This isopach map shows the thickness of the interval from the top of the Cotton Valley Group to the top of the Smackover Formation. It was necessary to contour this expanded interval, instead of just the upper part of the Cotton Valley Group, because of the limited availability of data. Ideally, just the part of the Cotton Valley Group above the Bossier Shale would have been contoured, but there are a limited number of Bossier picks in the database, and many of the Bossier picks are not at a consistent stratigraphic break (J.L. Ridgley, oral commun., 2002). Data for units below the Bossier, such as the Haynesville or Buckner Formations, are also limited on a regional basis. The Smackover Formation is the first unit below the top of the Cotton Valley that has abundant data available on a regional level. The data are provided as both lines and polygons, and the proprietary wells that penetrate this interval are graphically displayed as quarter-mile cells. The well information was initially retrieved from the IHS Energy Group, PI/Dwights PLUS Well Data on CD-ROM, which is a proprietary, commercial database containing information for most oil and gas wells in the U.S. Cells were developed as a graphic solution to overcome the problem of displaying proprietary PI/Dwights PLUS Well Data. No proprietary data are displayed or included in the cell maps. The data from PI/Dwights PLUS Well Data are current as of April 2001.