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The objectives of this chapter are threefold: (1) to provide a historical perspective on considerations of pervasive tight-gas accumulations, (2) to provide some observations on the present understanding of these accumulations, and (3) to anticipate where the industry is headed in the future.

From 1979 to about 1987, various workers (industry, government, and academe) discussed pervasive tight-gas accumulations and established important relationships for source rock, maturity, expulsion and migration, pressures, rock quality, and fluid content. Their main conclusion was that the hydrocarbons in these reservoir systems were dynamic and not static as in conventional structural and stratigraphic traps. The paradigm shift made by 1987 concluded that these accumulations were continually adjusting to existing conditions in both time and space. In more recent years, additional examples have been documented, and questions have arisen about the validity of the original model, noting the presence of more water in some systems than the model would predict.

The close proximity of the mature, gas-generating, and gas-expelling source rock to the reservoirs is critical. The amount and richness of mature source rock has to be adequate for the volume of reservoir rock being charged. The proper combination of these circumstances produces more gas than can be contained under normal pressure. The quantity of this gas charge relative to available pore space in the reservoir system will dictate the reservoir pressure.

Pervasive tight-gas accumulations have now been documented in more than 20 North American basins and are the targets for major ongoing exploration and development programs. The average reservoir porosity for these producing units is in the 8–9% range, with average in-situ permeabilities of hundredths of a millidarcy.

We believe the industry will likely move forward in four directions: (1) revisit older mature basins, (2) expand into new basins, (3) move into carbonate reservoirs, and (4) continue to develop tighter and tighter rock. With continuing technology improvements (especially in drilling and completing) and robust gas prices, the industry will access vast new reserves farther down into the resource pyramid.

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