Geological and Engineering Characterizations of the Upper Red River Formation in the Southwest Williston Basin

Abstract

Abstract The upper Red River Formation in Bowman County, ND and Harding County, SD consists of four porosity benches. The porosity in each bench is produced by selective dolomitization of depositional mud. Two of these porosity benches can be prolific oil reservoirs, while the other two benches are generally non-reservoir intervals. Different depositional settings and subsequent dolomitization in each of the porosity intervals explain the differences of their respective reservoir and productive characteristics. This paper describes depositional setting, petrographical observations from cores and engineering characterizations for each of the four porosity benches in the Red River, off-structure from the Cedar Creek anticline. The petrographical and engineering characterizations provide a side-by-side analysis and summary of core, electrical-log, drillstem-test and production statistics which can be used for development and exploration evaluations by analogy and computer simulation. Introduction The Red River Formation contains significant oil reserves in the Williston Basin and there are many excellent discussions of depositional and diagenetic controls of Red River rocks in the literature. This paper draws from much of that previous work. The purpose of this paper is to combine functionality with form for upper Red River zones in the southwestern portion of the Williston Basin. An attempt is made to be statistically rigorous and meaningful as data will allow. Where discussions concern permeability, flow rate and reserves, average or mean value is based on log-normal (geometric) distributions. Normal distributions for mean and standard deviation have been applied to values for thickness and porosity. The Ordovician Red River fields studied for this project lie in the southern Williston Basin on the northeastern flank of the southern end of the Cedar Creek Anticline. The study area includes portions of Bowman Co., ND and Harding Co., SD (Fig. 1). The area has gentle northeasterly regional dip of less than 10 at Red River depth. Oil entrapment occurs by up-dip porosity pinch-out, low-relief structural closures and low-displacement faulting. Reservoir rocks in all oil accumulations are dolomitized carbonate mudstones and wackestones which were deposited in open to restricted-shelf environments. In the Bowman-Harding area, Red River production is characterized mostly by accumulations of small areal extent at depths from 8500 to 9500 ft. Well spacing for initial exploration has been 320 acre and some areas, which have been unitized for enhanced recovery, are developed with 160-acre patterns. Horizontal wells have been spaced at two wells per 640-acre section. Reservoir studies indicate primary recoveries averaging 15 percent with a range of 6 to 28 percent. Depositional environments in the Red River Formation were constant across the study area and much of the Williston Basin which resulted in laterally continuous units with uniform sedimentation. Lateral changes in depositional environments cannot be used for explaining variable porosity within a given genetic unit. Subsequent diagenetic-dissolution processes and faulting are the source of most heterogeneity found in porosity units of the upper Red River. Reservoir Intervals The base of the Ordovician Red River is a gradational change to Winnipeg Shale while the top is overlain by the Stony Mountain Shale (Fig. 2). The total Red River Formation thickness in the study area is slightly greater than 500 ft between the Winnipeg and Stony Mountain shales but oil production occurs only in the upper 250 ft of the Red River interval. The Red River is informally divided here into upper and lower units based on the occurrence of hydrocarbon production and follows terminology used by Carroll. The local stratigraphic terminology used has undergone an evolutionary process and can present communication problems and mis-correlation. An attempt has been made to use common stratigraphic terminology employed by operators in the Bowman-Harding area. P. 217^