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Controls from Channel 
Formation
in Deep Water
Distributive Systems*
By
David Hoyal1, Benjamin A. Sheets1, Christopher M. Edwards1, and Roger B. Bloch1
Search and Discovery Article #50090 (2008)
Posted August 6, 2008
*Adapted from oral presentation at AAPG Annual Convention, Houston, Texas, April 20-23, 2008
1ExxonMobil Upstream Research, Houston, TX ([email protected])
Characterization of deep water distributive systems can
benefit from an understanding of the relationship between sand deposition and
slope in the early stages of distributive channel formation. In the simplest
case sand deposition is associated with a decrease in slope to below grade
conditions, as in the case of subaerial alluvial fans. However, under certain
conditions, deep water progradation appears to be related to a slope increase
suggesting the presence of more complex and subtle mechanisms of dynamic flow
inefficiency associated with higher slopes.
Over 80 deep water experiments were designed to
investigate the influence of slope on post-avulsion, incipient channel
extension. The results of the experiments are unequivocal but counter-intuitive.
Channel extension length is inversely related to slope over a wide range of
slopes (5-17 degrees). Additionally, channel extension is largely independent of
inlet flow density (salt concentration) over the experimental range (10-24
g/cc). Measurements of densimetric Froude number (Fr’) indicate that the flow
evolves to near-critical conditions at the channel-lobe transition. This
suggests that distributary channel length scale is strongly controlled by
hydraulic jump formation, a phenomenon already known to be a strong control on
intrinsic length scales in other morphodynamic systems.
Analogous patterns
of channel evolution are observed in a Quaternary deep water fan in the Gulf of
Mexico. Channels evolve from relatively short, narrow forms at the base of the
fan, to longer and wider forms towards the top. The transition is associated
with a decrease in slope over the fan surface that accompanies net progradation
of the entire system. Collectively, these datasets reveal an intimate link
between slope and deep water progradation that serves as an important tool in
deep water stratigraphic prediction.
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· There are flavors of avulsion cycles (e.g., supercritical, non-cohesive). · Modulation to critical flow is an important control on length scales (e.g., channel lengths) in supercritical distributive systems. · Inverse relationship between channel extension length and slope. o Channel extension is inhibited by hydraulic jumps, o But flow entrainment could also be an important mechanism for generating length scale in DW systems. · Suggests a new DW progradation mechanism – depositional ramp - not discontinuity like clinoform. · Minibasins demonstrate this effect strongly because slope changes are rapid due to confinement (mass conservation) – but should also occur in larger basins and on continental slope.
· Universality – may be similar mechanisms in alluvial fans. Grade is a scale dependent concept – complicated by intrinsic structure and fluid mechanics. |
