Abstract The successful search for new horizontal drilling targets will depend greatly on an understanding of the geologic aspects of HD-reservoirs. An examination of basins in North America reveals many potential horizontal drilling targets. Type A HD-reservoirs are found in Bakken equivalent units in the Anadarko and Appalachian basins. Type B HD-reservoirs, such as the Niobrara chalk in Colorado and Wyoming and the Viola Limestone in the Marietta Basin are currently associated with active horizontal drilling programs. Type C HD-targets similar to the Rospo Mare Lower Cretaceous strata includes the Ellenburger Group in the Permian Basin and Trenton-Black River strata in the Michigan Basin which are currently being exploited using horizontal technology. Results have been mixed primarily because many of these tests have been performed in depleted zones. There is an abundance of reservoirs with Type D HD-reservoir characteristics and the best North American example is the Sadlerochit Sandstone in Prudoe Bay Field. Several organic-rich “black” shales of Late Devonian and Early Mississippian age are present in the basins of the North American craton. Examples of similar age include the Antrim Shale of the Michigan Basin, the New Albany Shale of the Illinois Basin, the lower and upper members of the Bakken Formation of the Williston Basin, the Woodford Shale of the Mid-Continent, the Exshaw Formation of the Alberta Basin, the “Devonian” shales of the Appalachian Basin. Where thermally mature, these black shales are economically important as hydrocarbon source rocks. The Woodford Shale is Upper Devonian to Lower Mississippian in age