BARCLAY, JAMES E., Geological Survey of Canada, Calgary, Alberta, also Department of Geology and Geophysics, University of Calgary; and FEDERICO F. KRAUSE, Department of Geology and Geophysics, University of Calgary

ABSTRACT: Facies Models for Incised Valleys and Valley-fills

Valley-fill sandstones are prolific petroleum reservoirs in many basins and this study provides descriptive facies models forsuch deposits. Facies sequences and relative sea-level curves were compiled for 101 ancient and modern valley-fills to construct the models.

The valley-fills rest unconformably and transgressively on an incised surface with linear to sinuous valleys between exposed, soil-imprinted interfluves barren of sediment or with lags ofmarine-reworked fluvial sediments. Valleys are usually 1-5 km wide, 1-5 kms to 10[2] kms long and tens of m deep. Typical transgressive valley-fill sequences comprise fluvial sediments overlain successively by estuarine and brackish deposits, ravinement surface, shoreface and finally offshore deposits. Some fills are regressive or have a regressive cap. "Regional" sediments surrounding the fills often consist of regressive offshore-shoreline sequences.

A valley-fill facies model incorporates: scoured valleys and incised drainages, shoreline-perpendicular valleys, valleys truncating regional beds, subaerial interfluves, valley walls unrelated to valley-fills, and transgressive fluvial-estuarine-marine valley-fills encased in regional regressive beds. Transgressive sequences and fill discordance with encasing deposits distinguish valley-fills from generally regressive deposits such as estuary, tidal, distributary and fluvial channel-fills with laterally-equivalent channel-walls, relict and other deposits on non-incised unconformities, cyclothems, transgressive shoreface sands on ravinements, and offshore sands encased by marine shales.

Sequence stratigraphic principles imply the importance ofincision events and valley-fills to petroleum trapping and to unravelling stratigraphic relationships. The principles predict valley incision on marine shelves during lowstand and transgressive valley-filling during subsequent relative sea level rise. The fills correspond to sequence stratigraphic implications and the incision events correlate with interpreted global lowstands.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.