Lithofacies and permeability architecture of recent channel-belt and floodplain deposits of the lower Niobrara River, northeastern Nebraska provide a basis for evaluating heterogoneity in analogous fluvial sandstone reservoirs. The Niobrara River is a sand-bed braided river consisting of a sand-rich channel-belt, with vegetated islands, and floodplains adjacent to the channel. The channel-belt and floodplains occupy a 1.5 km-wide valley at the study location. Aggradation has occurred within the lower 14 km of the river since the mid 1950s due to a baselevel rise at the confluence of the Niobrara and Missouri Rivers.

Information from aerial photographs of surface environments and vibracores, ground-penetrating radar (GPR) profiles, and hydraulic conductivity measurements from the subsurface identify the distribution of “reservoir quality” facies in this braided river system. Heterogeneity within the valley fill exists between the channel and floodplain deposits. Juxtaposition of those deposits results from migration of the active channel-belt, crevassing, and channel avulsion. Differences in hydraulic conductivity between the channel and floodplain deposits range over several orders of magnitude (10^-1 to <10^-4 cm/sec). These differences are sufficient to partition the potential reservoir.

Heterogeneity within the channel-belt occurs at the interface between different elements of the channel fill. The lithofacies architecture of the channel-belt shows a series of amalgamated channel fills and bar complexes. Hydraulic conductivities within the channel-belt sands range over two orders of magnitude (10^-1 to 10^-3 cm/sec). This heterogeneity may create zones of preferential flow rather than a reservoir partition or baffle.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah