Capillary Exit Pressure as a Basin Sealing Mechanism

Jennifer Shosa and Lawrence Cathles

Abnormally pressured compartments in sedimentary basins require an efficient sealing mechanism. Most sealing mechanisms rely on either intrinsically low formation permeabilities or on the entry pressure of a non-aqueous phase into a fine-grained unit. However, the nanodarcy permeabilities required to maintain overpressures over significant geologic time are not plausible over wide areas. Entry pressures, while effective in trapping a non-aqueous phase in a local reservoir, can not prevent leakage where the non-aqueous phase is not ponded against the seal.

The capillary exit pressure required to displace water from a fine-grained formation into a coarse-grained formation which contains a non-aqueous phase provides an alternative sealing mechanism. Capillary exit pressure seals require contrasts in grain size and the presence of two phases in the coarse-grained unit, but do not require 100% saturation of the non-aqueous phase. These conditions can exist on all sides of a pressure compartment, and can account for sealing on the top, bottom, and sides of a compartment.

We have shown in the laboratory that capillary exit pressure seals under reservoir conditions allow no fluid flow across the seal until a threshold pressure is exceeded (e.g., the seal is not a relative permeability effect) and that exit pressures are additive over a series of fine/coarse interfaces. Capillary exit pressure seals can maintain the abnormal pressures observed in the South Eugene Island Block 330 field. Both a sufficient number of sand/shale layers and a gas phase are present in the pressure transition zone there. We believe capillary exit pressure seals are a general feature of sedimentary basins and are important in controlling large scale fluid flow.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California