Throughout geological history, organic-rich shales (ORS) show variable enrichment in uranium. These variations are both secular, related to global changes in the chemistry of the atmosphere and oceans through geological time, and spatial, related to local physical and chemical conditions of sedimentation and diagenetic processes. For a given time slice, uranium concentrations within ORS may range over two orders of magnitude (from < 5 to > 500 ppm).
In many ORS, there is a positive stable correlation between the uranium and total organic carbon (TOC) contents (e.g., Luning and Kolonic, 2003). With variable success, gamma and spectral gamma-ray logs and even measured uranium concentrations have been used to predict the TOC content of ORS. Further, a positive correlation between oil yield from Fischer retort assay and uranium concentration is observed for many ORS (e.g., Swanson, 1960). Rock-Eval data allow correlation of the pyrolysis yield (S2) from kerogen with the uranium content and relate uranium concentrations to kerogen type (oxygen index – hydrogen index classification) and maturation (Tmax). These data allow testing of the hypothesis that U-TOC correlation can be used for calibration of the hydrocarbon potential of ORS for predictive purposes. In the present study, we (1) review the factors controlling U-TOC correlations in general and (2) discuss some marine Australian ORS examples, deposited from the Mesoproterozoic to Cretaceous within the Greater McArthur (Mesoproterozic), Georgina (middle Cambrian), Eromanga (Early Cretaceous) and Bight (Late Cretaceous) basins.
Major processes resulting in incipient uranium enrichment of anoxic sediments are (1) precipitation of uranium transported with sinking particulate organic matter (U enrichment factor relative to sea water is 1 to 700) and (2) diffusion of seawater uranium (as U6+ complexes) into sediment pore water with subsequent uranium fixation by sorption and reduction. The diffusion process is generally considered to dominate the authigenic U flux even in anoxic settings. Primary variations in U-TOC ratios are affected by the concentration of U in oceanic water at the time of sedimentation and diagenesis, incipient concentrations of U in the deposited organic matter, sorption capacity of the organic matter during burial and diagenesis, and rates of sediment deposition and burial. These rates control the duration of U availability for sorption and reduction via U diffusion through pore water.
Original correlations between U and TOC content and pyrolysis oil yield are disturbed in a number of ways, including dissolution of U and oxidation of organic matter (OM) during intermittent sub-oxic and oxic episodes. In the post-depositional environment, U-TOC and U-oil yield correlations and ratios may be affected by diagenetic processes, maturation of the OM, migration of hydrocarbons, and contact with hydrothermal fluids. Further, high uranium concentrations can affect the process of the OM maturation via effects of irradiation. The challenge is to identify robust primary correlations from within generally disordered U-TOC scatter plots.
Despite the complexity of the processes resulting in uranium fixation and enrichment, primary enrichment of ORS in authigenic U can be approximated by linear dependencies. On a log-log plot, undisturbed U-TOC correlations will define a family of curves that can be approximated by straight lines with a unity slope at TOC > 1 to 3%; at these TOC concentrations, authigenic U starts to overwhelmingly dominate over the detrital U component. These lines will correspond to a number of different depositional and diagenetic environments that may be identified via inorganic (Fe, S, trace elements), organic (biomarkers) and isotope geochemistry.
The Australian marine ORS examples examined in this study adhere to the proposed correlations, demonstrating linear enrichment in U relative to TOC content (for samples with TOC > 1%) with a simultaneous increase in S2 values for the least mature samples dominated by oil-prone Type II (Cambrian -- Cretaceous) and Type I (Mesoproterozoic) kerogens. The concept presented here can be taken further to examine the application of combined spectral and neutron-induced gamma-ray spectrometry for continuous logging of U and TOC for predictive purposes.
Presented at the 20th Australian Organic Geochemistry Conference (AOGC 2018)
