3-D Architecture of Crevasse Splay and Point-bar Bodies: From Outcrop to Geologic Model
Anderson, Donna S. and Mary M. Carr
Colorado School of
Mines, Golden, CO
Crevasse splay bodies are likely effective reservoirs in many
thick tight-gas fluvial successions, yet they are incompletely
recognized and lack the same level of dimensional data as point bars
and channelbelts. North of Rangely Colorado, a 160-acre 3-D outcrop
area exposes 200 feet of the nonmarine part of the lower Iles
Formation (Neslen Formation of the Uinta basin, Utah). The outcrop
contains an upward stratigraphic change from isolated, stacked
crevasse channel/splay sandstone bodies to those of isolated, paired
point bars within a laterally migrating 1500-ft wide, sinuous
meanderbelt.
A largely deterministic 3-D model of the outcrop shows critical
differences in shape, facies architecture, areal distribution, and rock
volume between crevasse splays and point bars. It also shows a lack
of vertical and lateral connectivity among four crevasse-splay and
four point-bar bodies. All sandstone bodies show similar “average”
statistics: they contain the same types and gross proportions of grain
sizes and facies classified by sedimentary structures, with high net-togross
sandstone (over 90%), similar gross rock volumes (1000 to
1500 acre-ft), and average thicknesses (about 18 ft). By contrast, the
map-view dimensions and geometries and the internal facies
architecture and proportions are completely different due to
dissimilar, yet linked, depositional processes. In addition, individual
point bars are in poor lateral communication within the meanderbelt,
whereas individual crevasse-splay bodies are laterally widespread and
internally more contiguous. The geometry and connectivity of these
bodies has critical implications when considering whether well downspacing
is tapping new reserves or simply draining existing reserves
faster.
AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah