Timing, Morphology and Character of Lower Cretaceous Agat Deepwater Depositional Systems along the Norwegian Margin

Extrabasinal processes, such as hinterland drainage patterns and onshore tectonics, and intrabasnal processes, such as relative sea level changes and shelf processes, combine to control the timing, magnitude and longevity of sediment input from the basin margin. At the system scale, slope and basin-floor bathymetry will control sediment routing and storage, which in turn determines the morphology, sedimentology and stratigraphic architecture of deepwater depositional systems. However, the style in which these various controls interact to control the along-strike development of post-rift deepwater depositional systems is poorly understood. 3D seismic, wireline log and core data are integrated to assess the relative timing and contribution of the aforementioned controls on the deposition of post-rift, deepwater reservoir sandstones, offshore Norway. We focus on the Lower Cretaceous Agat Formation in the Slørebotn sub-basin and on the Måløy Slope; this unit comprises a series of slope fans and channel complexes that were fed by sediment routed via upper slope canyons.

Cored, sand-prone intervals from wells that penetrate the Agat Formation in the Måløy Slope area are dominated by: amalgamated units of structureless, fine- to medium-grained, commonly dewatered sandstones; debrites and ‘hybrid’ beds (0.1-1.5 m thick); structureless and stratified sandstones (0.05-0.5 m thick); and laminated, coarse siltstones (0.05-0.3 m thick). Most of the sandstones contain significant quantities of glauconite, implying that the sediment was reworked in and derived from a shallow-marine shelf environment. Lobe axis, off-axis and fringe sub-environments are the key depositional environments, with evidence of incision by channels. Typically, debrite-dominated intervals of the succession are overlain by amalgamated and non-amalgamated turbidite sandstones, reflecting the initiation of lobe complexes. Well correlations and seismic facies mapping indicate significant variation in the thickness and distribution of these primary depositional elements, which was controlled by remnant slope topography related to differential compaction across buried fault blocks.

The integrated dataset presented here provides a unique insight into the initiation, establishment and abandonment of post-rift deepwater systems along a continental margin, and the role that remnant topography plays in determining deepwater system behaviour and stratigraphic architecture.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California