Diagensis of Permian (Guadalupian) San Andres Formation, Central Basin Platform, West Texas

David A. Leary, Jay N. Vogt

Reflux of Guadalupian brines is interpreted to have been the dominant mechanism of dolomitization of San Andres subtidal carbonates on and near the Central Basin platform. Other important diagenetic processes include anhydrite emplacement and a late stage of diagenesis linked to sulfate reduction.

Supratidal dolomite has an average ^dgr¹⁸O of +4.5 ± 0.5 ^pmil (PDB, n = 9), suggesting stabilization in seawater evaporated to gypsum saturation. The strontium isotopic signature (⁸⁷Sr/⁸⁶Sr) for these dolomites is 0.7068 ± 0.0001 (n = 2) in the upper San Andres and 0.7071 ± 0.0001 (n = 2) in the lower San Andres, reflecting differences in the timing of dolomitization of these strata. Because these dolomites have strontium isotopic ratios that correspond with the depositional ages predicted from a strontium seawater curve from the Palo Duro basin, supratidal dolomitization is interpreted to have occurred penecontemporaneous with sedimentation.

The ^dgr¹⁸O of intertidal dolomite averages +3.6 ± 0.5 ^pmil (PDB, n = 12); this may indicate dolomitization by a less-evaporated seawater, although a later stabilization event is also possible. Gray, nonporous fusulinid wackestones typically have ^dgr¹⁸O values that average +4.7 ± 0.5 ^pmil (PDB, n = 15), again suggesting formation in the presence of gypsum-saturated brines. Patchy, brown, porous subtidal dolomites have ^dgr¹⁸O values that are about 1.6 ± 0.9 ^pmil (PDB, n = 11) less than adjacent unaltered samples. The lighter oxygen isotopes and intimate association of the two types of dolomite suggest the patchy dolomite recrystallized at a later time, either at a higher temperature or in an isotopically lighter fluid.

Anhydrite nodules in the subtidal facies of the lower San Andres have strontium isotopic values (⁸⁷Sr/⁸⁶Sr = 0.7071, n = 8) identical to the supratidal dolomite and anhydrite cements, suggesting early anhydrite emplacement. Because the mechanism of anhydrite emplacement is related to the release of calcium into a calcium sulfate-saturated brine during dolomitization, we conclude dolomitization in the subtidal facies was also early.

Late-stage diagenesis is marked by the association of radiogenic calcite and anhydrite, sulfur, pyrite, and authigenic quartz, kaolinite, and rare fluorite. Anhydrite cement and recrystallized and replacement anhydrite are radiogenic (⁸⁷Sr/⁸⁶Sr = 0.7074, n = 7) relative to unaltered, subtidal nodules, suggesting recrystallization in a radiogenic fluid. Radiogenic calcite [⁸⁷Sr/⁸⁶Sr = 0.7078 (n = 2), average ^dgr¹³C = -23.5 ± 4.8 ^pmil (PDB, n = 9); average ^dgr¹⁸O = -10.0 ± 0.5 ^pmil (PDB, n = 9)] was precipitated as a by-product of sulfate reduction and oxidation of hydrocarbons in meteoric water in an open system. Quartz, kaolinite, and possibly fluorite are related to the influx of the meteoric water, which is believed to have dissolved feldspars and introduced sulfate-reducing bacteria to hydrocarbon-bearing reservoir rock.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.