Clay Diagenesis and Sealing Capacities of Sequence Boundaries in the Cretaceous Section in Laramide Basins

JIAO, ZUN S. and RONALD C. SURDAM

In Rocky Mountain Laramide Basins major sequence boundaries play a key role in the Cretaceous petroleum system. A variety of lithologies characterize the major sequence boundaries such as paleosols associated with lowstand surface of erosion (LSE), clay-infiltrated sandstones beneath these LSE, and transgressive shales near maximum flooding surfaces. These low permeability rocks are fluid-flow barriers in single phase fluid-flow systems and evolve into capillary seals with progressive burial as the fluid-flow system changes from single phase to multiphase as hydrocarbons are added. Petrography, thermal maturation, and petrophysical studies show that with increased depth there is a significant increase in displacement pressure and sealing capacity along the sequence boundaries as a function of compaction, clay diagenesis, carbonate/quartz cementation, and conversion of the fluid-flows stem from single-phase to multiphase.

In the Cretaceous section paleosols and clay infiltrated sandstones associated with the LSE have displacement pressures ranging from 60 to 2000 psi and sealing capacities from 130 to 3000 feet (height of gas column). The transgressive shales have displacement pressures from 400 to 4000 psi and sealing capacities from 600 to 6000 feet. For each lithology, the displacement pressure and sealing capacity increases significantly with depth; coincident with an increase in the percent of illite in the mixed-layer S/I clay (e.g. 20% at approximately 3000 ft to 80% at 10,000 ft). The abundant carbonate filled fractures in these sealing rocks indicate that the carbonate cementation resulting from pressure drops is important in reestablishing the sealing capacity of the pressure seals during and/or after fracturing.

ACKNOWLEDMENTS
This study was funded by the Gas Research Institute under Contracts Number 5089-260-1894 and 5091-221-2146.