The Little Beaver, Badger Creek, and Middlemist fields located on the eastern flank of the Denver basin produce oil and gas from partly sorted, lenticular channel sandstones modified by wave action. These permeable sands lie within more or less continuous, impermeable, gray, impure, sandstone and siltstone zones deposited in a shallow marine shelf sea during the Cretaceous period. Structural closure is almost absent and oil is found on small structural terraces.

The producing sands may have originated as channel deposits on an irregular sea bottom resulting from minor tectonic movements, compaction over old erosion surfaces, or from slumping as a result of solution or flowage in the Permian evaporite section below. Local low areas on the sea floor were filled with channel sands later partly redistributed as bar-like features by wave action. Overlying Upper Cretaceous shale sediments were differentially compacted above these local sand lenses. Tectonic movements in the late Cretaceous and early Tertiary affected the entire Cretaceous section and modified previously formed minor Cretaceous structural elements. Shallow structural markers reflect structure present on the top of the producing sands.

Five critical isopachous intervals in addition to structural data are important in localizing producing sand lenses. These structural and isopachous criteria may be useful in extending present producing fields and in finding other lenticular sands in the Denver basin.

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