Sulfur isotopes in pyrite, anhydrite, elemental sulfur and carbonate associated sulfate as recorders of diagenesis in the San Andres Formation, West Texas

The Middle Permian San Andres Formation of West Texas has experienced a complex diagenetic history including paleokarst development, anhydrite cementation and carbonate replacement. This diagenesis has resulted in heterogeneous changes in porosity and permeability, impeding the ability to model fluid flow through the system. However, the occurrence of sulfur in a range of mineral phases and oxidation states opens the potential for diagenetic reconstructions of the system, providing constraint on the observed heterogeneity.

Within this formation, sulfur occurs in multiple phases including sulfate (anhydrite and celestine), sulfide (pyrite and sphalerite), elementary sulfur, and carbonate associated sulfate (sulfate that substitutes for the carbonate ion in the carbonate crystal lattice). Here we present combined SEM and sulfur isotope data of the different and carefully separated sulfur phases. Visual identification and isotopic analysis of pyrite framboids and euhedral pyrites indicate at least two stages of sulfate reduction. Additionally, carbonate associated sulfate and pore-filling celestine and carbonate replacing anhydrite allow different sulfate sources to be identified. This mix of oxidation states and minerals allows sulfur isotopes in the different phases to both preserve primary signals and record various diagenetic processes.

Overall, these data provide a framework that reflects the various depositional and diagenetic processes that formed and modified the San Andres formation. Our results demonstrate that sulfur analysis can be useful addition to other parameters indicating the degree of diagenetic overprint. This case study provides a larger framework for sulfur isotope applications in complex, heterogeneous formations.