Avulsion Stratigraphy in Ancient Alluvial Systems: A Tool for Predicting Fluvial Sand-Body Connectivity

Jones, Heather L. ¹, Elizabeth A. Hajek¹ (1) University of Wyoming, Laramie, WY

Fluvial reservoirs can be difficult plays for a number of reasons including non-uniform sandy-body distribution and subtle lithologic heterogeneities that can limit pressure communication between two neighboring sand bodies, even within a predicted flow unit. Predicting connectivity between channel-scale sand bodies in fluvial reservoirs is critical to maximizing sweep efficiency, but is difficult given the high degree of variability within fluvial systems. Avulsion is one of the main controls on basin-scale fluvial sedimentation and reservoir configuration. Based on recent work in four alluvial basins, we suggest that avulsion style may be a significant control on sand-body connectivity.

Current literature recognizes two avulsion styles: aggradational, characterized by an early phase of floodplain aggradation via crevasse splay deposition prior to the successful completion of avulsion, and incisional, characterized by flood plain scouring early during the avulsion. Both styles have been observed in modern rivers, however there is disagreement over what parameters promote one style of avulsion over another. In outcrop we recognize two stratigraphic expressions of avulsion - foreshadowed and stratigraphically abrupt - which may reflect end-member avulsion styles, and two different connectivity scenarios.

We present field observations categorizing avulsion deposits in four ancient fluvial systems. Our data suggest that in the areas studied a particular avulsion style dominates basin-wide. These preliminary results suggest basin-scale factors may dictate avulsion style rather than local conditions which vary within a basin. These observations may provide a process-based means of predicting the basin-scale distribution of avulsion style, which provides information about sand-body connectivity within a reservoir.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana