ABSTRACT

Pennel field has produced over 41 million barrels of oil from Mississippian, Silurian and Ordovician carbonate reservoirs. Approximately 29 million barrels of this production has been obtained from the U2 through Ug porosity zones in the upper 225 feet of the Red River formation, stratigraphically equivalent to the A, B, C, D zones now commonly designated by industry operators in the Central Williston Basin.

In Pennel field, the Red River is about 525 feet thick and is predominantly a limestone-dolomite sequence, with minor but significant anhydrite beds. Core suite studies show the upper Red River to consist of several cycles of marine through supratidal depositional facies. Bedded and nodular anhydrites, culminating the prograding tidal flat sequences of cycles, provide effective cap seals overlying porous dolomite zones. The lateral and vertical variations of dolomitization below the capping evaporites are largely responsible for the variations in reservoir properties and effective pay thicknesses. Porosity and permeability (pore distribution) appearto have been controlled by original rock facies and by early diagenesis.