Reserve Analysis and Economic Evaluation of Wasatch/Mesaverde Reservoirs, Greater Natural Buttes Producing Area, Southeastern Uinta Basin, Utah

Abstract

Abstract The Greater Natural Buttes (GNB) is located in the southeastern Uinta Basin of northeastern Utah. Cumulative production of the nonassociated gas of the producing area is 521 MMCF(1) and has been described as a continuous-type(2,3) or basin-centered gas accumulation. The GNB includes the Chapita Wells, Ouray, Natural Buttes, Island, and River Bend Federal Exploratory Units (Figure 1). The GNB field presently includes more than 880 producing wells, encompasses an area of 345,000 acres, producing from multiple fluvial sandstones in the Tertiary Wasatch and the Upper Cretaceous Mesaverde Formations. Since 1990, more than 350 additional wells have been drilled within GNB as infill development wells on an effective spacing of 80 acres, although the full development of the field area is not yet complete on this spacing pattern. There are three main operators in the field, with several additional operating companies around the perimeter of the field, as it is defined today. The eastern limit of the Natural Buttes field has been defined generally by the economic limit of Wasatch and Mesaverde gas production. Data presented in the Utah Geological Association's 1992 Uinta Basin Guidebook show that the economic limit of Natural Buttes production(4) is related to the thermal maturity of that field's source rocks: the coal-rich beds in the basal Mesaverde Formation(5,2). Drilling activity in the GNB in 1991 and 1992 increased dramatically over the previous five years, due in part to tax incentives allowed for gas produced from formations designated as "Tight." The Uinta basin contains some of the largest accumulations of oil and gas in any of the Western Interior basins, with production, indesending order, from the Tertiary Uinta Formation (gas), Green River Formation (oil and gas), Wasatch Formation (gas), and the Upper Cretaceous Mesaverde Group (gas). Figures 1 & 2 show diagrammatically the location of the significant gas producing fields: Natural Buttes and Island fields, as well as the other mlijor oil-producing fields in the Uinta and Piceance basins.