Wyoming, long the stronghold of anticlinal occurrence of petroleum, is now yielding to the stratigraphic trap. South Glenrock, a 1950 discovery on the southwestern margin of the Powder River basin, has developed a 50-million-barrel reservoir in Lower Cretaceous sands of the Dakota and Muddy zones, without the benefit of structural closure.

The term "Dakota" for the lower sand, fits current usage, but the sand is at a different stratigraphic level from other sands of the Wind River basin on the west, also called "Dakota." Dakota oil at South Glenrock is trapped by a westward facies change on a large eastward plunging nose off the prominent anticlinal structure of the Big Muddy oil field. Muddy oil is found in lenticular sands associated with prominent channeling at a point of unconformity, 100 feet above the Dakota zone.

South Glenrock structurally is integrally related to the Big Muddy anticlinal oil field, but produces from different sands. The latter exhibit depositional evidence of being influenced by an ancestral structural feature which was a pre-Laramide incipient "high" at the locale of South Glenrock. This was offset from the younger Laramide anticline which afforded structural accumulation at Big Muddy.

A close study of older sedimentation in other Wyoming localities probably will point to an earlier structural history of local uplifts. The ancestral structures may be offset from their more robust Laramide progeny.

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