Importance of Aquathermal Pressuring Effect in Gulf Coast: GEOLOGIC NOTE

Abstract

Study of subsurface pressure and compaction data in the Gulf Coast demonstrates the possible presence of aquathermal pressuring effect as proposed by Barker. The average rate of pore-pressure increase since isolation is estimated to be about 1.4 psi/ft, significantly higher than the average overburden-pressure gradient for this area which seldom exceeds 1 psi/ft. Therefore overburden pressure alone cannot explain the actual subsurface pore pressure; some other pressuring effect, such as aquathermal, also must be involved. The Gulf Coast data also suggest that aquathermal pressuring is more effective if the isolation of pore water started at a deeper depth or a greater compaction level. The leakage of pore water from the undercompacted shale then would be less, so that more of the generated abnormal pressure would be retained. The amount of water moved in the average undercompacted shale in this area is estimated by using Barker's temperature-pressure-density diagram for water; its ratio to the amount moved in normally compacted shales is in the order of 1:100, a very small quantity. However, such undercompacted shales usually are buried deeply and have reached high temperatures, so that the movement of such water--even this much--might be important in the migration of hydrocarbons, especially liquid hydrocarbons. If shale-pore fluids were isolated at a depth of 8,000 ft, and if the gradient of pore-pressure increase since isolation was 1.4 psi/ft, the pore pressure could have reached or exceeded the overburden pressure at about 20,000 ft. Thus, this event could have had a significant impact on the sediments, and may have contributed to structural deformation such as fracturing and/or shale diapirism.