Fibrous Illite Controls Productivity in Frontier Gas Sandstones, Moxa Arch, Wyoming

Abstract

Summary Core, log, and well-test analyses from two wells recently completed in the Frontier sandstone in the Moxa Arch area of Wyoming revealed that fibrous illite severely reduced gas productivity. In this study area, presence of fibrous illite currently cannot be predicted and effects can be recognized only through laboratory tests on preserved cores. Introduction This study is part of a research program sponsored by the Gas Research Inst. (GRI) to improve formation evaluation in lowpermeability gas reservoirs. The GRI program, initially focused on the Travis Peak sandstone in the East Texas basin, now is directed to other formations and basins, such as the Frontier sandstone in the Green River basin in southwestern Wyoming, to transfer technology previously developed. As part of this program, two wells recently have been drilled in which GRI, in cooperation with operating companies, funded the collection and analyses of conventional-core special logs and special completion, stimulation, and well tests. These two wells, the Terra Resources-Anderson Canyon No. 3-17 and the Enron-South Hogsback No. 13-8A, are situated as shown in Fig. 1 along the Moxa Arch of the Green River basin. Also in Fig. 1 is the site of the Enron Staged Field Experiment (SFE) No. 4, which was drilled after this study. Frontier core, log, and test data were provided to GRI from two other wells, the Texaco-State of Wyoming NCT No.1 and the Wexpro-Church Buttes No. 48, farther south along the Moxa Arch. This paper, however, focuses on the formation evaluation results obtained from the Anderson Canyon and South Hogsback wells. The Frontier sandstone is an Upper Cretaceous, low-permeability gas reservoir composed of marine and nonmarine facies deposited in a fluvial-deltaic depositional system. The Moxa Arch, Fig. 1, is a broad intrabasin uplift parallel to the Overthrust Belt on the west side of the Green River basin. At the north end of the Moxa Arch, the Second (lower) Frontier contains several sandstone "benches," and the laterally extensive marine shoreline sandstone of the Second Bench forms the main reservoir. In the Anderson Canyon well, the top of the Second Bench is at 9,074-ft core depth. The main productive reservoir interval is from 9,080- to 9,086.5ft core depth and consists of relatively clean, well-sorted and -rounded sandstone that formed in a foreshore or upper shoreface zone where bottom currents were strongest. Nonmarine mudstones and siltstones of the lower part of the First Bench overlay the Second Bench and form the seal for the reservoir. In the South Hogsback well, the top of the Second Bench is at 7,101-ft core depth. As with the Anderson Canyon site, the Second Bench consists of alternating zones of bioturbated muddy sandstones of low permeability from the lower shoreface facies and cleaner sandstones of the upper shoreface facies. The main productive reservoir interval is from 7,112- to 7,137-ft core depth.