GEOCHEMISTRY OF ARTIFICALLY MATURED ANNA SHALE: IMPLICATIONS FOR THE USE OF REDOX-SENSITIVE GEOCHEMICAL PROXIES IN MATURE ORGANIC-RICH SHALES

This study explores the validity of geochemical paleoredox proxies in mature organic-rich shales. Classically, Fe-S-Corg ternary diagrams and C-S crossplots have been used to differentiate between oxic, dysoxic, and anoxic/euxinic conditions, but should be limited to immature shales due to the loss of organic carbon during thermal maturation. Trace metal proxies are also used to elucidate paleoredox conditions, but their stability in mature shales has yet to be assessed adequately. As it would be difficult to find a shale bed with an immature and post-mature end-member, that was deposited under assumedly invariant depositional conditions, a modified version of open-system hydrous pyrolysis (OSHP) was developed. The design of this apparatus allows for a small continuous flow of only liquid-phase water to be maintained throughout the entire artificial maturation experiment, with the intention of experimentally mimicking maturation in a sedimentary basin. Using the OSHP apparatus, the Anna Shale (Pennsylvanian age) was matured artificially at experimental temperatures ranging from 300 to 360℃ for up to 72 hours. These conditions resulted in increases in vitrinite reflectance from 0.47 to 1.83% Rr and Tmax from 425 to 520℃, indicating the immature shale was matured through the oil window and into the gas window. With increased maturity, the shale exhibits an approximate loss in mass of up to 20%, likely due to the liberation of hydrocarbons and the depletion of sulfur from 2.03 to ~0.1 wt. %. The intention of the OSHP design was to mimic maturation as would occur in a sedimentary basin. However, comparison of the inorganic geochemistry of the artificially matured shale to that of shales matured by normal burial maturation versus intrusive events suggests that, based on the significant depletion of sulfur (i.e., pyrite), the current design and run parameters better replicates the extreme heating conditions associated with intrusion of organic-rich rocks. Under these conditions, the loss of sulfur and carbon demonstrates that Fe-S-Corg ternary diagrams and C-S crossplots would no longer be valid as redox indicators. Furthermore, increases in the concentrations of Ni and Co by nearly an order of magnitude and a doubling of Cr and V contents from the unaltered to artificially matured shales cannot be explained by increases in relative abundance due to mass loss alone. It is possible that the 316-stainless steel used to construct the OSHP reactor body became unstable during artificial maturation experiments and acted as a contaminant source of TMs. Therefore, further investigation is required to establish whether different run parameters could more closely mimic normal burial conditions and to explore the use of different materials for the construction of the OSHP apparatus that would better maintain resistance to corrosion