Bars to Valleys: The Hierarchy of Incised Valleys and Their Fills from the Grand Rapids Formation, Cretaceous, Alberta, Canada

Howard R. Feldman¹ and James Maynard^2
1 ExxonMobil Exploration Co, Houston, TX
² ExxonMobil Upstream Research, Houston, TX

High resolution seismic from the Iron River field reveals unprecedented detail from fluvial/estuarine incised valley fills in the Cretaceous Grand Rapids Fm. of Alberta, Canada. The rich data set includes over 60 wells, 850 m of core and 40 km² of high resolution seismic. At least 20 valleys were revealed in seismic of the 60-m-thick Grand Rapids Fm. For the first time we are able to define the true 3D geometry and connectivity of both reservoir and non-reservoir elements at the scale of bars to channel fills to valley fills.

The valleys are incised into shoreface parasequences and range up to 50 m deep. Valley fills range from complex with several generations of valley fills, to simple with only a single channel fill. Three types of channel fills were recognized: A) meandering channels with point bars, B) low sinuosity channels with common thalweg sand bars and overlying muddy lateral accretion, and C) small channels filled mostly with mud.

Importantly there is no nesting of valleys: 4th order valleys are not amalgamated within 3rd order incisions. There is no apparent relationship between depth of incision and stratigraphic position: third order sequence boundaries are composite surfaces made by many generations of criss-crossing valleys.

The lack of dendritic drainage networks and common changes in thalweg depth over short distances suggest that these valleys may not have formed by headward incision. The incision patterns are more consistent fluvial entrenchment and common avulsion to form new valleys (not avulsions within valleys).

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005