Fine Sediment Deposition and Storage in Sandy-Fluvial Systems

Open Access
Author:
Wysocki, Nathaniel P
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 25, 2014
Committee Members:
  • Elizabeth Ann Hajek, Thesis Advisor
Keywords:
  • fluvial sedimentology
  • fine sediment
  • sandy fluvial systems
  • sedimentology
  • surface processes
  • fluvial
Abstract:
Sandy-fluvial deposits are important hydrocarbon and water reservoirs. However, they can be difficult to predict and efficiently produce due to particle-size heterogeneities ranging from pore to basin scale. Fine sediment (silt and clay) is transported as suspended load and is either bypassed through a river as wash load or is deposited and stored in the channel bed or on the floodplain. Clay accumulations in subsurface-channel deposits can cause porosity and permeability reduction at the pore scale and act as baffles and barriers that compartmentalize flow at reservoir scales. Grain-size heterogeneity within fluvial deposits may record paleo-flow variability and/or paleo-sediment supply, but without improved understanding of the conditions under which clay can accumulate in sandy river beds, it is difficult to uniquely interpret these deposits. In order to better understand the processes controlling when and where fine sediment accumulates in sandy river beds, I conducted a series of physical experiments to test if there is a relationship between supplied-clay concentration and the amount of clay preserved in a river bed under constant- and variable-discharge conditions. In three constant-discharge experiments, clay was added in concentrations of 1,000 mg/l, 4,000 mg/l and 8,500 mg/l. An additional experiment with 1000 mg/l clay concentration was conducted with variable water discharge, where discharge was slowed to a stop every hour of run time and clay allowed to settle for one hour three times and once overnight. After each experiment, samples were collected from the dry bed and analyzed for clay content, and clay deposits observable in the bed were mapped in order to determine the nature and distribution of clay accumulations in the bed. Results demonstrate that under constant-discharge conditions, clay can accumulate in the bed and that the amount of clay retained in the bed is related to the amount of clay supplied to the flume. In experiments with a low clay concentrations, little to no clay accumulated in the bed, while under high clay concentration, as much as 5-10% of the bed is comprised of clay rich lenses and 40% of the bed is likely to have clay intercalated in pore space. Additionally, clay accumulations that form under constant-discharge conditions have a unique character and may be differentiable from those formed under variable-discharge flows. Clay accumulations deposited under variable discharge appear as discontinuous clay drapes, while under constant flow conditions they appear as continuous clay drapes, clay rich lenses, and intercalated clay. In all experiments, clay retention was highest in the main aggradational phase of the runs, with markedly less clay preserved during bypass. These results suggest the nature of clay accumulations in sandy channel deposits may be useful for reconstructing paleo-discharge conditions, and that subsurface reservoir quality may be predictable if these conditions can be constrained.