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Composition of Crude Oil and Its Relation to Stratigraphy in Wyoming: Abstract
Although the crude oils of Wyoming vary widely in their composition, nearly all of them can be classified into two major groups on the basis of the average composition of oils from each formation. These two groups of oils are found in rocks representing different environments of deposition. The high-sulphur, low-gasoline, aromatic-naphthene base crudes are associated with thick limestone, dolomite, and shale sequences which were deposited in broad shallow seas under conditions of prolonged crustal stability. The low-sulphur, high-gasoline, paraffin-naphthene base crudes are associated with sand-shale sequences deposited in areas of moderate tectonic activity with high rates of subsidence and deposition. Differences in oils within a formation are correlative with differences in environment of deposition and with depth of burial. In the Pennsylvanian-Permian formations of Wyoming the oils associated with the dolomite-evaporite facies at the east are more aromatic and naphthenic than the oils associated with the dolomite-shale facies at the west. Within the Frontier formation the oils from areas of low sand-shale ratios contain more paraffins and less sulphur than oils from areas of high sand-shale ratios. Oils from depths greater than 8,000 feet in the Tensleep sandstone of Wyoming generally contain more than 50 percent gasoline, whereas oils from depths less than 3,000 feet in the sand generally contain less than 25 percent gasoline. However, there is no change in the paraffin content of the heavier fractions of the Tensleep oils with depth. Also, there is no correlation between oil composition and depth for oils from all the Wyoming formations. No correlation was found between the composition of the Wyoming oils and their age, or the possible catalytic activity of their reservoir rocks.
Acknowledgments and Associated Footnotes
1 Carter Oil Co., Tulsa, Oklahoma
Copyright © 2006 by the Tulsa Geological Society