Basin-Floor Fans in the North Sea: Sequence Stratigraphic Models vs. Sedimentary Facies: Reply

Abstract

Examination of nearly 12,000 ft (3658 m) of conventional core from Paleogene and Cretaceous deep-water sandstone reservoirs cored in 50 wells in 10 different areas or fields in the North Sea and adjacent regions reveals that these reservoirs are predominantly composed of mass-transport deposits, mainly sandy slumps and sandy debris flows. Classic turbidites are extremely rare and comprise less than 1% of all cores. Sedimentary features indicating slump and debris-flow origin include sand units with sharp upper contacts; slump folds; discordant, steeply dipping layers (up to 60°); glide planes; shear zones; brecciated clasts; clastic injections; floating mudstone clasts; planar clast fabric; inverse grading of clasts; and moderate-to-high matrix content (5-30%). Many f the cored reservoirs either have been previously interpreted as basin-floor fans or exhibit seismic (e.g., mounded forms) and wireline-log signatures (e.g., blocky motif) and stratal relationships (e.g., downlap onto sequence boundary) indicating basin-floor fans within a sequence stratigraphic framework. This model predicts that basin-floor fans are predominantly composed of sand-rich turbidites with laterally extensive, sheetlike geometries. However, calibration of sedimentary facies in our long (400-700 ft) cores with seismic and wireline-log signatures through several of these basin-floor fans (including the Gryphon-Forth, Frigg, and Faeroe areas) shows that these features are actually composed almost exclusively of mass-transport deposits consisting mainly of slumps and debris flo s. Distinguishing deposits of mass-transport processes, such as debris flows, from those of turbidity currents has important implications for predicting reservoir geometry. Debris flows, which have plastic flow rheology, can form discontinuous, disconnected sand bodies that are harder to delineate and less economical to develop than deposits of fluidal turbidity currents, which potentially produce more laterally continuous, interconnected sand bodies. Our core studies thus underscore the complexities of deep-water depositional systems and indicate that model-driven interpretation of remotely sensed data (i.e., seismic and wireline logs) to predict specific sedimentary facies and depositional features should proceed with caution. Process sedimentological interpretation of conventional cor is commonly critical for determining the true origin and distribution of reservoir sands.