SIMULATING HETEROGENEITY IN CHANNEL-BELT DEPOSITS – DEPOSITIONAL MODEL AND ARCHETYPAL UNITS

Many important aquifers and hydrocarbon reservoirs are composed of gravelly sediments deposited in the channel belts of braided rivers. For these aquifers and reservoirs, the true three-dimensional heterogeneity in hydraulic properties (porosity and permeability) is generally unknown. But, as in most sedimentary deposits, heterogeneity is related to the sedimentary architecture, of which important aspects are the proportions, geometry, and spatial relationships of sedimentary units. To represent important aspects of heterogeneity in gravelly channel-belt deposits, we utilized a geometric simulation approach to create a synthetic deposit. The region represented is ~2.5 km in length and width and ~10 m in thickness. The value of this synthetic deposit depends on the fidelity with which it represents the sedimentary architecture of channel-belt deposits. A companion presentation focuses on the geometric simulation method but here we focus on the nature of the sedimentary units it represents.

Recent work on modern rivers and ancient sediments has led to a conceptual depositional model of the channel-belt deposits of braided rivers. Importantly, these deposits can be organized in a hierarchy such that those at one scale comprise mutually exclusive spatial associations of those at the next smaller scale. At the largest scale, channel belts are distinct from floodplains; our focus is on channel-belt deposits. Channel belts are composed of compound bars and channels. Contrary to most representations, the volumetric proportion of compound-bar deposits greatly exceeds that of channel fills. Compound bars are composed of unit bars and are often cut by cross-bar channels. Unit bars are built from the deposits of various bedforms, with those of dunes representing the greatest volumetric proportion.

For the geometric simulation, archetypal shapes of the depositional units at each level of this hierarchy were created based on observations and measurements, particularly those of Lunt et al. (2004). They are combined in ways that fill space and conform to rules that reflect whether their boundaries in nature are erosional or depositional.