Underpressured Reservoir--The Other Part of the Story

Charles W. Spencer

For many years considerable interest has been shown in overpressured reservoirs because of drilling problems and the association of abnormally high pressure and oil and gas production. The significance of underpressured reservoirs has been somewhat neglected because underpressuring is difficult to detect.

Oil and gas reservoirs with abnormally high or abnormally low pressures are present in many basins in the United States and worldwide. The above-normal (overpressured) reservoirs are generally deep (> 10,000 ft or 3,050 m), commonly gas bearing, and, in onshore basins in the United States, usually occur in low-permeability (tight) reservoir-rock sequences. Reservoirs with below-normal pressures are mostly shallow (< 6,000 ft or 1,830 m), usually contain gas and/or oil, and occur in both tight and permeable (conventional) reservoirs.

Abnormally low reservoir pressures can be caused by gaseous diffusion and gas shrinkage in reservoirs that have been uplifted or otherwise cooled. Recharge of these low-pressure reservoirs by meteoric water does not occur rapidly enough to repressure the reservoirs to a normal, hydrostatic pressure. Underpressuring can occur regionally as a result of reservoir water drainage to a low-elevation discharge area. Examples of underpressuring can be found in Rocky Mountain oil- and gas-bearing sandstone and carbonate reservoirs that range in age from the Tertiary to early Paleozoic.

Abnormally low reservoir pressure can cause exploration and completion problems. The most serious problem is that deep mud and mud-filtrate invasion complicates well-log identification of hydrocarbon saturation. Also, underpressured gas-bearing reservoirs typically yield poor or no shows during drilling, even where mud-logging equipment is used. Drill-stem tests (DSTs) are frequently inconclusive because of deep fracture and/or matrix invasion by mud and mud filtrate. If DSTs in low-pressure reservoirs are inconclusive, then inflatable open-hole packers should be used, and the reservoir should be swabbed until formation fluid is recovered.

Undoubtedly, many wells have been unknowingly drilled through underpressured accumulations. A better understanding of their unique characteristics is needed in order to increase discovery rates.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988