Abstract: A Genetic Stratigraphy for the Valhall Formation (Cretaceous), Outer Moray Firth, UK Sector, North Sea

Steven J. Johansen

The VaIhall Formation in the Outer Moray Firth consists of interbedded marls, shales, calcareous sandstones (reservoir seals and flow barriers), and porous siliciclastic sandstones (reservoir units). All were deposited by pelagic marine and turbidite processes. It is divisible into five genetic sequences, with the base of each sequence defined by an apparent transgressive event.

A generalized genetic sequence consists of: (1) transgressive units of organic-rich shales, 5 to 15 feet thick, overlain by pelagic limestones, 5 to 30 feet thick; (2) relative highstand deposits(?), 10 to 200 feet thick, of pelagic carbonate marls and carbonate-rich turbidites; and (3) relative lowstand deposits(?), 10 to 300 feet thick, of carbonate-poor shales, sandstones, and debris flows. Reservoir sandstones are present in both the inferred highstand and lowstand portions of the sequence. Smaller subdivisions (parasequences) are also recognizable and define individual flow units in producing fields. Reservoirs are within Mutti's type II to I turbidite systems, and nonreservoir units are Mutti's type III systems, debris flows, pelagic marls, and carbonate-rich turbidites shed fro local carbonate platforms.

The Valhall is a retrogradational unit, characterized by a decreasing siliciclastic sediment supply and increasing pelagic carbonate sedimentation. The shelf-slope margins were not defined by progradational or aggradational sedimentation equilibria, but were instead controlled by faulting. Sandstone influx during both inferred highstands and lowstands either reflects tectonic controls on sand supply or else reflects the ability of submarine canyons to supply sand to the deep basin during transgressions.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994