Overview of Worldwide Occurrence of Abnormal Reservoir Pressures

Charles W. Spencer, Ben E. Law, Fred F. Meissner, Viacheslav I. Slavin

Abnormally high reservoir pressures may be encountered in tide deeper parts of many basins and areas worldwide. These overpressured or geopressured reservoirs have several characteristics in common. They most often occur at depths greater than 3,000 m (10,000 ft) and where bottomhole temperatures are 93°C (200°F) or higher. However, overpressuring shallower than 3,000 m (10,000 ft) does occur. Regional overpressures are commonly associated with hydrocarbon production or shows of hydrocarbons, especially gas, but may coproduce water.

No single mechanism can explain all occurrences of regionally high pressure, but some mechanisms are (1) undercompaction (incompletely dewatered sediments), (2) hydrocarbon generation, (3) water release from dehydration of clays and other minerals, (4) (aquathermal) water expansion in well sealed aquifers due to heating, (5) tectonic stress, (6) fluid movement up faults from deeper rocks, and (7) osmosis. Some local overpressuring can be attributed to long hydrocarbon columns in fields and by hydrodynamics related to high-elevation recharge areas.

Underpressuring is common at depths less than 1,800 m (6,000 ft) and temperatures of less than 65°C (150°F). Some suggested causes of underpressuring include (1) gas cooling, (2) gas diffusion, (3) rock dilation caused by erosion, (4) aquathermal cooling, (5) low-water table in deserts, (6) topographically low ground-water discharge area, and (7) osmosis.

Overpressuring is generally easily identified because of fluid flow into wells during drilling; however, underpressured reservoirs are usually difficult to detect without actual pressure measurements. Years ago, shallow underpressured reservoirs were easily detected during cable-tool drilling, but since the advent of rotary drilling, many of these accumulations have been inadvertently bypassed. Undoubtedly, there are major worldwide resources of shallow, underpressured by-passed gas.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995