Abstract: Heterogeneity in Deposits of an Aggrading, Sandy Braided River: Implications for Fluvial Sandstone Reservoirs
SKELLY, RAYMOND L. and FRANK G. ETHRIDGE, Department of Earth Resources, Colorado State University.
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