Abstract: Capillary Seals and Their Evolution in the Cretaceous Section in the Powder River Basin, Wyoming

Zun Sheng Jiao, Ronald C. Surdam, Randi S. Martinsen, William P. Iverson

In contrast to the basin-wide pressure compartments in the Cretaceous shale section of the Powder River basin, the anomalously pressured compartments in the Cretaceous sandstones are much smaller in scale and are isolated from each other. The anomalously pressured sandstones are subdivided stratigraphically and diagenetically into relatively small, isolated pressure compartments (largest dimension 1 to 10 miles) by low-permeability rocks that

were converted to impermeable capillary seals as the fluid-flow system evolved into a multiphase system with the addition of hydrocarbons during progressive burial.

A variety of lithologies serves as capillary seals and sandstone pressure-compartment boundaries: paleosols associated with lowstand surfaces of erosion (LSEs), clay-infiltrated sandstones beneath these LSEs, transgressive shales, and carbonate-cemented sandstones associated with fractures and perhaps faults. The capillary seals associated with sandstone pressure compartments have displacement pressures of 400 to 4,000 psi and sealing capacities of 1,100 to 11,000 feet (height of gas column). For each lithology, the displacement pressure and sealing capacity increases significantly with depth, coincident with a mixed-layer illite/smectite (I/S) compositional change with progressive burial, from approximately 20% illite in the I/S clays at approximately 900 m (3,000 ft) to 80% at 3,000 m (10,000 ft), and a structural change from random to ordered. Pore throats are primarily in the subnano and nano categories (<0.01 to 0.05 m), and permeabilities are from 0.02 to 0.08 md. Petrographic and thermal maturation studies show that increase of displacement pressure and sealing capacity with depth is a function of compaction, clay diagenesis, carbonate/quartz cementation, and the conversion of the fluid-flow system from single-phase to multiphase.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994