Predicting Variability Of Basal Scour And Quantifying Vertical Connectivity For Channel-belt Reservoirs Using Observations From The Holocene Mississippi River And Channel-belt
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Assessment of potential for draining unpenetrated but closely stacked channel-belt reservoirs from an existing belt penetration depends upon an understanding of the methods of and the likelihood for connectivity between otherwise isolated reservoirs. Knowledge of both the variability in scour depth beneath a channel-belt and the distribution of channel-belt lithofacies is critical for making such assessments, yet is very poorly understood. In order to evaluate these two properties, a collection of 314 driller’s records, 245 cross section points, and over 300 hand-augered boreholes along a 75 mile stretch of an active Mississippi River meander belt were reviewed, along with a detailed historical record of active river morphology recorded by the Mississippi River Commission. Three lithofacies units were identified within the channel-belt, the depths of were recorded so that the thickness, volume, and three-dimensional distribution of lithofacies could be examined. Two trends are evident: 1) The variability of the Mississippi River channel-belt basal scour surface mimics the variability of the active Mississippi River bankfull-thalweg depth, and 2) Whereas the Mississippi River is the classic example of a mud dominated fluvial system with abundant abandoned meanders, much of the lower abandoned channel fill is not composed of passive-fill clays, but rather a grittier mix of active-fill mud and sand of variable permeability. Sand was also found to be present nearly everywhere above the basal scour surface, providing at least a degree of connectivity between sand dominated bar deposits.A key result of connectivity modeling in this study is the development of areal connectivity curves between an overlying belt and the various lithofacies components of an underlying belt based on the vertical separation between the two. Results obtained in this study suggest that sand-to-sand connectivity begins to diminish almost immediately with channel-belt separation. At vertically separation equal to 50% of the average channel-belt thickness, the upper channel-belt should be able to connect through the scour surface and into the sand lithofacies of the lower channel-belt below across 73% of the overlapping area. As the vertical separation approaches the equivalent of the average channel-belt thickness, the likelihood of connectivity between potential reservoirs decreases to less than 6%. However, sand-to-sand connectivity persists through vertical separation of approximately 200% of the average channel-belt thickness due to deep scour. Once connected by the scour surface, communication between stacked channel-belts depends on the reservoir quality. Our study shows that, even in muddy systems, the material into which a fluvial surface scours is likely to have sufficient quality to permit reservoir communication over various degrees of vertical channel-belt separation.