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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 46 (1996), Pages 25-39

The Formation and Behavior of "Vapor Lock" Pressure Seals and Associated Hydrocarbon Accumulations in Geologically Young Basins

W.M. Benzing (1), G.M. Shook (1), S.D. Leroy (2)

ABSTRACT

This paper demonstrate a mechanism for the formation of geopressure and hydrocarbon accumulations in geologically young basins (e.g., Gulf of Mexico). Gas exsolving from solution in response to changes in temperature create a relative permeability barrier that restricts vertical fluid flow locally. Continued gas dissolution in source rock, transport and subsequent exsolution at shallower depths maintain the "vapor lock" seal. Any sands that are found within the pressure seal fill preferentially with the non-wetting gas and oil. Further migration of the trapped hydrocarbons is restricted by the capillary entry pressures of the surrounding shales; thus, capillary entry effects enhance the sealing nature of the pressure barrier. Sands outside the zone of exsolution may be overpressured, but do not contain free gas.

Seismic modeling indicates that the creation of pressure seals and the occurrence of abnormal fluid pressure reduces the seismic velocity and distorts the true depth to geologic horizons below the pressure seal. The potential therefore exists for improper seismic interpretation if the geopressure phenomena is not accurately identified.


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