ABSTRACT

In the Big Muddy-South Glenrock field area, Natrona County, Wyoming, probably 100 million bbl of oil will be produced ultimately from 3 separate sandstone reservoirs within the Lower Cretaceous section; from oldest to youngest, the Dakota, the lower Muddy, and the upper Muddy. Entrapment of oil in each of these producing zones is the result of updip pinchout or facies change from porous and permeable sandstone to nonreservoir shale, siltstone, or sandstone, assisted by a favorable hydrodynamic environment.

Each of these sandstone reservoirs occupies a flank position around and across the east plunge of the Big Muddy anticline. Isoliths of permeable Dakota sandstone in the producing area and the regional pattern of sandstone distribution, together with the lithologic characteristics of the Dakota sandstones, suggest deposition within northeasterly flowing rivers that drained an incipient Big Muddy uplift and emptied into the sea near Glenrock. Sand delivered to the sea was distributed along a northwest-trending shoreline by relatively low-energy, destructive, marine processes accompanying the upper Dakota transgression. The Dakota pool at Big Muddy-South Glenrock has a continuous oil column more than 2,500 ft in length and an anomalous, inclined oil-water contact.

The lower Muddy sandstone pool also appears to be a single, continuous reservoir with a vertical oil column of at least 2,500 ft. This sinuous sandstone reservoir trends northeast along the southeast flank and around the east plunge of the Big Muddy anticline and has the classic meander morphology of a fluvial river deposit. The physical dimensions of this northeasterly flowing river are comparable with the upper reaches of the present Mississippi River. The influence of probable structural growth along the Big Muddy axis on the radius of meander curvature and depth of the scour channel in the lower Muddy river is particularly evident.

In the upper Muddy sandstone interval 2 nearly parallel sandstone pools trend essentially north-south across the east-plunging Big Muddy axis, and have the typical lithologic and morphologic characteristics of marine shoreline or barrier-bar deposits. Each pool has a vertical oil column of about 1,500 ft.

Regional mapping of the potentiometric surface of the 3 Lower Cretaceous producing intervals and the unusually long oil columns in each of the Lower Cretaceous sandstone pools at Big Muddy-South Glenrock indicate that a favorable downdip hydrodynamic flow must exist in the area of oil accumulation and must be enhancing the oil-holding capacity of the updip barrier zones.

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