The Devonian gas shales of the Appalachian basin constitute a sequence of dark brownish–gray to black laminated rocks that contain 0.5 to 2 0 weight percent (wt%) organic matter, the source of the gas in the shale. The gas shales underlie about 170,000 mi2 (440,300 km2) of the basin, mainly under the Appalachian plateaus. Their total volume exceeds 12,600 mi3 (52,517 km3), and they contain more than 3.3 trillion tons (3.6 X 1012 Mg) of organic matter. The gas shales are low–permeability, low–porosity rocks having permeabilities of 0.1 to 10 microdarcys (µd) and porosities in the 1 to 3 % range. They have produced slightly more than 3 trillion cubic feet (8.5 X 1010 m3), mainly from the Big Sandy area of eastern Kentucky and adjacent West Virginia, but their adsorbed gas in place is large; estimates range from less than 200, (5.66 X 1012 m3) to more than 1,860, (5.27 X 10‘3 m3). Near outcrops on the west side of the basin, the gas shales yield gases of low thermal maturity that have a large component of biogenic gas; deep in the basin at depths of 8,000 to 11,000 ft (2.438 to 3.352 km) below sea level, the shales yield high–maturity dry gas at approximately the upper limit of gas generation.
Because the gas shales have low permeability and most of the generated gas is adsorbed on organic matter, the Devonian gas shales must be broken by extensive natural fracture systems before the shales will yield gas in commercial volumes. Shallow gas wells near Lake Erie yield 5,000 to 100,000 cubic feet (5 to 100 mcf) (142 to 2,832 m3) per day at nearly atmospheric pressure, whereas the deeper wells of the Big Sandy field may yield as much as 5 million cubic feet (mmcf) (1.416 X 105 m3) per day at normal rock pressure. Production rates even in the larger wells usually drop to about 20% of the initial rate before stabilizing to a nearly flat rate. The ensuing slow decline over several decades is attributed to a nearly steady–state flow of gas from the shale matrix through the fracture system to the well bore. The size and geometry of the fracture system may be the most important factor in determining a well’s productivity.
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.