--> Abstract: Rethinking the Classic Oxbow Filling Model: Some Hope for Improved Reservoir Connectivity, by John Holbrook and Neal Alexandrowicz; #90124 (2011)
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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Rethinking the Classic Oxbow Filling Model: Some Hope for Improved Reservoir Connectivity

John Holbrook1; Previous HitNealTop Alexandrowicz1

(1) Earth and Environmental Sciences, University of Texas at Arlington, Arlington, TX.

The idea of the oxbow lake originating from meander neck cutoff and subsequently filling with clay is a well-established mainstay of fluvial sedimentary models. Though not necessarily intended, this model has lead to a widely held assumption that preserved oxbow channel fills are typified by impermeable mud from base to top, which can strongly partition sandy point-bars and otherwise compartmentalize channel-belt reservoirs. These models originate from modern analogs that largely birthed in the Lower Mississippi Valley.

We have reexamined this model over a 100 km reach of the Mississippi Holocene meander belt using surficial mapping and borehole data. Over 500 boreholes penetrate the approximately 20 oxbow fills in this reach. These data are then compared to a late 1800’s survey of channel depth which is here used as a standard of typical pre-modification channel morphology. Averaging of data reveals that the bankfull thalweg depth of the Mississippi River from the 1800’s survey is 78 ft. Borehole data yield an average channel-belt thickness of 85 ft. Channel fills average in ascending order 29 ft of sand, 34 ft of heterolithic sand and mud, and 22 ft of mud-dominant strata. This argues for the following sand-rich fill process. Approximately 10% of the channel depth is preserved as bed-load sand prior to cut off. Sand then fills approximately one quarter of the open channel to a level approximately at the inflection of the curve defining area vs. depth (app. 55 ft). This is because the deeper part of the channel defines minimal channel volume and is easily filled during the decades long period of channel cutoff lasting up until the end is fully plugged. Above this, channel area expands greatly and filling moves to a heterolithic splay-fill mode that persists until the fill/channel slope is too low to support bedload transport into the channel through tie channels penetrating the end plugs. Filling then moves to the more classic muddy oxbow filling. This occurs at approximately the low-water stage of river flow (app. 30 ft below bankfull).

Though mud is presumed to fill the entire open channel during neck cutoff, the actual mud fill is only about the upper channel third. The substantial sand content typical of the lower part of the channel fill means that petroleum has a high probability of passing beneath muddy parts of the fill through the more permeable basal sands in channel-belt reservoirs.