Print this pageFluid 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.