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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. |
