Modeling Crevasse Splay Versus Point-Bar Bodies: Relative Roles in Characterizing Tight-Gas Fluvial Reservoir Successions

Anderson, Donna S.¹, Mary M. Carr¹ (1) 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 within channelbelts. A 200-ft thick vertical succession of strata within a 160-acre outcrop area within the nonmarine part of the Upper Cretaceous, lower Iles Formation (Neslen Formation of Utah), north of Rangely, Colorado, shows the dramatic differences between crevasse-splay and point-bar bodies. The outcrop contains an upward change from isolated, stacked crevasse channel/splay sandstone bodies to those of point bars within a 1500-ft wide, sinuous meanderbelt. 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-to-gross 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 for the splay and point bar deposits due to dissimilar, yet related, depositional processes. In addition, individual point bars are in poor lateral communication within the meanderbelt, whereas individual crevasse-splay bodies are laterally widespread and laterally contiguous with other crevasse-splay bodies.

3-D modeling clarifies body types and dimensions, vertical and lateral geometric relationships, and yields insights regarding the interplay of sedimentologic processes within different architectural elements. Modeling combined with outcrop observations yield clues as to why crevasse splay bodies are under-recognized from limited subsurface data. For example, crevasse channels can be confused with multistory channels when observed in an isolated outcrop or other laterally confined dataset. In addition, a high proportion of mudclast lags at the bases of proximal crevasse-splay channels could be misinterpreted in 1-D data as reflecting “braided stream” conditions.