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Pinedale Field: a Giant New Gas Resource, Green River Basin, Wyoming, U.S.A

Robert W. McDermott, Ultra Petroleum Corp, 304 Inverness Way, Suite 295, Englewood, CO 80112, phone: 303-645-9807, [email protected] and Troy G. Graham, Ultra Petroleum Corp, 304 Inverness Way, Suite 295, Englewood, 80112.

The Pinedale Field is developing into one of the largest gas fields in the Greater Green River Basin of southwest Wyoming and along with Jonah Field, a significant new natural gas resource in America. Production is from over-pressured, tight-gas sandstones of the Upper Cretaceous Lance Formation and upper part of the undifferentiated Mesaverde (“Lance Pool”), coincident with the Pinedale Anticline. Current development along the Pinedale Anticline spans an area over 3 miles in width and 30 miles in length. The gross hydrocarbon-bearing column, which exceeds 5,000 feet in total thickness, consists mainly of fluvial and flood plain facies rocks deposited in a broad alluvial valley. In order to better understand the Lance Pool reservoir characteristics and to optimize the development of the Pinedale Field, Ultra Petroleum Corp embarked on a coring program in which 10 wells were cored and a total of 853 feet of Lance Pool rock was recovered. Detailed analysis of the core integrated with Ultra's geological, petrophysical and geophysical models has led to a unique understanding of the Lance Pool. The alluvial sandstones of the Lance Pool were deposited in a rapidly subsiding basin by modest-sized rivers reflecting constant channel-belt migration. The resulting reservoirs are laterally- and vertically-discontinuous, multi-story sandstones. Effective reservoir characteristics appear to include lower porosities and permeabilities, a greater range of effective water saturations and a thicker net pay section than previously believed.