Abstract

The Glorieta and San Andres formations represent some of the most important carbonate platform oil reservoirs in the Permian Basin. For oil exploration correlation between faulted blocks belonging to these formations is essential, and a task that remains a challenge despite extensive studies. This project aims to test a new geochemical tool – isotopes of Carbonate Associated Sulfate (CAS) – that will aid in the correlation of faulted rock packages belonging to the Glorieta and San Andres formations. The study area, known as West Dog Canyon, is located just north of the Guadalupe Mountains National Park on the Texas/New Mexico border. It comprises well-exposed outcrops of faulted rock packages belonging to the Glorieta and San Andres formations.

Using sequence stratigraphy, we will correlate faulted rock packages in the study area. Simultaneously, we will use these same correlated rock packages with two additional, independent geochemical approaches, collectively known as chemostratigraphy. The first geochemical approach, carbon and oxygen isotope analyses of carbonates is an established method, whereas the second approach, sulfur and oxygen isotope analyses of CAS is novel. Earlier attempts to use the carbon and oxygen isotope composition of carbonate rocks as geo-chemical identifiers, turned out to be ineffective, because the original carbon and oxygen isotope fingerprints of the different carbonate rock packages were overprinted during chemical weathering of the rocks. An alternative chemostratigraphic tool, which is less prone to be affected by weathering, is needed.

We hypothesize that sulfur and oxygen isotope signatures of CAS is a promising chemostratigraphic tool for correlating carbonate formations and sequences in the Permian Basin. Carbonate associated sulfate – i.e. dissolved sulfate that is trapped in carbonate rocks at the time the rocks form – can be partially lost from the carbonate rocks during weathering. However, the sulfur and oxygen isotope signatures of the CAS that remain in the rock are much less affected by weathering than the carbon oxygen isotope composition of carbonates, mainly because little sulfate is added to the CAS pool during such processes. Thus, while the concentration of CAS may be lowered during carbonate weathering, the sulfur and oxygen isotope composition of CAS continues to reflect the isotope composition of sulfate that was dissolved in the water bodies that once covered the Permian Basin. Notably, the water exchange between the Permian Basin and the global ocean may have been restricted to various degrees during the deposition of the Glorieta and San Andres formations. Such restrictions are expected to result in rapid deviations in the isotope signature of CAS, which makes it very likely that different intervals of these formations can be identified by their CAS isotope fingerprint.