MODELING HIERARCHICAL SEDIMENTARY ARCHITECTURE OF BRAID-BELT DEPOSITS USING GEOMETRIC SIMULATION

We are developing a digital representation of the hierarchical sedimentary architecture of fluvial braid-belt deposits, which are important components in some aquifers and petroleum reservoirs. The digital model is intended to have realistic architecture from the scale of centimeters to the scale of kilometers. Once completed, the model should be a useful tool in interpretive simulations to study how hierarchical sedimentary architecture affects flow and transport within reservoirs. The model is based on the architecture of strata quantified at a field site of an active modern river. The stratal architecture is organized into four hierarchical levels. Hierarchical level IV, the highest of these levels at the largest scale, comprises compound bars and major channel fills that are 100s to 1000s of meter in length and width and up to 10 meters thick. Hierarchical level III comprises units that define the internal structure of level IV units. For example, compound bars have internal structure created by the accumulation of unit bars and cross-bar channel fills. These units are hundreds of meters in length and width and up to 2 meters thick. Hierarchical level II comprises units that define the internal structure of level III units. These unit types are defined by their internal sedimentary structure and include small-scale trough sets, medium-scale trough sets, and planar strata. Hierarchical level I comprises units contained within level II units. These unit types are defined by their texture and include sand, sandy gravel, and open-framework gravel. Units of levels I and II are decimeters to meters in length and width and decimeters thick.

We are creating the digital model using geometric simulation. This involves representing geometric shapes of unit types at each hierarchical level with piecewise planar elements, each defined by a set of length and angle parameters. Values for these parameters are then drawn from appropriate statistical distributions quantified from field data. The units are simulated in descending order through the hierarchical levels, combining sets of level N-1 units to fill space within each level N unit. Where two or more units at the same hierarchical level overlap, rules are applied to determine which of the units to assign to each grid location. These rules follow from the sedimentary processes by which the units build.