Coal-Bed Methane and Tight Gas Sands Interrelationships
Published:January 01, 1986
Coal beds are known to exist in parts of most major sedimentary basins in the conterminous United States, from outcrop to depths in places exceeding 15,000 ft (4,572 m). The environments of deposition conducive to the formation of organic-rich swamps are the same as those favoring deposition of regressive sand bodies presently identified as underlying many coal beds. Many of these sand bodies are present in the San Juan and Piceance basins and are currently identified as tight gas reservoirs.
Methane generated during the coalification process exceeds 5,000 ft3 of gas per ton of coal (ft3/ton) (1 56.2 5 cm3/g) through the rank of low-volatile bituminous coal. Seldom have coal samples been collected, however, that contain more than 600 ft3/ton (18.75 cm3/g) of gas in place. The excess gas—generated gas minus in-place gas—must have escaped, possibly into adjacent tight gas reservoirs.
The bulk of the coal in the San Juan basin, estimated to contain more than 31 tcf (0.9 X 1012 m3), of coalbed and gas, is found in the Upper Cretaceous Fruitland Formation. An unknown volumeof gas has been fed by the coals into the underlying Pictured Cliffs Sandstone and other nearby reservoirs.
Coals in the Piceance basin of western Colorado are found in the Mesaverde Group immediately above the Rollins Sandstone or its equivalent.
The coal-bed methane resource for the Piceance basin is estimated to be about 60 tcf (1.7 X 1012m3).
An assessment of the coal-bed methane resources in the Colorado-New Mexico area indicates the presence of 7 4 tcf (2.1 X 1012 m3) in the San Juan, Piceance, and Raton basins, all within 125 mi (2 01 km) of the center of a high thermal gradient area.
Only a small portion of the large volumes of gas generated during coalification is found within the coal beds themselves.Additional gas has migrated into adjacent reservoir rocks or escaped to the atmosphere. Because of depositional environment continuity, tight gas sand reservoirs are a ready recipient for the large volumes of excess coal-bed gas.
Figures & Tables
Geology of Tight Gas Reservoirs
Tight gas reservoirs occur in low-permeability, gas-bearing formations that are present to some extent in all gas-producing basins worldwide. This is the first volume to bring together data on tight reservoirs for a variety of basins and different geologic settings. The papers in this volume discuss characteristics of some of the most significant tight gas areas in the United States; however, these data are equally applicable to many other recognized and unrecognized tight gas provinces in other nations. In general, tight reservoirs in the United States are grouped into tight gas sandstones and eastern Devonian shales. The Devonian shale sequences are dominantly marine shale but include some siltstone and sandstone. Tight gas sandstone formations of other than Devonian age are present throughout the United States and consist primarily of fluvial and marine sandstones and siltstones. In addition, gas also occurs in low-permeability marine carbonate reservoirs. The 14 papers in this volume cover such topics as: coal-bed methane and tight gas sands interrelationships; gas-bearing shales in the Appalachian basin; exploration and development of hydrocarbons from low-permeability chalks; and geologic characterization of low-permeability gas reservoirs.