Cotton Valley Lime (Oolitic Shoal) Play
Geology
Figure 9 shows a good portion of the play, as well as the location for a published core description of the Pioneer Willeford No. 1 well in Upshur County, Texas (Figure 10). This description is typical of the play, with porosity in the 15% neighborhood, and perms equal to or less than 1 md. Notice that Sw increases down section. The main reservoir is in the oolitic lime facies, which can be dolomitized. The sequence may grade up-section into tite lime and anhydrite, and grade down-section into tite biomicrite.
Figure 11 shows the overall Jurassic section and where the Cotton Valley Lime occurs below the Bossier Shale, and above the Smackover Formation (lime and dolomite), or Buckner Formation (anhydrite).
During the Cotton Valley Lime time of deposition, the oolitic bars developed down dip of tidal flats, and up dip of biohermal reefs (Figure 12). These bars followed long belts, or fairways, along the same position of the Cotton Valley Lime shelf, or ramp. A 3D depiction of these shoals is schematically illustrated in Figure 13.
It should be noted that, on occasion, the oolitic shoal is dolomitized and porosity is enhanced. Porosity values above 20% appear on some logs, and these areas tend to have exceptional production – often yielding over 5 BCFGE for these areas. They are the “Sweet Spot” of the bar or field. This type of rock dwindles away on the flanks of the bar, where porosities and perms are lower, and wells gradually die off toward lower values. It is this 1 to 3 BCFGE “rind” that is often left to exploit in these fields.
Fields have also been developed along strike, following bar trends along the shelf edge. Over time, fields have tended to merge together along depositional strike.