Ronald C. Surdam
Many of the most productive Cretaceous hydrocarbon reservoir rocks in Rocky Mountain Laramide basins are in close stratigraphic proximity to organic-rich source rocks (e.g. Muddy Sandstone/Mowry Shale). The spatial attributes of mass transfer processes characterizing the diagenesis/maturation of these reservoir/source rock systems during basin evolution are interrelated.
Typically, that portion of the Cretaceous shale section in a Laramide basin below a present-day depth of 9,000 feet is anomously pressured, a gas saturated, basin-wide, dynamic fluid-flow compartment. The driving mechanism for the compartmentalization is the generation, storage, and subsequent reaction of hydrocarbons. As these processes proceed in the shales, the fluid-flow system is converted to a multiphase regime, low-permeability rocks are converted to capillary seals, and closure of the seals creates the compartment. Above the anomously pressured, gas saturated shales the fluid-flow system remains single phase and typically under water drive (influenced by the meteoric water regime).
The vertical compartmentalization of the fluid flow system has a pronounced effect on mass transfer characteristics in the sandstones within and above the pressure compartment. Mass transfer in the sandstones, as reflected in alteration of framework grains and cementation/decementation reactions, in those portions of the basin characterized by a single-phase fluid-flow system is large scale and regionally significant. In contrast, in those portions of the basin characterized by a multiphase fluid-flow system, mass transfer in the sandstones, while commonly intense, is on a much smaller scale and typically confined to relatively small, isolated fluid-flow compartments.
AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994