Title: Underpressured Gas Accumulations with Pressure Control at the Outcrop
Speaker: Philip H. Nelson, U.S. Geological Survey, Denver, Colorado
Publication: The Outcrop, January 2013, p. 25
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 valid only in water-dominated formations. 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, whether conventional or continuous, is determined by the hydrologic setting at the scale of a sedimentary basin.