Ordovician Red River cores were examined from 16 wells across western North Dakota and eastern Montana plus core from 8 wells in Brush Lake field area, Sheridan County, Montana. Several distinctive basinwide carbonate lithofacies record at least three cycles of upward shoaling or restricted conditions. The oldest and best developed of these cycles consists of a fossiliferous burrowed carbonate mudstone member overlain by a non-fossiliferous, finely laminated micro- or crypto-crystalline dolomite member. The cycle is completed by an anhydrite of regional extent. The thickest development of porous dolomite is in the lowest cycle of the Red River. This cycle contains most of the Red River oil reserves of the study area.

Porosity distribution is directly related to patterns of dolomitization in the carbonate members of each cycle. In northeast Richland County, Montana, an area of dense well control, net porosity isopach maps of each member of the lowest cycle show alternating bands of porous and nonporous carbonates. These bands are oriented northeast-southwest. Bands of good porosity development in the burrowed member occur between bands of good porosity in the overlying laminated member. A classification system based upon these patterns of dolomitization is used to analyze statistically the occurrence of porosity in the lowest cycle.

Regional anhydrites, nonporous carbonate rocks and kerogenites within these cycles have formed adequate seals for the trapping of Red River oil.

Fifty percent of Red River structural growth occurred during upper Interlake time in northeast Montana. Rejuvenation of this growth occurred through Cretaceous time.

Black kerogenous limestones in the burrowed member of the lowest cycle may be the source of Red River oil.

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