Conceptual model for predicting mudstone dimensions in sandy braided-river reservoirs

Abstract

Sandy braided-river deposits with high net-to-gross sand ratios are commonly attractive reservoirs, yet internal lithologic heterogeneities, particularly the presence of low-permeability mudstone deposits, significantly complicate the development of such units. Previous work has focused on measuring the scale and distribution of mudstone deposits in outcrop analogs; however, because of extreme differences in scale, discharge, sediment load, and geologic history, the results of these studies are difficult to apply with confidence to a wide range of sandy braided-river reservoirs. Based on work in modern braided rivers (Niobrara and North Loup rivers, Nebraska) and ancient braided-river deposits (Kayenta Formation, Jurassic and lower Castlegate Sandstone, Cretaceous, Colorado and Utah), we propose a process-based conceptual model for understanding and predicting the distribution and geometries of fine-grained (mudstone) intervals in sandy braided-river deposits. This model is an idealized channel-fill unit composed of five fine-grained lithofacies (mud plugs, channel-lining muds, interbar muds, inclined heterolithic strata, and flood-plain and overbank material) that scale proportional to channel-thread dimensions, including depth, cross-stream width, and downstream length. Each lithofacies is found in a different region in an individual channel fill, and lithofacies found low in a fill may be preferentially preserved. Within braided-river deposits, extrinsic depositional factors, such as aggradation rate, available accommodation, and avulsion-return time, produce different channel-fill stacking arrangements, preserving fine-grained lithofacies in different, relative proportions. This conceptual model provides an approach to reservoir characterization that deductively constrains the dimensions and distribution of fine-grained barriers to flow and may help account for the inherent variability in sandy braided-river deposits.