StephenStephen C.C. Ruppel,Ruppel, SUBREGIONALSUBREGIONAL DEVELOPMENTDEVELOPMENT OFOF RESERVOIRRESERVOIR POROSITYPOROSITY ATAT AA MAJORMAJOR PERMIANPERMIAN UNCONFORMITY:UNCONFORMITY: BureauBureau ofof EconomicEconomic Geology,Geology, SANSAN ANDRESANDRES FORMATION,FORMATION, WESTWEST TEXASTEXAS TheThe UniversityUniversity ofof Texas,Texas, Austin,Austin, TX,TX, 78713-892478713-8924 ABSTRACTABSTRACT Most porosity development in carbonate reservoirs can be attributed in part or in total to diagenesis. This is especially true in Permian reservoirs of the Permian Basin, nearly all of which are dolomitized. Linking this diagenesis and associated porosity to unconformities representing major falls in sea level, INTRODUCTIONINTRODUCTION STRATIGRAPHYSTRATIGRAPHY ANDAND FACIESFACIES however, is commonly problematic using typical reservoir data sets. New subsurface data from the San Andres Formation, a major oil-producing reservoir in Texas and New Mexico, coupled with relationships previously defined from SANSAN ANDRES-GRAYBURGANDRES-GRAYBURG STRATIGRAPHYSTRATIGRAPHY ANDAND FACIESFACIES CYCLECYCLE STRATIGRAPHYSTRATIGRAPHY ANDAND FACIESFACIES ARCHITECTUREARCHITECTURE equivalent outcrops, illustrate both the style and expression of such porosity SANSAN ANDRESANDRES STRUCTURESTRUCTURE FUHRMAN-MASCHOFUHRMAN-MASCHO FIELDFIELD Fuhrman-MaschoFuhrman-Mascho FieldField Fuhrman-MaschoFuhrman-Mascho SanSan AndresAndres Field,Field, AndrewsAndrews County,County, TexasTexas development. MIDDLE PERMIAN SEQUENCE STRATIGRAPHY ty Northwest Southeast MIDDLE PERMIAN SEQUENCE STRATIGRAPHY i -1100 PSL A-43 PermianPermian BasinBasin Gamma ray (API) Neutron porosity (%) PSL A-47 Depth Although the San Andres is characterized by an upward-shallowing succession Oil FMU 12A FMU 211 FMU 607 FMU 605 FMU 705 FMU 806 (ft) WFMU 110 WFMU 109 aturation WFMU 124 GR N GR N GR N GR N High Western Central Western 0 100 30 20 1010 0 -10 s GR N GR N of outer- to inner-ramp carbonate lithofacies in most platform settings, pervious Relative Algerita Western Permeabil GR N GR N ft GDP ft ft ft ft freq. Delaware Basin Midland S W P G SGR N PSL A-42 sea level Escarpment Escarpment SGR ft ft ft studies have demonstrated that the section is broken by a major unconformity Series seq. Fall Basin Platform Basin ft 4250 4200 4250 G16 Queen Queen Queen 4250 4250 D D that represents a sea-level fall of at least 100 m and a hiatus of 0.5 to 1 my. D 4150 4150 G15 4200 -1200 Grayburg Grayburg Cherry Grayburg Grayburg FUHRMAN-MASCHO FIELD G14 Fm. Grayburg A Canyon 4300 A Both the evidence and the result of this sea-level fall event are clearly expressed G13 upper Cherry Canyon upper upper G12 San Andres San Andres San Andres CGR in the San Andres reservoir at Fuhrman-Mascho field. Although cryptic in some Z G11 CGR 4300 Z core sections, the hiatus is indicated by an abrupt shift from outer ramp G10 4300' G9 Grayburg Brushy Brushy 1 deposits, composed of open marine fusulinid wackestones and packstones, to G8 Canyon Canyon GUADALUPIAN G7 equivalent exposed tidal flat deposits. Porosity is developed in solution-enhanced vertical upper San Andres -1100 composite sequence G6 2 -1200 burrows below the unconformity. Although highly heterogeneous, the porosity G5 G4 in this burrowed zone reaches 15 m in thickness and extends for many G3 -1300UNIVERSITYBLOCK LANDS 9 G2 Cutoff Cutoff McKnight 3 lower lower cycles Upward-shallowing kilometers. The widespread porosity development at this unconformity is a Guadalupian HFS 12-13 Upper San Andres Fm. G1 San Andres lower UNIVERSITY LANDS San Andres potential new target for both field exploitation and regional exploration BLOCK 10 L8 upper -1200 San Andres Holt comp. sequence comp. operations. San Andres lower L7 Victorio Pk Bone -1100 L6 lower Spring Glorieta Glorieta -1100 Yeso LEONARDIAN Victorio Pk Clear Fork Clear Fork Structural high STUDY QAb3743c -1000 (>-1150') AREA Hiatus 4400' 4600 3. Comprehensive study of the San Andres and Grayburg Formations in outcrop in the Guadalupe 4600 EMMA FIELD 0 10,000 ft 4592 Mountains by Kerans and co-workers shows that the San Andres platform carbonate succession is 0 300 m divided by a major uniformity representing at least 100 m of sea-level fall. QAc1874c C.I. 50 ft 4700 1700 ft Guadalupian HFS 1-4 2. Structurally, the field exhibits a local relief of about 4650 Lower San Andres Fm. Lower 350 ft. Mapping of the McKnight shale, below, shows 4700 1300 ft Cochran Hockley Lubbock Crosby Dickens that the early San Andres paleotopography bears no 4700 3000 ft 2600 ft Study 4700 2700 ft relationship to current structure. 4700 area NORTHERN SHELF NEW Slaughter Depth MEXICO 1700 ft 1900 ft 1800 ft ft m 0 0 TEXAS Yoakum Terry Lynn Garza Kent upper San Andres MEXICO Inner ramp–tidal flat Ramp crest grain– Core Peritidal tidal flat mud-dominated facies dominated packstone–grainstone Gaines LOWERLOWER SANSAN ANDRESANDRES PALEOTOPOGRAPHYPALEOTOPOGRAPHY North South Vacuum FUHRMAN-MASCHOFUHRMAN-MASCHO FIELDFIELD Western Delaware LSA high porosity/ Middle ramp peloidal– Outer ramp fusulinid Hanford Dawson Borden Scurry Fisher Algerita Escarpment Brokeoff Mts 4500' permeability Siltstone–sandstone 30 Guad. 13 Escarpment Mts skeletal wackestone–packstone wackestone–packstone 100 QAc1877c upper Guad. 12 West EASTERN Seminole Seminole PSL A-43 San Andres MIDLAND North PSL A-47 Guad. 4 Robertson Gaines Depth 6. Cross sections through the field demonstrate (1) long correlation lengths of Grayburg cycle-base siltstones, (2) poor continuity and predictability of upper San Andres cycles and Martin Howard component facies, and (3) major facies offset marking the unconformity at the upper San Andres–lower San Andres boundary. Outer ramp fusulinid wackestones at the top of the 0 20 40 mi Means Mitchell Nolan Guad. 3 Guad. 4 Andrews ft m LSA are sharply overlain by pisolitic, tidal-flat facies of the USA suggesting partial erosional truncation of the LSA. PSL A-42 Guad. 1 50 0 20 40 60 km Martin Fuhrman-Mascho lower Guad. 2 15 CENTRAL San Andres Leon. 8 Mabee SHELF Emma Leon. 7 NEW MEX Lea FUHRMAN-MASCHO FIELD Ector Goldsmith Glasscock Sterlng Coke TEXAS BASIN BASIN Foster Harper ft Loving m Reeves Winkler Midland LITHOSTRATIGRAPHIC Penwell NOMENCLATURE 1000 300 Ward Upton Reagan Tom Green 0 0 Waddell Shallow Water San Andres Formation Guad. 13 4600 PLATFORM Dune Irion Inner Ramp STUDY platform carbonates DELAWARE McElroy AREA Upper Victoria Peak Formation platform carbonates Pecos Guad. 5 Lower Cherry Canyon Formation 5. The reservoir section at Fuhrman-Mascho comprises an overall upward- Cutoff Formation shallowing succession of lower San Andres, upper San Andres, and Grayburg 5 10 mi Crockett Schleicher deep ramp mudstones deposits. UNIVERSITY LANDS San Andres lower OZONA BLOCK 9 Brushy Canyon Formation 0 UNIVERSITYBLOCK LANDS10 BASIN -2200 0 10 km Grayburg Formation deposits are composed of restricted inner platform cycles N ARCH Guad. 1- High-frequency sequence After Kerans and Ruppel (1994) 5-11 Guad. made up of siltstone-rich bases and tidal-flat caps. Unconformity Sutton Terrell -2100 Upper San Andres (USA) rocks comprise a complex cyclic succession of inner Top of -2000 platform, shallow water subtidal carbonates (dominantly mud-dominated McKnight -2000 packstones), and cycle capping exposure tidal-flat facies. Cored well QAa1719c Peloidal grain- San Andres/Grayburg Siltstone–sandstone Gypsum Lower San Andres (LSA) rocks are dominantly outer ramp fusulinid fields ≥ 10 MMbbl production 4. Lower San Andres carbonates comprise six Leonardian and Guadalupian high-frequency sequences (L7-L8, G1-G4), dominated packstone QA12908(a)c Deeper Water wackestones with local capping peloid/ooid grain-dominated packstones and whereas the upper San Andres contains two high-frequency sequences (G12 and G13). During the long mid-San Mudstone Tidal flat Cycle set EMMA FIELD 0 10,000 ft grainstones. Outer Ramp Andres lowstand platform hiatus, deposition was limited to lowstand sandstones of high-frequency sequences G5 Peloidal wackestone– Fusulinid packstone High-frequency 0 300 m - G11 (Brushy Canyon Formation). packstone cycle boundary QAc1876c 1. The Fuhrman-Mascho field is located on the Central Basin Platform, an early Permian constructional QAc1875c C.I. 100 ft platform. During San Andres deposition, the field area lay on a generally eastward-dipping carbonate ramp. Bureau of Economic Geology QAc?? c.