BASIN-FLOOR FANS IN THE NORTH-SEA - SEQUENCE STRATIGRAPHIC MODELS VS SEDIMENTARY FACIES

Examination of nearly 12,000 ft (3658 m) of conventional core from Pal eogene and Cretaceous deepwater 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-tran sport deposits, mainly sandy slumps and sandy debris flows. Classic tu rbidites are extremely rare and comprise less than 1% of all cores. Se dimentary features indicating slump and debris-flow origin include san d units with sharp upper contacts; slump folds; discordant, steeply di pping layers (up to 60 degrees); glide planes; shear zones; brecciated clasts; clastic injections; floating mudstone clasts; planar clast fa bric; inverse grading of clasts; and moderate-to-high matrix content ( 5-30%). Many of the cored reservoirs either have been previously inter preted as basin-floor fans or exhibit seismic (e.g., mounded forms) an d wireline-log signatures (e.g., blocky motif) and stratal relationshi ps (e.g., downlap onto sequence boundary) indicating basin-floor fans within a sequence stratigraphic framework. This model predicts that ba sin-floor fans are predominantly composed of sand-rich turbidites with laterally extensive, sheetlike geometries. However, calibration of se dimentary facies in our long (400-700 ft) cores with seismic and wirel ine-log signatures through several of these basin-floor fans (includin g the Gryphon-Forth, Frigg, and Faeroe areas) shows that these feature s are actually composed almost exclusively of mass-transport deposits consisting mainly of slumps and debris flows. Distinguishing deposits of mass-transport processes, such as debris flows, from those of turbi dity currents has important implications for predicting reservoir geom etry. Debris flows, which have plastic flow theology, can form discont inuous, disconnected sand bodies that are harder to delineate and less economical to develop than deposits of fluidal turbidity currents, wh ich potentially produce more laterally continuous, interconnected sand bodies. Our core studies thus underscore the complexities of deep-wat er depositional systems and indicate that model-driven interpretation of remotely sensed data (i.e., seismic and wireline logs) to predict s pecific sedimentary facies and depositional features should proceed wi th caution. Process sedimentological interpretation of conventional co re is commonly critical for determining the true origin and distributi on of reservoir sands.