The aquathermal-pressuring concept shows that isolated, water-filled reservoirs become abnormally pressured when temperature rises owing to increasing depth of burial. When reservoirs contain free gas, the situation is more complex and abnormal or subnormal pressures may develop depending on the gas/water ratio and the

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initial and final depth of burial. Because zones that have nonhydrostatic pressures must be effectively isolated from their surroundings (or the pressure should be equalized), a model would be a sand lens encased in shale. Consider, for example, where isolation occurs at 4,000 ft (1,200 m) where temperature is 123°F (51°C), and subsequent burial moves it to 8,000 ft (2,400 m) at 178°F (81°C). The pressure in the gas will start at 1,860 psi (12,815 kPa) hydrostatic at 4,000 ft (1,200 m), but the temperature rise will increase it to 2,035 psi (14,021 kPa), at 8,000 ft (2,400 m) is 3,720 psi (25,630 kPa), so the gas reservoir will be 1,685 psi (11,610 kPa) underpressured. Real gas reservoirs contain both gas and water. Calculations show that for trapping at 4,000 ft ( ,200 m) followed by burial to 8,000 ft (2,400 m) the reservoir will show various amounts of underpressuring if it contains more than 3 vol.% gas. With less than 3 vol.% gas it will overpressure. At greater trapping depths, high percentages of gas are needed to produce underpressuring, for example, 16 vol.% at 12,000 ft (3,600 m). Temperature decrease owing to uplift and removal of overburden produces the opposite effects, and reservoirs containing high percentages of gas develop abnormally high pressures.

This theoretical model provides an explanation for the common occurrence of underpressured gas, particularly in stratigraphic traps with low water contents. It also explains the underpressured gas in the bottom of basins (such as San Juan, Wattenberg, and western Canada "Deep Basin") and shows how abnormal and subnormal pressures can be developed in adjacent gas reservoirs in a restricted geographic area (such as the Appalachians). Regional tilting may bury a formation in one area but uplift it in another leading to regional trends from subnormal to abnormal pressures. An example of this is provided by the "gas sands" of the Morrow in western Oklahoma.

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