Is the Uranium Content in Black Shale a Suitable Parameter to Assess the Alteration of Organic Matter and Petroleum Potential?

Abstract

Summary The simultaneous occurrence of high TOC and high uranium contents in black shale indicates that uranium is concentrated in the organic matter. Nevertheless, such uranium-OM correlations may be misleading as uranium-bearing minerals also occur in black shale. Thus, the spatial uranium distribution in black shale, but also the diagenetic timing of uranium fixation or possible later remobilization are critical factors, and control when and to what degree the OM may have been degraded due to the natural decay of high uranium contents. Previous studies only considered the bulk uranium content of samples as a parameter in estimating the degree of irradiation effects on the OM, and the sample sets were normally from different locations with different thermal maturities. Moreover, vertical and lateral variations in lithology and mineralogy as result of depositional environments as well as thermal maturation during burial are additional controls for different uranium contents and phase occurrences. In this contribution, published data from Schulz et al. (2019) will be presented about the approximately 4 m thick and thermally immature western Russian Koporie Formation (“Dictyonema” Shale) using an interdisciplinary analytical approach in order to answer questions about the source of uranium, its occurrence in the shale, both spatially and temporarily, and the consequences for the OM and the petroleum potential.