--> Abstract: The Formation and Behavior of "Vapor Lock" Pressure Seals and Associated Hydrocarbon Accumulations in Geologically Young Basins, by W. M. Benzing and G. M. Shook; #90950 (1996).

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

W. M. Benzing, G. M. Shook

This paper demonstrates 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 nonwetting 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. Numerical simulations demonstrate the viabilit of this model for the formation of geopressured reservoirs in young basins, and show that two distinct pressure seals can form under certain conditions.

The model incorporates salient features from the Gulf of Mexico, including spatially-variable heat flux, methane and CO2 generation and transport, and a low-permeability shale as the continuous rock fabric. Steady state isotherms exhibit significant relief due to the variable heat flux, and gas exsolves from solution along two distinct isotherms. The gas forms a relative permeability barrier that restricts vertical fluid flow. As the gas saturation exceeds a critical saturation, any sands found within the pressure seals fill with the non-wetting hydrocarbons. Gas is trapped within the sands by capillary entry pressures of the surrounding shales, improving the sealing nature of the pressure barrier. Sensitivity studies show that specific seal geometry is sensitive to the var ability in heat flow, gas content of the aqueous phase, and relative permeability and capillary pressure characteristics of both the sand and shale; however, one or more vapor lock seals form under a variety of conditions.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas