Fluvial channel sandstones of Early Pennsylvanian (Morrowan) age comprise regional reservoirs in parts of Colorado, Kansas, Texas, and Oklahoma. In southwestern Kansas, these reservoirs commonly exist at depths of 4000-5000 ft (1200-1500 m) and have reserves of 150,000-200,000 bbl of oil per well, making them highly economical. Reservoir sandstones form part of transgressive valley-fill sequences deposited within channels incised into underlying Mississippian carbonates. Thickness of the fill varies up to 60 ft (18 m), is commonly 10-30 ft (3-9 m), and displays rapid changes along channel length. As a result, detailed mapping of channel trends is difficult. Stewart field, located in Finney County, Kansas, is a good example of this type of reservoir. Maximum reservoir qual ty exists in very fine to fine-grained fluvial sandstones reworked by tidal action. Early attempts to extend the field to the east failed because existing two-dimensional seismic and well data did not help workers properly resolve channel orientation. A three-dimensional (3-D) seismic survey, shot prior to initiation of waterflood operations, helped (1) locate the channel between existing dry holes and (2) identify prospective locations that were then successfully drilled. Further extrapolation of the 3-D data resulted in a dry hole that established the limits of interpretation in this area. Stewart field thus provides an important case study regarding the capabilities and limitations of 3-D data in exploring the interwell frontier.