SURDAM, RONALD C., ZUNSHENG JIAO, RANDI S. MARTINSEN, and WILLIAM P. IVERSON, Univ. of Wyoming, Laramie, WY

ABSTRACT: Capillary Seals and Pressure Compartment Boundaries in the Cretaceous Section in the Powder River Basin

A wide variety of lithologies serve as capillary seals and pressure compartment boundaries in the Powder River basin (PRB). Paleosols associated with lowstand unconformities (LSE), clay infiltrated sandstones beneath these unconformities, shales associated with transgressive surfaces of erosion (TSE), transgressive shales, and carbonate cemented sandstones that are associated with fractures and perhaps faults all serve as bounding compartment seals.

The capillary seals associated with pressure compartments in the PRB havedisplacement pressures ranging from 1,000 to 4,000 psi and sealing capacities from 1,500 to 6,000 feet (height of gas column). For each sealing lithology, the displacement pressure and sealing capacity increases significantly (up to one order of magnitude) with depth. This increase is a function of clay diagenesis (progress of the smectite to illite reaction in mixed-layer clays and the kaolinite to chlorite reaction), carbonate/quartz cementation, and the conversion of the fluid-flow system from single-phase to multiphase.

Any rock type that has relatively low permeability and is involved with a multiphase fluid-flow system can form a capillary seal and be part of a pressure compartment boundary. The low permeability can be caused by the original depositional environment, compaction, clay diagenesis, or diagenetic cements, or a combination of these elements. All of these low-permeability rock types will serve as capillary seals in a multiphase fluid flow system; and if three-dimensional closure of the capillary seals occur, fluid isolation and pressure compartmentalization will result, and abnormal pressure will occur with burial.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.