Deposition, Porosity Occurrence, and Reservoir Properties of San Andres Formation at Suniland Field, Lynn County, Texas
James L. Beaver, Jr.
Suniland field is located in the west Texas Permian basin. Production is primarily from peritidal shelf dolomites of the Permian (Guadalupian) San Andres Formation. San Andres deposition was highly cyclical, with at least two upward-shoaling megacycles comprising the lower 600 ft (183 m) of the formation; many smaller cycles occur within the megacycles. The field consists of an elongate, low-relief, east-west-trending anticline formed from draping over a pre-San Andres structural high. Hydrocarbon trapping is controlled primarily by structural closure and discontinuity of San Andres reservoir facies. Anhydrite cement precipitation also may cause an updip (northerly) permeability loss. Reservoirs occur in thin, permeable oolitic grainstone intervals possessing primary inte granular porosity in combination with vugular and moldic pore systems. These grainstones interfinger with porous but generally impermeable oolitic packstones. Adjacent wackestones and mudstones are generally nonporous and impermeable. Porosity occlusion is primarily from precipitation of late dolomite cements. Anhydrite cements do not occlude porosity to a high degree.
Oil production from the San Andres Formation at Suniland field is from nine separate reservoirs comprising two major pay categories, designated San Andres and lower San Andres. Pay depths range from 3,755 to 4,211 ft (1,144 to 1,283 m). Cumulative oil production to January 1986 is 7.35 million bbl from 69 wells. Permian (Leonardian) Glorieta production accounts for about 16% of total production. Reservoirs produce from solution gas expansion at low gas-to-oil ratios and high water cuts. Effective porosity in pay zones averages 14%; permeability averages 17 md.
AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.