Recent oil and gas discoveries in sandstone lenses of the Gallup formation or its stratigraphic equivalent in the San Juan Basin have stimulated subsurface exploration in a search for similar stratigraphic traps. The purpose of this article is to discuss briefly the Gallup formation of upper Cretaceous age and its gradation from a porous massive sandstone on the southwest flank, into a block calcareous shale on the northeast side of the San Juan Basin.