Oil is produced from lenticular sandstone bodies in the Tyler Formation of Pennsylvanian age in central Montana. The source of this oil is believed to be petroliferous limestone strata in the underlying Heath Formation of Mississippian Age. Thhe contact between the Tyler and the Heath is unconformable and favorable relations for oil accumulation occur where sandstone of the Tyler is in beveled contact above inclined limestone beds of the Heath as in the Sumatra, Stensvad, Ivanhoe, Keg Coulee, Bascom, Melstone, and Big Wall fields

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