About This Item
Share This Item
The San Andres Formation of Permian age (late Leonardian-early Guadalupian) is the most prolific producer of hydrocarbons within the Permian basin. The Levelland-Slaughter trend, located in the northwestern shelf province, is a series of east-west-trending fields characterized by stratigraphically controlled, updip porosity pinchouts from porous dolomites to nonporous dolomites and evaporites. Equivalent facies crop out in Chaves and Lincoln Counties, New Mexico.
Outcrop study and examination of subsurface data from shallow pay zones within the Diablo, Linda, and Twin Lakes fields, indicate that a variety of high- and low-energy subtidal facies serve as reservoirs. These include (1) oolite packstones and grainstones, (2) wispy-laminated crinoid wackestones, (3) fossiliferous wackestones, and (4) ripple-laminated pellet grainstones. These facies were deposited as a mosaic in channels, lagoons, shoals, and shallow open-marine environments. Vertical and lateral facies relationships are therefore highly variable and play a major role in reservoir heterogeneity.
Secondary porosity is wholly responsible for oil production, although the depositional setting controls its type and abundance. Intercrystalline porosity is characteristic of the mud-rich facies and is crucial for economic production. Sucrosic textures associated with coarse crystalline dolomite and large intercrystalline pores provide the highest production potential. Moldic porosity is characteristic of the oolitic and fossiliferous facies, but pores are disconnected and only marginally productive unless combined with fractures.
Producing intervals are sealed by tight dolomite mudstones and a thick (5-10 ft), laterally continuous bed of nodular mosaic anhydrite. Unequivocal evidence for supratidal sabkha deposition of this lithofacies is absent. The location of the sealing anhydrite and the presence of high intercrystalline porosity in the underlying subtidal facies are the most important factors determining San Andres production.
End_of_Article - Last_Page 253------------