Sulfur isotope analysis of bitumen and pyrite associated with thermal sulfate reduction in reservoir carbonates at the Big Piney–La Barge production complex

Sulfur isotopes of solid bitumen and associated pyrite from the Madison Limestone in the Big Piney–La Barge production complex were measured using a Secondary Ion Mass Spectrometry (SIMS) method. The solid bitumens, a product of thermochemical sulfate reduction, yielded δ34S values of +18.9 ± 3.9 that are consistent with inferred values for native Mississippian sulfate. In contrast, coarse and fine grain pyrite grains were found to be 34S depleted, with values similar to that of the produced H2S (δ34S ∼ +10‰). We interpret these results to indicate that two different sources of sulfate were involved with TSR within the Madison Limestone–autochthonous anhydrite, which is now completely replaced with calcite, and Permian age sulfate dissolved in the aquifer. While checking for inclusions within the bitumen that could lead to erroneous measurement, we found the bitumen possesses a ∼5 μm rim and internal “worm-like” features enriched in organic sulfur. We hypothesize that the rim is the result of back reaction of the late forming H2S with the solid bitumen and that the <1 μm diameter wormy features may result from liquid–liquid immiscibility occurring at the high temperatures of formation.