Three-Dimensional Connectivity of Point-Bar Deposits

摘要

The geometry of heterogeneities within fluvial channel-belt deposits is predicted using an existing model of flow and sediment transport in river channel-meander bends. The thickest and coarsest-grained sediment accumulations are deposited near a channel-bend apex, and finer-grained sediments accumulate higher on the downstream end of a channel bar. Shale drapes deposited on individual beds during low flows are preserved mostly on downstream-dipping surfaces in finer-grained bar-top deposits. Extensive finer-grained deposits also accumulate in concave-bank areas in the lee of point bars when meanders migrate downstream. The coarsest deposits occur as elongate bodies parallel to the channel-belt axis when channel bends migrate mostly by downstream translation and are more circular when channel bends simply increase in sinuosity. The character of deposits preserved in concave-bank areas of the channel, the style of channel cutoff and filling during abandonment, and the amount of channel-belt aggradation during bar migration influence whether coarser-grained bodies of adjacent bar deposits are well connected. This 3D gridded model allows reservoir simulation studies that predict effects of types of deposit heterogeneity on patterns of subsurface flow through reservoirs and aquifers. Subsurface flow simulations suggest that channel belts with coarser-grained abandoned fills and significant vertical aggradation during bar migration constitute reservoirs with appreciably higher recovery factors than those with finer-grained channel abandoned fills and no vertical aggradation. Model results highlight the need to better understand connections between different types of facies associations in fluvial channel belts to improve predictions of fluvial-reservoir behavior.