Recent application of horizontal drilling technology to the Ordovician Red River "B" zone in the southern Williston basin has resulted in a successful oil play, with more than 100 wells drilled in 1995 and 1996. The Red River "B" reservoir is a dolomitized laminated carbonate with microsucrosic porosity of 8-25% and permeabilities in the range of 1-66 md. It occurs within the middle of three depositional cycles ("A," "B," and "C") that form the upper Red River Formation. Each cycle consists of a lower burrowed limestone, middle laminated member, and capping anhydrite or lime mudstone. The "B" reservoir is confined to the "B" laminated member and consists of an upper portion, characterized by better reservoir quality, and a lower, less permeable portion. Horizontal drilling has the advantage of significantly increasing well-bore exposure to the upper, more permeable portion. Well data indicate the total Red River "B" porosity zone has remarkable extent over parts of southwestern North Dakota, southeastern Montana, and northwestern South Dakota. Productivity from horizontal wells displays considerable variation that can be correlated with structure/tectonic patterns and with reservoir petrophysical character.