INVESTIGATING THE USE OF NITROGEN ISOTOPES AS PALEOREDOX PROXIES IN SEDIMENTARY SYSTEMS AND HYDROCARBON RESERVOIRS

On geological time scales, the nitrogen cycle consists of three main processes: nitrogen fixation, nitrification, and denitrification/anammox, all of which are affected by the amount of oxygen present in the water column. The balance between nitrogen fixation and denitrification/anammox controls the pool of fixed inorganic nitrogen, and can be traced using nitrogen isotopes. This can be illustrated using a conceptual model that relates sedimentary δ¹⁵N to deep water oxygen concentrations using a simple, non-linear curve. As a result, the nitrogen isotopic profile at a given location should reflect the history of the relative contributions of nitrogen fixation to denitrifiation and thus the past redox environment. Our goal was to test this conceptual model in different sedimentary systems, including hydrocarbon reservoirs.

We measured sedimentary δ¹⁵N values in several different sedimentary systems, including hydrocarbon reservoirs, in order to assess the conceptual model and evaluate the use of δ¹⁵N as a paleoredox proxy. Measurement of sediments from the Black Sea indicated that the measured δ¹⁵N profile corresponds to the changes in redox state as supported by trace metal concentrations and micropaleontology; this supports our hypothesis that the nitrogen cycle is regulated by oxygen concentration. Analysis of samples from the Woodford Shale provided information regarding the fidelity of the proxy in an unconventional hydrocarbon play, and for samples of different thermal maturities. Overall, δ¹⁵N values appear to be reliable proxies for depositional water column paleoredox conditions in both well-preserved and hydrocarbon systems, as the δ¹⁵N signal does not seem to be preferentially overprinted by diagenetic/catagenetic processes.