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Abstract:

Two cores from the Permian (Guadalupian) Yates Formation in Winkler County, Texas, were analyzed using thin-section petrography, scanning electron microscopy/energy dispersive X-ray, stable-isotope geochemistry, and 40Ar/39Ar laser step heating. The Yates was deposited in a coastal-sabkha environment. The sandstone facies is the hydrocarbon reservoir; the dolostone and anhydrite facies are impermeable. Sandstones are very fine- to fine-grained arkoses, subarkoses and lithic arkoses. The major authigenic phases in the sandstones are corrensite and dolomite. Mg-rich sabkha-based pore fluids were responsible for their formation. Other modifications to the sandstones include K-feldspar and quartz overgrowths, and unstable-grain dissolution.

The clay-mineral suite in the Yates Formation consists of corrensite, illite and chlorite. Corrensite (Rl ordered, trioctahedral chlorite/smectite, with approximately 50% smectite layers) is found in the three major facies; however, it is most prominent in the sandstones and is absent from the interbedded black shales. Chlorite-to-smectite ratios do not vary with changes in stratigraphic position or lithology. Clay-mineral suites in dolostone and anhydrite layers, where present, are similar to those in the sandstones. Illite is more prevalent in dolostones than in sandstones, however. Elemental analysis indicates that Mg is a major component in the corrensite, although Fe is also present. The persistence of corrensite in the sandstones is interpreted as the result of relatively uniform porewater salinity and Mg levels.

Stable-isotope values were determined on dolostones and dolomite cement in sandstones. In well #269, dolostone values range from δ13C +4.97 to +5.94%o and δ18O from –1.74 to +1.94‰ PDB. In well #270, δ13C ranges from +4.46 to +5.97‰ and δ18O from +0.15 to +2.39‰. Dolomite cements in sandstones from well #269 range from δ13C +0.67 to +4.92‰ and δ18O from –1.68 to +0.91‰. In well #270, δ13C ranges from +0.97 to +3.26‰ and δ18O from –6.09 to +1.23‰.

The 40Ar/39Ar laser step-heating method was used to determine absolute ages on three clay-size mineral separates. In these samples, the clay-mineral suite consists of mixtures of corrensite, illite, and chlorite. Feldspars and mica are also present. The analyses revealed a low-retentivity phase with an apparent age of 275 to 250 Ma, which may represent the age of corrensite formation.

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