in a sand-rich, channelized lobe system with a high net-to-gross (>70% sand), and good lateral and vertical connectivity. Seismic mapping revealed that sediments were transported from the Utsira High into the SVG via long-lived antecedent southeast-northwest-trending canyons during the Oxfordian to Early Cretaceous. Draupne Formation 3 was deposited in a channelized slope system (20–40% sand), resulting in higher heterogeneity and thus lower connectivity compared to Draupne Formation 1. Also, Draupne Formation 3 records the onset of inversion of the Gudrun hanging wall and the subsequent growth of the Gudrun fold, marking thus a fundamental change in seafloor topography. Progradation of Draupne Formation 2 and 3 suggests increased sediment flux from the Utsira High, which may be triggered by the observed shift toward more arid conditions or uplift of the Utsira High by lithospheric folding related to basin inversion. Finally, the turbidites of the Draupne Formation were deposited during a prolonged relative sea-level rise in the Late Jurassic. Therefore, eustatic sea-level fluctuations are considered to be of minor importance in controlling the depositional architecture of the Upper Jurassic turbidite system in the Gudrun area.