The Manderson field on the Manderson anticline, which is a plunging structural nose about 12 miles long, is located on the eastern flank of the Big Horn basin in Big Horn County, Wyoming.

Seismic and subsurface information indicates no effective structural closure along the axis of this anticline to form a trap for the accumulation of oil and gas.

The facies change in the Phosphoria formation, from carbonates to redbeds takes place in an easterly direction across the southern end of the Big Horn basin.

By establishing a chert horizon as the dividing line between the productive and non-productive zones of the Phosphoria formation, the problem of locating stratigraphic traps is simplified. Isopach

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maps of the productive part of the Phosphoria formation show the relation of the Manderson field to this stratigraphic trap.

The reservoir in the Manderson field is a stratigraphic trap formed by the facies change in the Phosphoria formation.

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