This study related the distribution of pore geometry to enhanced recovery within the Upper Ordovician Red River

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Formation, Cabin Creek field, Montana. The field is located on the Cedar Creek anticline in southeast Montana. The Red River Formation is a sequence of alternating limestones and dolostones. Lateral and vertical variations of dolomitization are mostly responsible for reservoir heterogeneity. Production is from the U2, U4, and U6 dolostones, whereas the interstratified U1, U3, and U5 limestone units are nonproductive. Cumulative Ordovician and Silurian production was 61,574,000 bbl of oil as of September 1979, with reserves of 13,426,000 bbl. Waterflood began in 1964, and the field is a good candidate for tertiary recovery by the carbon dixoide miscible process.

Studies of thin sections, mercury capillary pressure curves, and resin pore casts have shown that several different types of pore systems occur, each associated with a particular depositional environment and diagenetic regime. Pore-system geometry is a function of the size and shape of the dolomite crystals composing the reservoir rock matrix. The size, sorting, and shape of pore throats determine the reservoir characteristics of each pore system. In addition to dolomitization, post-depositional leaching of calcite and evaporitc sulfate minerals was important in reservoir porosity development.

Mean pore-throat size, a statistical measure of pore geometry, was found to increase as porosity increased. Using this relation and electric log porosity values, it is possible to predict pore geometry and, as a consequence, recovery efficiency, if lithofacies distribution, porosity type, and diagenetic history are known. Since residual oil saturation is strongly dependent on the pore-system characteristics of the reservoir rock, it is possible to identify parts of the reservoir which have potentially high residual oil saturation, thus establishing targets for enhanced recovery.

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