Geological Heterogeneities Important to Future Enhanced Recovery in Carbonate Reservoirs of Upper Ordovician Red River Formation at Cabin Creek Field, Montana

Several different pore systems are present in dolomite reservoir rocks of the Red River formation (Upper Ordovician) at Cabin Creek field, MT. Each system is associated with particular depositional environments and diagenetic regimes. Pore geometry is mostly a function of the size and shape of the dolomite crystals composing the rock matrix. Mean pore-throat size, a statistical measure of pore geometry, increases as porosity percent increases, depending on the type of dolomite. This relationship permits prediction of reservoir pore geometry and a better assessment of recovery efficiency once lithofacies distribution, porosity origin, and diagenetic history have been determined for the reservoir by study of cores and rock thin-sections. Introduction The reservoir characteristics of any rock type depend on the arrangement of the pore space and how the pores are interconnected. The pore-system geometry of a reservoir rock must be understood to determine fully its response to primary or enhanced recovery. To predict pore geometry trends, it is necessary to establish relationships between measures of pore geometry and petrophysical parameters. which are measurable by electric-log surveys of boreholes. This is because cores, which are necessary for pore geometry determination, are usually available for select wells of any given field. Pore geometry is mostly a function of depositional environment and diagenetic processes such as cementation, recrystallization, mineralogical alterations, and selective leaching of rock components. This study presents an approach for determining heterogeneities of carbonate-reservoir pore geometry and for delineating pore geometry throughout the reservoir. The application to future enhanced recovery also is discussed. The formation under study is the Red River formation (Upper Ordovician) of Cabin Creek field, a producing oil field located in southeast Montana (Fig. 1). The Red River formation is a major producing reservoir in the area, and Cabin Creek is a potential candidate for tertiary recovery. Structurally, Cabin Creek is on the Cedar Creek anticline, a long asymmetrical feature on the southwest margin of the Williston basin (Fig. 1). Fig. 2 is a structure contour map of Cabin Creek field. The Red River formation averages about 500 ft (153 m) in thickness and consists of a sequence of alternating limestones and dolostones (Fig. 3). Production is from the U2, U4, and U6 dolostone units in the upper 150 ft (46 m) of the formation. The interstratified U1, U3, and US limestone units are nonproductive and nonporous (Fig. 4). Lateral and vertical variations in degree of dolomitization are mostly responsible for variations of reservoir properties. Commingled Ordovician and Silurian oil production was 61,570,000 bbl as of Sept. 1979, with reserves of 13,425,000 bbl (2 134 405 m ). The field has been on waterflood since April 1964. Approximately 1,450 ft (444 m) of core from 12 different wells was studied to delineate field stratigraphy, distribution of lithofacies, and depositional environments. Core slabs were ground with abrasive grit, then etched in diluted hydrochloric acid to enhance sedimentary structures and aid in identification of carbonate grains and matrix material. Staining methods were used to aid mineralogical identification. Diagenesis, porosity types, and origin of porosity were determined by petrographic analysis of thin-sections. Values of porosity percentage, permeability, and saturations are from core-plug analyses. Size and shape of pore throats were determined from mercury capillary-pressure data and from scanning-electron micrographs of resin pore casts, respectively. Plots were made of porosity percentage vs. parameters of pore geometry for producing zones within the Red River formation. SPEJ P. 429^