Abstract

In the Big Muddy-South Glenrock field area, Natrona County, Wyoming, probably 100 million bbl of oil ultimately will be produced from three separate sandstone reservoirs within the Lower Cretaceous section—from oldest to youngest, the Dakota Formation, the lower Muddy sandstone and the upper Muddy sandstone. 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 importantly 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. The regional pattern of permeable Dakota sandstone distribution, and the internal characteristics of the Dakota rocks 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 probably was distributed along a northwest-trending shoreline by relatively low-energy, destructive, marine processes. The Dakota oil pool at Big Muddy-South Glenrock has a continuous oil column of more than 2,800 ft 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 3,000 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 river deposit. The physical dimensions of this northeasterly flowing river are comparable with the upper reaches of the present Mississippi River. Incipient structural growth along the Big Muddy axis may have produced the local decrease in the radius of meander curvature and increase in the depth of the scour channel of the lower Muddy river where it crossed the paleogeographic high.

In the upper Muddy sandstone interval, at least two nearly parallel sandstone pools trend essentially north-south across the east-plunging Big Muddy axis, and display 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 three Lower Cretaceous producing intervals and the presence of unusually long oil columns in each of the Lower Cretaceous sandstone pools at Big Muddy-South Glenrock indicate that a favorable, downdip hydrodynamic flow exists in the area of oil accumulation and has enhanced the oil-holding capacity of the low-displacement-pressure, updip barrier zones.

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