DOWNSTREAM GRAIN-SIZE CHANGE IN A MODERN SAND-BED RIVER AND IMPLICATIONS FOR REGIONAL QUALITY OF RESERVOIRS AND AQUIFERS: CANADIAN RIVER DRAINAGE, OKLAHOMA, USA

Recent developments in reservoir-quality technology suggest that improved prediction of reservoir quality requires better pre-drill estimates of sandstone texture and composition. In response to this need, we are integrating some GIS-based topographic analysis and mapping tools with conventional techniques for the characterization and analysis of sediment in modern sand-bed rivers. The intent of this work is to identify and document the controls on changes in bed material with distance along the drainage system. The study design enables us to evaluate changes in sediment as functions of a) transport distance, b) bedrock lithology, and c) changes in near-channel slope. In contrast to gravel-bed rivers, few studies of this nature have been conducted in sand-bed rivers even though sand-bed rivers constitute the bulk of hydrocarbon production from fluvial systems. We are particularly focused on reasons for changes in particle size because of the strong control that grain size exerts on the permeability of granular aggregates.

Our results indicate that fining does occur in the Canadian River (1,027km along-channel-length). However, the fining is best recognized in sediments obtained from positions high on the sand bars. These samples correspond to high-discharge events when most particle-size attrition occurs. Samples obtained at lower positions on the bars also show a downstream decrease in grain size, though this along-river change is less pronounced. Samples from the main channel (mean low-flow channel) show no change. Other findings from this work with implications for permeability of sandstones: 1) grain size appears to vary systematically with bedrock lithology. This relationship suggests that changes in bedrock character beneath an incisement can influence the texture of the overlying valley fill; 2) mean minimum particle size in the downstream portion of the riverbed is about 0.1mm. This mean grain size is the same documented to occur in the lower reaches of the Mississippi and Amazon drainage systems. Similarity in grain size for all three drainages suggests the occurrence of a physical lower limit to particle size in active channelized systems; and 3) similar to studies conducted in gravel-bed rivers, we document a very rapid transition from sand to gravel in the up-dip reach of the study area.