Abstract

Reexploration of old fields in mature hydrocarbon provinces, such as the Permian Basin of West Texas and New Mexico, may significantly increase oil and gas reserves. Reevaluation of data from old wells in the context of current oil and gas prices and using new geologic and engineering reservoir models can result in discovery of untapped hydrocarbon-producing zones. Reservoirs of the Permian (Guadalupian) San Andres and Grayburg Formations are the most prolific hydrocarbon producers of the Permian Basin and provide a vast data base for reexploration.

The San Andres and Grayburg Formations in the Penwell, Jordan, Waddell, Dune, and 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 deposited under open-marine conditions 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 principally 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 have demonstrated gas production from the Grayburg Formation. Because this gas was not commercial at the time of discovery, the wells were not depleted and were subsequently plugged. At current prices, however, the gas could be profitable. Grayburg gas may indicate a gas cap over downdip oil. The presence of downdip oil in the Dune field Grayburg reservoir suggests the existence of an untapped oil reservoir on the east flank of Penwell and Jordan fields.

York field lies east (basinward) of Penwell and Jordan fields and produces oil from an Ellenburger reservoir. York has structural closure at the Grayburg stratigraphic level and is on strike with proven Grayburg production at McElroy and Dune fields in the PJWDM field complex as well as proven Grayburg production at Edwards and Harper Extension fields, which are east of the PJWDM field complex. Thus, York is a prospect for Grayburg production.