As the cumulative production of oil from the Big Horn basin approaches the 2 billion bbl mark, it is appropriate that we look at the geology once again.

The Big Horn basin, located in northwestern Wyoming and south-central Montana, is bordered on the east by the Big Horn Mountains; on the south by the Owl Creek Mountains; on the west by the Absaroka and Beartooth Mountains; and on the north by the Nye-Bowler lineament. The combination of a great reservoir-source duet in the Paleozoic rocks and the creation of large anticlines during the Laramide orogeny has been the key to the Big Horn basin’s success as an oil producer.

Stratigraphy of the Big Horn basin can be divided generally into (1) the Middle Cambrian clastics, (2) the Paleozoic shelf carbonates, (3) the Mesozoic clastics, (4) the Late Cretaceous to Tertiary synorogenic clastics, and (5) the Tertiary post-orogenic clastics and volcanics.

By far the most economically important formations have been the Permian Phosphoria and Pennsylvanian Tensleep Formations. This dynamic duo consists of porous eolian and shallow marine, quartz sandstones of the Tensleep overlain by shallow marine, oil-rich carbonates of the Phosphoria. The two have combined to produce over 1.5 billion bbl of oil in the Big Horn basin alone.

The draping of the Tensleep and Phosphoria over large Laramide structures (closures of over 5,000+ ft, 1,500 m, and areal extents up to 15 mi2, 40 km2) was the final key. An upcoming afternoon session of papers will explore the proposed anatomies and mechanisms of folding in the Cordilleran foreland.

There is a controversy over the morphology of the folds in the foreland. Put simply, there is some disagreement over how much the horizontal or thrusting component contributes to these folds. Stearns, who described the fold at Rattlesnake Mountain in the 1971 Wyoming Geological Association guidebook, favors drape folding over near-vertical faults in the Precambrian. Berg in the 1962 AAPG Bulletin and Gries in the 1983 AAPG Bulletin favor a more thrusted model with reverse fault planes dipping 60° to 30°. Sales in the 1968 AAPG Bulletin agrees with the morphology of Berg and Gries, but feels that the folds in the foreland were generated by faults with strong lateral components.

Concerning other problems, Stone in the October 1967 AAPG Bulletin pointed out that the Paleozoic reservoirs often have a common oil-water contact (OWC) within individual structures. He attributed the common OWC to fractures joining the reservoirs. The OWC is commonly tilted; this he attributed to hydrodynamic flow. An understanding of fractures and tilted oil-water contacts is imperative for successful exploration and production programs in the Big Horn basin.

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