Underpressured Gas Accumulations with Pressure Control at the Outcrop
Underpressure is manifested in oil and gas wells by pressure-depth ratios less than hydrostatic. If the ratio of the pressure measured in a drillstem test to the depth of a gas reservoir, for example, is 0.32 psi/ft, this is a value substantially less than a nominal fresh-water ratio of 0.433 psi/ft and is an indicator of underpressured conditions. Explanations for extensive underpressured systems commonly invoke a loss of gas and drop in temperature during uplift and erosion, causing an originally overpressured continuous gas system to evolve to an underpressured state. An alternative explanation relies on hydraulic continuity within a confined aquifer that sets the pressure reference for the gas accumulation. Underpressure in confined aquifers can be studied at the scale of a sedimentary basin by converting pressure to hydraulic head. The conversion is useful only in the water-dominated part of the aquifer. Hydraulic head accounts for both the pressure and elevation of the drillstem test. Where hydraulic head of a confined aquifer is substantially less than the surface elevation, the aquifer is underpressured and gas accumulations supported by the aquifer will also be underpressured. Rocks of confined aquifers crop out on the eastern flanks of the Anadarko, Raton, and Denver Basins. The area of discharge in the east is located where hydraulic head and outcrop elevation are equal. However, recharge from the surface or from outcrop in the west is either restricted or nonexistent due to confining aquifers and geologic structures. In this view, underpressure of oil and gas reservoirs is determined by the hydraulic setting at the scale of a sedimentary basin.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012