Dolomitization and anhydrite diagenesis of Permian San Andres Formation, Central basin paltform, west Texas

Conference · Fri May 01 04:00:00 EDT 1987 · AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States)

OSTI ID:5865885

Vogt, J N

Normal-marine, subtidal carbonate facies of the San Andres Formation on the Central basin platform in west Texas are completely dolomitized and contain several types of accessory anhydrite. Oxygen isotopes indicate pervasive dolomitization occurred by reflux of CaSO/sub 4/-saturated brines. Strontium isotopes and electron microprobe analysis suggest that anhydrite precipitated early, probably as a by-product of dolomitization, and has undergone subsequent alteration by strontium-depleted, radiogenic fluids. Unaltered dolomite is generally a gray, nonporous wackestone containing anhydrite-filled fusulinid molds. Altered dolomite is light brown, porous, and contains fusulinids that are generally well preserved. Around anhydrite nodules, however, the altered dolomite is fabric destructive. Porosity is mostly intercrystalline and intraparticle. X-ray diffraction data indicate that both dolomite types are well ordered. Electron microprobe analysis of the unaltered dolomite indicates that it is stoichiometric. Oxygen isotopes for the unaltered dolomite average delta/sup 18/O = + 4.7 per thousand (PDB). Assuming near-surface conditions, the unaltered dolomite formed from a fluid enriched about 4 per thousand in /sup 18/O relative to Permian (Guadalupian) sea water, suggesting a hypersaline origin for the dolomitizing fluid. CaSO/sub 4/-saturated brines originating from an overlying anhydrite/dolomite sequence were the likely dolomitizing fluids. The altered dolomite is depleted by about 1.3 per thousand in /sup 18/O relative to the unaltered dolomite, indicating stabilization at higher temperatures and/or by an isotopically depleted fluid. Anhydrite probably precipitated as calcium was released to CaSO/sub 4//sup -/ saturated brines during dolomitization. Anhydrite occurs as replacement nodules, interparticle cement, and as a replacement cement.

• Research Organization: Shell Western Exploration and Production, Inc., Houston, TX
• OSTI ID: 5865885
• Report Number(s): CONF-870606-
• Conference Information: Journal Name: AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States) Journal Volume: 71:5
• Country of Publication: United States
• Language: English

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CALCIUM SULFATES
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CARBONATE MINERALS
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