Figure 9. Distribution and thickness of net lower Purdy (upper Morrow) sandstone, NE Rice Field, Texas County, Oklahoma (Harrison, 1990). Areas of sand accumulation were isolated by mud and silt deposition in the incised valley. These shale plugs separate fields and reservoirs. Shale valley walls further confine these sandstones. This results in total encasement of valley fill sandstone in clay-rich, sealing rock and creation of an ideal disposal zone. Cross-section A-A’ is shown by Figure 10.