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Fluid
Flow/Pressure Compartmentalization in Rocky Mountain
Laramide Basins
SURDAM, RONALD C.
In the Tertiary and Mesozoic stratigraphic section, fluid-flow systems in the
Rocky Mountain Laramide Basins are regionally compartmentalized. In the upper
7000 to 9000 ft, the fluid-flow system is single phase (dominated by meteoric
water) and is normally pressured. Hydrocarbon accumulations in this upper
compartment are confined by structural closure or stratigraphic traps. Below a
present-day depth of 7000 to 9000 ft, there exists a second, regional fluid-flow
compartment that is typically anomalously pressured. This second regional
compartment occurs in Cretaceous shales and is characterized by a multiphase
fluid-flow system with a pervasive hydrocarbon phase. Hydrocarbon accumulations
in this anomalously pressured compartment are not necessarily confined to
structural or stratigraphic traps, but instead are dependent on zones of
enhanced porosity and permeability. In the anomalously pressured regional
compartment, sandstones are either underpressured or overpressured. Below the
anomalously pressured compartment-typically below the lowest organic-rich shale
in the Mesozoic section-there is a third fluid-flow compartment that is normally
pressured and dominantly single phase.
Within this region anomalously pressured compartment sandstones are
subdivided structurally, stratigraphically and diagenetically into relatively
small, isolated compartments (largest dimension 1 to 10 miles). The driving
mechanism of pressure compartmentalization in both the shales and sandstones is
the generation an storage of liquid hydrocarbons that subsequently partially
react to gas, converting the fluid-flow system from a single-phase regime to a
multiphase regime in which capillarity controls permeability. In the sandstones,
three-dimensional closure of capillary seals lateral, above, below, and commonly
within sandstones results in manifold isolated fluid-flow
compartments.
This study was funded by the Gas Research Institute under Contracts Number 5089-260-1894 and 5091-221-2146.