Geological Considerations of the Weber Sand Reservoir, Rangely Field, Colorado

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract The heterogeneity of the Weber Sand Reservoir required that a geological evaluation be considered with reservoir engineering to enhance primary and secondary recovery. Stratigraphic: studies delineate a multilayered distribution of discrete sand bodies. Lenticularity, lithology, crossbeds, and fractures cause preferential drainage, prohibiting a favorable balance of production and injection wells in many sands. These geological parameters have been considered in analyzing parameters have been considered in analyzing reservoir performance with subsequent increased production. production. GENERAL GEOLOGY AND RESERVOIR CHARACTERISTICS Rangely Field is located in northwestern Colorado on the north plunge of the Douglas Creek Arch between the Piceance and Uinta Basins (Fig. 1). The principal production is from the Permo-Penn Weber formation. Accumulation is Permo-Penn Weber formation. Accumulation is localized on an anticline of Laramide age. The Weber formation interfingers across the structure into the arkosic Maroon formation to the south. The stratigraphy and depositional history of the Weber is pertinent as it relates to the reservoir of the field. The section here consists of some 600 to 800 ft of interbedded sands, silts, and shales of the Weber formation with tongues of shale, silt, and arkose of the Maroon formation. In general, the Weber is a buff, clean, well-sorted, fine-grained sandstone with variable calcareous cementation. A littoral marine environment apparently predominated in this area during Weber time. This caused sand bodies of various depositional modes to form complex multilayered and laterally coalescing sand units. The texture indicates a reworked mature deposit. Separating or forming laterally equivalent facies to these sand units are gray and red shales, red silts, and coarse maroon arkoses generally considered deposited in a deltaic environment with a source attributed to the Uncompahgre Highlands to the south. Depth to the top of the Weber in the field ranges from about 5,500 to 6,500 ft, and penetration varies from less than 100 to more than penetration varies from less than 100 to more than 700 ft, depending on structural position. Fig. 2 depicts the structure as mapped on top of the Weber formation. An initial gas cap of about 130 ft was present on the crest of the structure. An oil column of about 800 ft was encountered above the generally recognized oil-water contact of -1,150 ft. Porosity, permeability, and saturations vary within the Weber and within the field. In general, porosity in the sand is estimated to average near 15 percent.