Geology and Production Performance of the Niobrara Low-Permeability Reservoir in the Denver-Julesburg Basin

Abstract

Abstract This paper presents geologic and reservoir parameters of the Niobrara Formation In Weld County, Colorado. With the use of computer generated contour maps, It is possible to predict favorable areas of profitable possible to predict favorable areas of profitable Niobrara pay. This predictability Is further enhanced when combined with Scanning Electron Microscopy (SEM) analysis and historical production analysis. The SEM results show that the Niobrara In this region is a micrite and not a true chalk. The porosity Is, therefore, lower than would be expected in a chalk. The thickness of the second bench and the pore volume appear to have a better relationship to known faults In the Niobrara than present day structure. These parameters were analyzed in order to predict areas of parameters were analyzed in order to predict areas of faulting and fracturing, since these areas are known to have the best potential for Niobrara production. Use of these techniques Indicates that the northern portion of the study area has the highest potential portion of the study area has the highest potential for successful Niobrara wells. Based on the limited amount of production history available in this region and current market oil and gas prices, the average Niobrara well in this region appears to be uneconomic unless supported by additional production from other horizons. However, computer mapping suggests that current production Is not located in the most promising areas. Greater Niobrara production potential may be found In local areas characterized by greater porosity, thicker benches, and proximity to faults. Introduction The development of cost effective predictive techniques for petroleum exploration has been a continuing quest in the petroleum geology industry. This paper presents one such technique found useful in the Weld County, Colorado, Niobrara Formation of the Denver-Julesburg Basin. More specifically, the data were derived from the Second Chalk Bench of the Niobrara. This bench was selected as It constitutes the most continuous bench across the study area and has the highest potential for commercial hydrocarbon development. The geographical study area consists of 56 townships located in Weld County, Colorado, T1-7N, R61-68W (Figure 1). The purpose of this paper Is to Identify favorable areas for Niobrara hydrocarbon exploration. Seven critical variables from publicly available well logs were input Into a computer and used to generate a series of contour maps showing present day structure, paleostructure, porosity, thickness, and pore-volume. paleostructure, porosity, thickness, and pore-volume. Two further techniques, Scanning Electron Miscroscopy (SEM) analysis of sidewall cores and historical production analysis, were employed to assist In production analysis, were employed to assist In interpreting and predicting potential reserves. Evaluation of the computer maps, SEM data, and historical production data provided the basis for predicting favorable areas for hydrocarbon exploration predicting favorable areas for hydrocarbon exploration in the Niobrara Formation. GENERAL GEOLOGY The Niobrara Formation was deposited during the Late Cretaceous Period in the Western Interior Seaway. The Niobrara is divided Into two members: the Smoky Hill Chalk and the Fort Hays Limestone. The upper member, the Smoky Hill Chalk, consists of gray to white chalky shale with three locally massive chalk benches, referred to as benches 1, 2, and 3 (from top to bottom). The Fort Hays Limestone, the lower member, is composed of 25 to 85 feet of chalk and shaly chalk interbedded with thin beds of chalky shale (see Figures 2 and 3), The Niobrara produces gas from low-relief structures on the east flank of the Denver-Julesburg Basin and the north flank of the Las Animas Arch in Colorado, Kansas, and Nebraska (Smagala, 1981). In Yuma County, Colorado, and portions of the Las Animas Arch, Bench 1 is a high porosity, low permeability reservoir. This differs from the correlative chalk bench in the Weld County Denver-Julesburg Basin which is of lower porosity. This reduced porosity is thought to be due porosity. This reduced porosity is thought to be due to greater depth of burial. The Niobrara produces biogenic gas In low volumes ranging from 20 to 300 thousand cubic feet of gas per day (MCFGPD) (Lockridge, 1978). Niobrara wells are commonly stimulated with a foam fracture treatment. P. 353