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Petroleum Geochemistry and Paleontology Establish Age Relation of Tyler Formation in Central Montana and North Dakota


In August 1971, my exploration effort at Shell Oil Company, Denver was focused on the Tyler Formation production in central Montana and North Dakota. An earlier effort by Shell geologists interpreted the Tyler Formation in North Dakota to be a series of cyclothems. My effort was directed to the Tyler in both places to see if there is any way to focus Shell's exploration effort in Central Montana? In 1972, I attended the AAPG Annual Convention in Denver when Wally Dow of Amoco presented his paper on “The Application of oil correlation and source rock data to exploration in the Williston basin.” On the basis of paleontology and oil-source rock correlation, the Tyler Formation of central Montana can be differentiated from the Chester-Morrow cyclothems of North Dakota. The age of these units had been a long-standing problem. However, J. F. Meacham using conodonts and L. D. Holcomb using foraminifera demonstrated that the Tyler in Montana is Chester in age, whereas the cyclothems in North Dakota are Chester-Morrow. The oil-source rock correlation is based on the composition of hydrocarbons in the low boiling range (<114 0C). The triangular graph for both the oil and rock data of the Tyler-Heath in central Montana and the Chester-Morrow cyclothems in North Dakota illustrates these two geochemical types. The three end members are straight-chained C7 paraffins, branched C7 paraffins, and C7 napthenes or cyclic staturates. The Tyler-Heath oil-rock group is designated napthenic because it tends toward the C7 napthene end member. Similarily, the oil and rock samples from the Chester-Morrow cyclothems of North Dakota are paraffinic because they tend toward the straight-chained paraffins. None of these oil samples are napthenically transformed or high in sulfur. In this basis, Shell focused its exploration in central Montana.