David E. Tabet¹, Jeffrey C. Quick¹

(1) Utah Geological Survey, Salt Lake City, UT

ABSTRACT: Potential Neslen Formation Fairway for Coalbed Gas in the Eastern Uinta Basin, Utah

The thickest coal beds of the Upper Cretaceous Neslen Formation (Campanian) were deposited in a ten-mile-wide fairway behind the northeast-trending shoreline of the underlying Sego Sandstone; this fairway roughly parallels the southeastern margin of the Uinta Basin. Within the 175- to 670-foot-thick (53 - 204 m) Neslen Formation, one to five lenticular beds of coal are commonly found in the lower 250 feet (76 m) of the formation, with a maximum net coal thickness of about 25 feet (7.6 m). Evaluation of thermal maturity data published by the U.S. Geological Survey indicates that the coal rank along the fairway is high volatile C bituminous, or too immature for major gas generation. However, the updip stratigraphic pinch-out of the Neslen coals, which occurs along the western flank of the Douglas Creek arch, provides an ideal trap for migrated gas. The pinch-out in the southern half of the fairway is exposed along the Book Cliffs, whereas the pinch-out in the northern half is in the subsurface. These coals could contain gas that has migrated up dip from mature Mesaverde source beds deeper in the Uinta Basin like the gas found in the nearby Natural Buttes field. Measured gas contents of near-surface coals along the Book Cliffs range from 0 to 48 cubic feet per ton (0 to 1.5 cc/gm). Methane sorption capacity of the coals, based on coal quality data, indicates the coals could hold between 270 to 380 cubic feet per ton (8.4 to 11.9 cc/gm), or economic quantities.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado