Yeso rocks contribute a high percentage of the oil produced in the Permian basin. The Yeso was deposited as three distinct facies in late Leonardian time. The shelf deposits of slightly anhydritic dolomite are divided into the Glorieta and the upper and lower Clearfork rock units; the shelf-edge deposits around the basin margins consist of massive reef-like dolomite; and the basin deposits are predominantly

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shale, sandstone, siltstone, and limestone of the Spraberry Formation.

The basin sediments appear to be ideal petroleum source rocks and limited fracture development restricts the occurrence of basin oil to rock of Permian age. Shelf or platform oil may be mixed Permian and pre-Permian due to structural deformation, faulting, fracturing, and deposition across truncated surfaces.

Correlations of oil analyses with reservoir rock types suggest that differences in the chemical composition of basin and shelf oils may be related to the amount of evaporites in the reservoir rocks.

Oil groupings can be explained geologically through differences in sources, environment, paths of migration, or mixing of oils.

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