ABSTRACT: Ravinement Surfaces and Stratigraphic Architecture in Coastal Plain to Offshore Facies of Transgressive Systems Tracts: Examples from the San Juan Basin (Mesaverde Group, Campanian, USA) and North Sea Basin (Brent Group, Middle Jurassic)

ESCHARD, R., Institut Francais du Petrole, Rueil-Malmaison, France, B. TVEITEN, Saga Petroleum, Sandvika, Norway, F. VAN BUCHEM, Institut Francais du Petrole, Rueil-Malmaison, France, and R. WRIGHT DUNBAR, Rice University, Houston, TX

A transgressive systems tract comprises multiple progradational/aggradational units, each of which records a complete cycle of base-level rise and fall, and which step landward as relative sea level rises. During the transgressive (base-level rise) phase of each progradational unit, facies architecture is controlled by three main processes: (1) continuous creation of continental accommodation space, which permits aggradation of coastal-plain sediments; (2) entrapment of sediments in estuaries and tidal channels; and (3) shoreface retreat, which induces an erosional flooding surface, the ravinement surface. As the shoreline shifts landward, shoreface and coastal-plain sediments are eroded by wave processes down to the rising storm wave base. Eroded sediment is redeposited offshore as " ransgressive sands," and/or backshore as washover fans. During the regressive (base-level fall) phase of the progradational unit, the littoral system progrades over the ravinement surface. The turn-around point between transgressive and regressive phases is marked by the rapid aggradation of shoreface, foreshore, back-barrier, and coastal-plain deposits.

Landward-stepping progradational/aggradational units were studied in excellent, laterally continuous outcrops of the Menefee, Cliff House, and Lewis Shale formations (Mesaverde Group) in the San Juan basin, and recognized in the subsurface in the North Sea basin (Brent Group). In the Mesaverde Group, lateral facies changes from coastal-plain through offshore environments were followed along and between time surfaces over distances of kilometers within progradational units averaging 30 m thick. In the transition between coastal-plain and littoral environments, ravinement surfaces are deeply incised (local relief up to 25 m) within coastal-plain strata. Below the ravinement surface, estuarine sandstone/mudstone complexes interfinger with tidal marsh deposits and are arranged in a geomet ic landward-stepping pattern. Storm-dominated shoreface sandstones prograded over the ravinement surface.

The outcrop model of this type of transgressive systems tract was important in reconstructing reservoir geometries within the Ness and Tarbert Formations in the Brent Group of the North Sea. Reservoir compartmentalization, connectivity of fluid-flow units, and principal permeability barriers within reservoirs are best explained and their geometries predicted through high-resolution correlation between coastal-plain and littoral deposits. To adequately characterize the internal geometries and external limits of these reservoirs, it was essential to recognize the different stratigraphic architectures and facies associations that occur within the transgressive and regressive phases of progradational/aggradational units.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)