Reservoir Heterogeneity Resulting From Bottomset Deposition in the Lee of Dunes and Unit Bars

Abstract

Dune and unit bar migration often forms sets of high angle cross-strata (foresets) and may also form bottomsets and topsets. Of these three components, bottomsets have the highest preservation potential but are less well documented and relatively poorly understood. Bottomsets vary greatly in thickness and internal structure, and their petrophysical properties may be greatly different to the foresets. Bottomsets are often finer grained with very different fabrics compared to the associated foresets. Consequently in sandstones, bottomsets can act as small local or laterally extensive, partial barriers to flow and lead to reservoir heterogeneity and anisotropy. This presentation presents some flume and rock record examples from research aiming to improve understanding of bottomset formation, structure and characteristics. A large number of controls influence bottomset development including host bedform character (size, morphology, height relative to flow depth), sediment character (grain size, shape, density, sorting) and flow conditions during deposition (depth, velocity, turbulence intensity, steadiness and uniformity). The characteristics of bottomsets vary more than foresets. They may be planar, ripple cross-laminated or massive. For example, under relatively low flow velocities with high sediment flux, bottomsets can be dominated by co-sets of thin fine-grain, ripple cross-lamination formed by back-flow ripples in the flow separation zone (in the bedform lee) and co-flow ripples further downstream. Because bottomsets vary with discharge and sediment characteristics, their importance within fluvial reservoirs depends on the palaeoenvironment. In sandstones deposited in rivers with strongly unsteady flows, bottomset persistence, internal structure and thickness will vary over meters to hundreds of meters. The lateral extent and variation in character of foresets and bottomsets are generally greater in unit bar deposits than dunes because they migrate over a wider range of flow conditions, persist over multiple discharge events and are more laterally extensive.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015