Application of horizontal wells has increased tremendously and had significant and positive results in the: U.S. over the past decade. The pace of horizontal drilling for oil has remained high since 1990 (approximately 600 to 1,000 wells/yr). Most of these wells have been drilled in three formations: the Austin chalk in Texas (79%), the Bakken shale in North Dakota (5%), and the Niobrara in Colorado and Wyoming (2%). Wells drilled in other formations constitute approximately 14%. The Dept. of Energy Natl. Petroleum Technology Office (DOE/NPTO), working closely with industry developed a unique analytical system (including models and database) to assess the potential for wider application of this technology to the balance of remaining U.S. oil resources across a variety of formations. The findings of the DOE effort are the primary scope of this paper.