Estimation of the dimensions of fluvial geobodies from core data is a
notoriously difficult problem in reservoir modeling. To try and improve
such estimates and, hence, reduce uncertainty in geomodels, data on
dunes, unit bars, cross-bar channels, and compound bars and their
associated deposits are presented herein from the sand-bed braided South
Saskatchewan River, Canada. These data are used to test models that
relate the scale of the formative bed forms to the dimensions of the
preserved deposits and, therefore, provide an insight as to how such
deposits may be preserved over geologic time. The preservation of
bed-form geometry is quantified by comparing the Alluvial architecture
above and below the maximum erosion depth of the modem channel deposits.
This comparison shows that there is no significant difference in the
mean set thickness of dune cross-strata above and below the basal
erosion surface of the contemporary channel, thus suggesting that
dimensional relationships between dune deposits and the formative
bed-form dimensions are likely to be valid from both recent and older
deposits.
The data show that estimates of mean bankfull flow depth derived from
dune, unit bar, and cross-bar channel deposits are all very similar.
Thus, the use of all these metrics together can provide a useful check
that all components and scales of the alluvial architecture have been
identified correctly when building reservoir models. The data also
highlight several practical issues with identifying and applying data
relating to cross-strata. For example, the deposits of unit bars were
found to be severely truncated in length and width, with only
approximately 10% of the mean bar-form length remaining, and thus
making identification in section difficult. For similar reasons, the
deposits of compound bars were found to be especially difficult to
recognize, and hence, estimates of channel depth based on this method
may be problematic. Where only core data are available (i.e., no outcrop
data exist), formative flow depths are suggested to be best
reconstructed using cross-strata formed by dunes. However, theoretical
relationships between the distribution of set thicknesses and formative
dune height are found to result in slight overestimates of the latter
and, hence, mean bankfull flow depths derived from these measurements.
This article illustrates that the preservation of fluvial cross-strata
and, thus, the paleohydraulic inferences that can be drawn from them,
are a function of the ratio of the size and migration rate of bed forms
and the time scale of aggradation and channel migration. These factors
must thus be considered when deciding on appropriate length:thickness
ratios for the purposes of object-based modeling in reservoir
characterization.