Reexploration of old fields in mature hydrocarbon provinces, such as the Permian Basin of West Texas and New Mexico, may significantly increase known oil and gas reserves. Data from old wells are being reevaluated in light of current oil and gas prices using new geologic and engineering reservoir models, in discovery of untapped hydrocarbn-producing zones. The San Andres and Grayburg (Guadalupian) Formations in the Penwell, Jordan, Waddell, Dune, McElroy (PJWDM) field complex exhibit a regional depositional pattern of basinward (eastward) progradation, resulting in en echelon reservoirs with Grayburg reservoirs located basinward of San Andres reservoirs. Reservoir facies comprise porous dolomitized grainstones and packstones in open-marine facies rocks within upward-shoaling carbonate cycles. Nonporous tidal-flat facies thicken toward the platform (westward) and form an updip seal. Current production at Penwell and Jordan fields is from the San Andres reservoir; current production at Waddell, Dune, and McElroy fields is mainly from the Grayburg reservoir. The Grayburg oil reservoir at Dune field contains a gas cap. Three wells on the east flanks of Penwell and Jordan fields in West Texas have demonstrated gas production from the Grayburg Formation. Because this gas production was not commercial at the time of discovey, the wells were not depleted and were subsequently plugged. At current prices, however, the gas would have been profitable. This Grayburg gas production may be from a gas cap over downdip oil, and the presence of downdip oil in the Dune field Grayburg reservoir suggests a potential untapped oil reservoir on the east flank of Penwell and Jordan fields. Edwards, York, and Harper Extension fields lie basinward (east) of Penwell and Jordan fields and occur along strike with the Grayburg reservoir at Dune and McElroy fields. Edwards, York, and Harper Extension fields have structural closure at the Grayburg stratigraphic level, and Edwards and Harper Extension fields are currently producing oil from the Grayburg Formation. York field produces oil from an Ellenburger reservoir and is a prospect for Grayburg oil production.