The lower San Andres Formation (Permian), Levelland and Slaughter fields, northern shelf of the Midland Basin, west Texas
Well logs, cores, and petrographic analyses of selected wells in the Levelland and Slaughter fields have revealed that the lower San Andres Formation consists of up to five vertically repeating depositional cycles. Each depositional cycle primarily consists of a shoaling upward sequence of subtidal, intertidal, and supratidal facies. Subtidal sediments contain dolomudstones, packstones, wackestones, and grainstones with bioturbation and stylolitization. Fossil allochems include bivalves, crinoids, foraminifers, brachiopods, and lesser quantities of corals and bryozoans; nonskeletal grains include pellets, oolites, and lesser oncolites. Intertidal sediments are primarily composed of packstones, wackestones, and grainstones with laminites, burrows, and lesser quantities of algal stromatolites. Skeletal grains include bivalves, crinoids, and lesser foraminifers; nonskeletal grains include pellets and intraclasts. Supratidal sediments are typically anhydrite interlaminated with dolomite. Algal stromatolites, fenestrae (bird's eye) structures, tepee structures, desiccation cracks, pisolites, and collapse breccia occur in this facies.^ Petrographic analyses of the lower San Andres reveal the following facies: crinoidal, mudstone, spiculitic, crinoidal-bioclastic, fusulinid-bioclastic, bioclastic, pelletal, oolitic, peloidal-intraclastic, pisolitic, algal-laminated, and anhydrite.^ Petrographic and geochemical analyses have revealed that the most common diagenetic processes within the lower San Andres are as follows: (1) micritization, (2) cementation, (3) dolomitization, (4) compaction, (5) dissolution, (6) anhydritization, (7) silicification, (8) fracturing, (9) stylolitization, and (10) pyritization.^ Porosity and permeability are variable in the study area. The commonest porosity types are the moldic, vuggy, interparticle, intraparticle, intercrystalline, and minor cavernous. Anhydritization resulted in lowering primary and secondary porosities in the study area.^ Sedimentary structures indicating possible subaerial exposure surfaces are common in the study area. These include tepee structures, desiccation cracks, fenestral fabric, and breccia.^ Stable oxygen and carbon stable isotopes were tested for the first time in the study area as possible indicators for subaerial exposure surfaces. A test suite of selected samples from cycles 4 and 5 of the Mobil 360 (Levelland Unit) were analyzed for oxygen and carbon stable isotope content. Shifts in the abundance of oxygen isotopes were recognized along subaerial exposure surfaces. Comparison of the core description with stable isotopic analysis reveals that the latter may be a valuable technique in recognizing subaerial exposure surfaces in the dolomites of the lower San Andres Formation, although, further work and tighter sampling are required in order to establish the validity of this approach. ^
Abushagur, Sulaiman A, "The lower San Andres Formation (Permian), Levelland and Slaughter fields, northern shelf of the Midland Basin, west Texas" (1992). ETD Collection for University of Texas, El Paso. AAI9311278.