Many Cretaceous stratigraphic oil accumulations of Wyoming may be mapped with log resistivity, because the clay-rich edges of sandstone bodies have lower resistivity and the clay poor centers of the sandstones have higher resistivity. Although many factors affect log resistivity, variation of the clay content of a sandstone body causes several orders of magnitude change in resistivity in and around a given stratigraphic trap.

Resistivity maps generally agree with SP isopach maps, but tend to show the location of the most porous and permeable clay-poor sandstones, rather than the total sand thickness. Resistivity maps are also useful when no SP is evident outside the developed sand body or when clay fill makes SP unreliable.

Although the presence of carbonate cement, coals, and lignites, etc., complicate interpretation of resistivity maps in some formations, the tool is usually effective in simple clay-quartz formations. The tool is particularly useful during development drilling of a stratigraphic trap.

The use of resistivity mapping is illustrated with a number of Muddy and Dakota stratigraphic oil fields in the Powder River basin of Wyoming at Hilight, Recluse, South Glenrock, Gas Draw, and Coyote Creek.

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