Geometry of Heterogeneities within Fluvial Point Bar Deposits

Brian J. Willis¹ and Christopher D. White^2
1 Texas A&M University, College Station, TX
² Louisiana State University, Baton Rouge, LA

Flow and sediment transport is modeled for multiple channel bends in a three-dimensional gridded format to examine the geometry of heterogeneities within meandering fluvial channel deposits. In the models, the coarsest grained sediments are deposited near a channel bend apex and finer grained sediments accumulate higher on the down stream end of a channel bar. The coarsest deposits occur in elongate bodies parallel to the channel belt axis when channel bends migrate mostly by down valley translation and are more perpendicular to this axis where bends increase in sinuosity during migration. Style of channel bend cutoff and channel belt aggradation rate also controls whether coarser grained bodies in adjacent bars deposits are connected along the channel belt. Shale drapes deposited on beds during low flows are preserved more commonly on upper parts of bars during rising flow stages and on downstream ends of bar deposits as channels migrate. They are thus predicted to occur on down valley dipping surfaces within the finer grained sediments near the top of a bar deposit. Three-dimensional gridded models have significant advantages over past models that compared two-dimensional cross-sections through simulated deposits to outcrop exposures, as the results can be used in reservoir simulation studies to predict affects of deposit heterogeneity on patterns of subsurface flow through reservoirs and aquifers.

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