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Abstract

Drilling for coalbed gas in the Upper Cretaceous Ferron Sandstone of central Utah during the 1990s has resulted in one of the most successful plays of this kind. Development through the year 2003 has resulted in three fields, as well as a potential fairway 6-10 mi (10-16 km) wide and 20-60 mi (32-96 km) long, corresponding to shallow coal occurrence at depths from 1100-3500 ft (330–1060 m) in the Ferron, a sequence of interbedded fluvial-deltaic sandstone, shale, and coal in the lower part of the Cretaceous Mancos Shale. The major reservoirs in this interval consist of thin to moderately thick (3-30 ft [1-10 m]) coalbeds of relatively low rank (high-volatile B bituminous) and variable gas content, ranging from 100 scf/ton (3.0 cm3/g) or less in the south to as high as 500 scf/ton (15.6 cm3/g) in the north. Other lithologies also contain gas and contribute a minor portion of the produced gas. Productive wells have averaged over 500 mcf/day and, after several years of production, continue to typically show increases in gas production. In the major productive area, Drunkards Wash unit, the first 33 producers averaged 974 mcf and 85 bbl of water per day after five years of continuous production. Estimated ultimate recoverable reserves for individual wells in this unit average about 1.9 bcf, with one standard deviation about that mean of ±1.5 bcf.

Based on several criteria, including gas content, thermal maturity, and chronostratigraphy, the play is divided into northern and southern parts. The northern part is characterized by coals that have the following characteristics: (1) high gas contents; (2) moderate thermal maturity (e.g. vitrinite reflectance [Ro] values of 0.6-0.8%); (3) good permeabilities (4-20 md); (4) lack of exposure; and (5) overpressuring, due to artesian conditions. Southern coals have much lower gas contents (<100 scf/ton [3.0 cm3/g]), lower thermal maturity (Ro = 0.4-0.6%), and they are partially exposed along an extensive, 35-mi (56-km) outcrop belt that may have allowed a degree of gas flushing. These coals, however, are slightly thicker and more extensive than those to the north and thus may retain some potential. Northern coals appear to contain a mixture of gas from three sources: in-situ thermogenic methane, migrated thermogenic methane from more mature sources, and late-stage biogenic gas. Current development is focused on the northern portion of the stated fairway, where well control and an existing infrastructure are present. Indications are that coalbed methane development in the Ferron will increase by at least one hundred in the near future from the 754 wells producing as of the end of 2003.

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