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Abstract

Many of the most productive Cretaceous hydrocarbon reservoir rocks in Rocky Mountain Laramide basins are in close stratigraphic proximity to organic-rich source rocks; an example is the Muddy Sandstone/Mowry Shale system in the Powder River Basin. The spatial attributes of mass-transfer processes characterizing the diagenesis and maturation of these reservoir rocks and source rocks during basin evolution are interrelated.

As is typical in Wyoming Laramide basins, that portion of the Cretaceous shale section in the Powder River Basin below a present-day depth of approximately 2700 m (9000 ft) constituïes one anomalously pressured, gas-saturated, basinwide, dynamic fluid-flow compartment. The driving mechanism for the compartmentalization of these shales was the generation, Storage, and subsequent reaction of hydrocarbons. As these processes proceeded in the shales, the fluid-flow regime was converted to a multiphase regime; low-permeability fluid-flow barriers were converted to capillary seals; and three-dimensional closure of these seals created the compartment. Above these anomalously pressured, gas-saturated shales, the fluid-flow regime remains single-phase and typically under water drive (influenced by the meteoric water regime).

The vertical compartmentalization of the fluid-flow regime has had a pronounced effect on the different mass-transfer characleristics in the sandstone bodies within and above the basinwide pressure compartment in the shales. Mass transfer in the sandstones in those portions of the basin characterized by a single-phase fluid-flow regime is large-scale and regionally significant, as reflected in the alteration of framework grains and cementation/decementation reactions. In contrast, in those portions of the basin characterized by a multiphase fluid-flow regime, mass transfer in the sandstones, while commonly intense, is on a much smaller scale and typically is confined to relatively small, isolated fluid-flow compartmenls. These compartments in the sandstone have trapped the hydrocarbon whose presence led to their formation.

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