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Reports: DNI854492-DNI8: Morphologic Prediction of Reservoir Quality in Deltaic Stratigraphy

Brandon J. McElroy, PhD, University of Wyoming

This project is focused on testing the hypothesis that longitudinal profiles of delta fronts are a function of the distribution of particle excursion lengths over delta brinks in the absence of other types of sediment transport in a receiving basin. Particle excursion lengths are determined by their advection velocities in the transport direction and their settling velocity. If excursion lengths are long enough, particles will travel in suspension beyond the topset, and be deposited on the delta foreset or bottom set, building a low-slope delta front or ultimately making an onlapping stratigraphic package.  If excursion lengths are very short, then gilbert-style deltas will form with angle of repose foreset slopes because grains will aggrade on the upper portions of the foreset until failure occurs. A series of experiments were conducted wherein 2-D delta profiles were constructed under varying depth and discharge conditions, using two separate sediment grain types – crushed corn cob and plastic.  Our goal was to predict the longitudinal profile of these deltas as a function of the sediment characteristics and transport conditions.  The process began with the selection of a suite of water discharge values, with multiple respective flow height values.  These values were designed to allow for sediment transport as bed load and as suspended material.  We also varied sediment type in order to achieve independent variability of the sediment settling velocity.  In all, eleven experiments were run with the same initial conditions.  Discharge varied between 35 and 120 cm3/s, and water height at the delta brink varied between 0.8 and 5 cm. Velocities ranged from 17.5 to 80 cm/s.  We used plastic sediment (1.5 g/cm3) for four experiments, and less-dense corn cob (1.05 g/cm3) for six experiments. The final experiment used a mix of the two sediment types. Both the plastic and corn have a similar median grain size (600 μm for plastic, 680 μm for corn).  Parameters for individual experiments are given in table 1.

#

Discharge [cm2/s]

Depth @ brink [cm]

Sediment

1

90

4.5

plastic

2

120

3.5

plastic

3

120

1.5

plastic

4

60

2

corn cob

5

60

3

corn cob

6

90

2

corn cob

7

35

0.8

corn cob

8

35

2

corn cob

9

60

1.5

corn cob

10

120

5

plastic

11

90

3

mix

At the start of each experimental run, a master time was started and was then used to record the start time for all cameras and sediment input.  The sediment was fed in roughly 45 centimeters up stream of the original delta brink point. The timer and cameras ran until the system reached a steady state, where the delta simply prograded at a constant rate through the flume. While we have not fully analyzed the experimental results, no deltas were created with less than angle of repose foreset slopes.  Instead, it was observed that each delta typically self-organized as it prograded such that the flow depth above the topset was either shallower or deeper than that which was set.  This might be related to some optimal transport condition.  It was clear from experimental observations that there was some suspension of sediment along the topset, as well as some suspended sediment which was transported beyond the topset-foreset break. However it was observed that in most instances sediment transport occurred proportionally slightly more as bedload. Our focus now is on capturing detailed description of individual particle motion and connecting that back to flow conditions in order to better understand the role of motions of individual particles and their overall distributions as related to topographic forcing of hydraulics.

Initial deposition at start of experiements.

Fully developed Gilbert Delta aggrading above original topset elevation.