A study of the Heath Formation (Mississippian) and the Lower member of the Tyler Formation (Pennsylvanian) in central Montana suggests a close relation between oil accumulations and the relative positions of the Limestone member of the Heath Formation and sandstone beds in the Lower member of the Tyler Formation. Reconstruction of the pre-Amsden structure and the Tyler-Heath paleogeology shows that oil accumulations at Sumatra, Stensvad, Ivanhoe, Keg Coulee, Bascom, Melstone, and Big Wall fields occur where the paleostructural position of the lower Tyler sandstone beds was updip from, and in direct contact with, the Limestone member of the Heath Formation. The foregoing relations are strengthened further by a study of Alice and Porcupine domes. These domes have barren san stone reservoirs in excellent structural-stratigraphic traps. The paleostructure in the dome area was flat and the limestone source rock was not well developed.

Application of these reservoir-source relationships should be useful in exploring for new oil reserves in the sandstone bodies of the Lower member of the Tyler Formation. The same principles should be applicable to other geologic provinces with similar problems.