Near the close of Desmoinesian time, regional uplift toward the west elevated the Tensleep of the Big Horn basin above sea-level. Broad, low-relief, northeast-trending folds developed during this orogenic uplift. Streams entrenched a well-developed drainage pattern on the exposed Tensleep surface and furnished sediment to the upper portion of the Minnelusa in the east and southeast. During Wolfcampian time, the Phosphoria sea transgressed the area and the incised stream channels were filled with shale and re-worked Tensleep sandstone; later Phosphoria deposition onlapped post-Tensleep hogbacks and low hills.

The majority of the oil that has been produced, and that will undoubtedly be produced, from the Tensleep has been from traps which are structurally controlled. However, accumulation in a significant number of these traps is the result partly or wholly of three stratigraphic variables; (1) an intra-formational change in permeability and(or) lithofacies providing a facies trap; (2) incised channeling with later infilling of basal Phosphoria shale providing a truncational subcrop trap; and (3) a combination of (1) and (2) above with later Laramide anticlinal folding superimposed on or near these primary traps, an effect which commonly causes the effect of tilted oil-water contacts. Either this type of tilt was not great enough to cause secondary migration farther into the fold or the dow dip flow of ground water caused a "tar seal" to be formed at the oil-water interface and froze the oil in place.

There seems to be a depth-temperature-porosity relationship in the Tensleep. Thus far in the Big Horn basin, porosities are known to decrease progressively with increasing depth and temperature. Siliceous overgrowths form on the rims of the quartz grains, because of the increased compaction load and temperature, thereby reducing primary porosities. Ground water invading the Tensleep at shallower depths will also cause a similar phenomenon; therefore, the Tensleep will not always be porous even at shallow depths. Possibilities for finding adequate porosity at greater depths will be enhanced in

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the future by exploring in those areas more favorable to the accumulation of oil in primary traps that have not been modified greatly by Laramide folding. Oil initially in place may retard secondary silicification of the Tensleep sandstones.

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