DEPOSITIONAL ORIGIN OF ANHYDRITE WITHIN THE TRIASSIC THAYNES FORMATION, SW WYOMING

The Triassic Thaynes Formation in western Wyoming is an approximately 355 m thick layer of gray, red, or brownish-red mudstone that contains extensive evaporites, both anhydrite and halite. It is interpreted to have been deposited in open marine and non-marine environments during the Olenekian of the lower Triassic. In southwest Wyoming, fluid inclusion volatile analyses by ExxonMobil indicate that Triassic formations on the Moxa arch, including the Thaynes, are a confining unit for sour gas, and potentially also for future geologic carbon dioxide sequestration in deeper Paleozoic formations. This study uses Sr isotopes to examine and evaluate the nature of the caprock in this unit, the depositional origin, and potential continuity of the lithology.

Anhydrite from the Thaynes Formation, if deposited in a marine setting, should record ⁸⁷Sr/⁸⁶Sr of seawater determined for Late Permian and Triassic time. Eight samples from a 30m section of core from a well on the Moxa arch near the La Barge platform in southwestern Wyoming yielded ⁸⁷Sr/⁸⁶Sr from 0.70820 to 0.70927. Only one sample, the shallowest in the core, records a ratio for Triassic seawater. All of the other samples record higher than accepted values. All samples were collected from anhydrite layers that are parallel to bedding except one that was taken from a cross-cutting vein of almost pure anhydrite, which we interpret to be of secondary origin.

If the anhydrite in layers parallel to bedding is primary, then the brine from which it precipitated was not solely marine, but also included continental runoff. Alternatively, the anhydrite could be secondary, precipitating from younger fluids. We favor the first alternative because the ratios generally become less radiogenic higher in the section. This suggests that the environment is transitioning to a marine setting and is less influenced by meteoric waters up-section. We suggest that the majority of the unit records restricted marine settings with variable proportions of marine and continental input. We conclude that this section of the Thaynes Formation alone should not be relied upon to provide a good regional seal for carbon dioxide sequestration, however the marine signal near the top of the described section may indicate a more laterally continuous sealing unit higher in the section.