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