The West Texas (Permian) Basin is a complexly structured intracratonic basin with prolific oil and natural gas
production from conventional reservoirs and from thick, prolific organic
mudrocks. The basin has two phases of subsidence; a lesser phase in Ordovician-Devonian time ('Tobosa basin'), and the main phase in Pennsylvanian and Permian time (the 'Permian basin' proper). It is built on diverse Proterozoic crustal units, including a ca. 1120-Ma layered mafic complex that may be related to basin
origins. The crust was fractured during Cambrian rifting of the southern margin
of North America; the pattern of this fracturing in the deep Delaware Basin is obscure but may have governed late Paleozoic
and folding occurred in the Pennsylvanian to early Permian, as part of the
Ancestral Rocky Mountains (ARM) orogen. In the basin, the ARM contains a variety of small to medium basement- cored uplifts, folds, thrust
faults, and two trends of strike-slip faults, with an overall pattern that is
consistent with SW–NE compression. This SW–NE compressive stress could not have originated from the
northwestward convergence of the Ouachita-Marathon thrust belt southeast of the ARM, but may have
originated either from the Pacific (by flat subduction) or from strong
continental collision in the Appalachian Orogen. Lines of weakness generated
during the Proterozoic and/or Cambrian concentrated stress and created the complex structures.
The West Texas branch of the ARM is buried by more than 2.5 km of post-deformational Permian strata—the Permian Basin. Subsidence began during ARM
deformation, then increased in rate and continued to the end of the Permian.
Permian subsidence resulted in the maintenance of the deep marine Midland and Delaware basins that were not filled in until Late Permian time. These basins were only connected to the world ocean by narrow and relatively shallow straits. The subsiding basins were poorly oxygenated, and thus were able to accumulate great thicknesses of organic mudstone and other basinal sediment. Despite Mesozoic basin-margin modifications to the east, south, and southwest, the Permian subsidence shows a bowl-shaped pattern centered on the Central Basin axis of uplift. This axis is the site of the Pecos mafic complex, which was subjected to compression in Pennsylvanian time. Sinking of a mafic crust or
its subjacent lithosphere, begun during compression, may have been a driving
force for Permian subsidence.
The Permian subsidence was also responsible for putting source rocks into the oil window in the Midland and northern
Delaware basins. Further maturation to gas occurred within the deep basins
generated by ARM deformation and Marathon thrust loading.