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Abstract: The "Vapor-Lock" Pressure Seal--A Possible Connection between Fluid Pressure Distribution and Structure in Young, Clastic Basins

William M. Benzing

Most young sedimentary basins, such as the Gulf of Mexico, contain zones of anomalously high fluid pressure that may provide clues for finding major new hydrocarbon reserves. Evidence is mounting that such basins are dynamic systems with fluids flowing across "leaky" pressure seals. The anomalously high pressure gradients within the seals define the geopressured zones. Given that fluid flow exists, the observed seal gradients are evidence of flow through a low permeability barrier.

One form of low permeability barrier, a gas charged sediment, has been demonstrated in the lab. Gas in the sediment pores impedes the flow of liquids. This type of seal behaves much like the more traditional concept of a lithology seal. Since the "vapor-lock" seal depends upon factors which control gas solubility, such as pressure, temperature and salinity, it can form within a uniform lithology and cut across sedimentary layers.

Assuming the "vapor-lock" seal is more likely to follow isotherms than bedding planes, the potential exists that the 3-D surface represented by the seal may have local highs which would produce localized minima in the effective stress. This would produce a mechanically unstable situation with the localized, high pressure, low density sediment creating buoyant forces within the sediment. These vertical forces, coupled with lateral variations in compaction due to geopressure, lead to sediment deformation and the formation of structural traps.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana