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Symposium on the Geology of Rocky Mountain Coal, October 2-4,
ABSTRACT: ORIGIN AND CONTROL OF METHANE IN A COAL BED OF THE PALEOCENE FORT UNION FORMATION, JOHNSON COUNTY, WYOMING
Six core samples from a thick, low-rank coal bed in the Powder River basin, Wyoming, have gas contents ranging from 56 to 74 cubic feet per ton of coal . The gas consists of 88-94 percent methane, 0.4-3.0 percent heavier hydrocarbons, and 4-11 percent carbon dioxide. The samples were collected at depths between 1,054 and 1,223 feet in core hole B23BG1, in the SE¼SE¼ sec. 7, T. 48 N., R. 77 W., which was drilled as part of a cooperative program between the U.S. Bureau of Land Management and the U.S. Geological Survey. The coal bed is within the upper part of the Paleocene Fort Union Formation. It is 202 feet thick in the core (1,052-1,254 feet) but thins to about 50 feet at drill sites in adjacent townships. Structure maps drawn on top of the coal show a general westward dip complicated by small structural features. Core hole B23BG1 penetrates the coal at the top of an anticline, a feature which may help to localize the gas.
The gas most likely is derived from the coal; stable carbon-12/carbon-13 ratios of -58.6 to -60.9 per mil show that it is biogenic. Several factors may account for its retention in the coal bed:
(1) Core and geophysical logs show the coal is overlain by fine-grained strata in the vicinity of the core hole. These may act as a seal, restricting gas migration out of the bed.
(2) Cleats are poorly developed in the upper 69 feet of coal, but are better developed below, giving the coal two layers of differing permeabilities. The highest gas contents were in samples from directly below the transition zone, suggesting that the upper, less permeable coal layer may also inhibit migration.
(3) The anticline and other small structures mentioned above may restrict migration of the gas updip