ABSTRACT: Sequence Stratigraphy, Depositional Facies and Reservoir Continuity of a Storm-Wave-and Tide-Dominated Delta: An Example from the Lower Cretaceous Ben Nevis and Avalon Formations, Jeanne D'Arc Basin, Grand Banks, Canada

Hassan Sabry

The Avalon Formation is recognized as a regressive/transgressive sequence of Barremian to late Aptian age, overlain unconformably (sequence boundary) by the Aptian to Albian Ben Nevis transgressive sequence. Sedimentation took place in a Mesozoic failed-rift basin setting dominated by an inferred delta complex that occupied the southern end of the Jeanne D'Arc basin. Geostrophic events, namely storm waves and tidal currents, dispersed and fractionated sediments northward into mainly thin-bedded clean sands with interbedded argillaceous, silty beds.

The Avalon and Ben Nevis formations contain collectively four reservoir zones separated by two shale markers representing marine-flooding surfaces. Based on detailed sedimentologic investigation of over 6000 ft of Avalon and Ben Nevis cores, the following geologic history of events and depositional model are proposed.

Prior to the Avalon sequence, a lowstand prograding wedge of the Eastern Shoals Formation terminated with lagoonal red mudstones and was locally emergent and rooted.

During deposition of the basal Avalon (late Barremian), a slow transgression, marked by brackish water sediments, started, resulting in the formation of transgressive barrier bars and migrating tidal inlets in the Hibernia area. These transgressive shoreline sands display good reservoir continuity across the basin with large oil recoveries, e.g., Hibernia 0-35 field.

A subsequent slow drop in relative sea level in the central part of the basin resulted in deposition and development of a prograding delta-front platform. Close to the end of Avalon deposition (middle Aptian), an ensuing rapid drop in relative sea level initiated very rapid seaward progradation of the coastal zone and incision of fluvial/estuarine channels into the older now-exposed delta-front platform.

During Ben Nevis deposition (late Aptian to Albian), a major rise in relative sea level resulted in the landward retreat of the shoreline, aggradation of fluvial/estuarine channels, and the reworking of the coastal sands by tides and storms to form over 400 ft of thick shoreface detached bar complex and beach ridges. These host the best reservoirs and have the highest oil recoveries, e.g., Hibernia 0-35 field. These thick transgressive sequences are attributed to a high rate of sediment supply, which kept up with continuous subsidence during transgression.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990