FACIES AND STRATIGRAPHY OF THE SAN ANDRES FORMATION (MID-PERMIAN) PETROLEUM PROVINCE, NORTHWEST SHELF OF THE PERMIAN BASIN, WEST TEXAS: A RESURGENT PLAY

World-class carbonate reservoirs of the Leonardian-Guadalupian stage San Andres Formation on the Northwest Shelf of the Permian Basin in West Texas have reemerged as a target for unconventional development. These reservoirs have produced more than 4 billion barrels of oil conventionally and still contain the greatest recoverable reserves of any carbonate reservoir in the Permian Basin. In this region, the San Andres represents a ~1,500 ft. thick section of carbonates and evaporites that were precipitated in shallow lagoon and sabkha environments on a broad, low-relief, rimmed shelf. During subaerial exposure, porosity developed in carbonate facies by reflux dolomitization, followed by the precipitation of nonporous anhydrites and porosity-occluding anhydrite cements that formed seals and traps. Low-relief structures overlie deep seated paleo highs and were prone to secondary dolomitization, forming the highly porous and permeable lithofacies of legacy oil fields. Transgressive-regressive sea level pulses and rapid progradation of the shelf resulted in cyclical stacking of porous and tight lithofacies; stratigraphic traps developed in the form of updip porosity pinchouts that offset basinward, occurring in increasingly younger strata. Updip migration of hydrocarbons from source rocks of the northern Midland Basin was facilitated by compaction drape and vertical fracturing of strata atop a lower Permian shelf margin, where hydrocarbons filled-to-spill structurally low-relief reservoirs. Laramide tectonism exposed Guadalupian strata in SE New Mexico, allowing an influx of meteoric water and subsequent lateral (downdip) flushing of oil from the lower oil column, where secondary vuggy porosity is pervasive. Architecture and reservoir properties of off-structure strata are favorable for unconventional recovery of primary oil from the transition zone, a 150-300 ft. thick interval that is characterized by a depth dependent gradational oil-water saturation, where meteoric waters only lightly swept, or bypassed, in situ petroleum. The oil-water contact, at the top of the transition zone, defines the vertical extent of the main pay zone (target for conventional development in legacy fields). Below this contact, the economic play limits of the transition zone extend beyond the flanks of legacy fields.