URANIUM GEOCHEMISTRY IN THE MARCELLUS SHALE: EFFECTS on METAL MOBILIZATION

The Marcellus Shale is one of the largest sources of natural gas in the United States. It is also naturally enriched in uranium and extraction of the hydrocarbons may generate significant environmental problems related to metal mobilization. Uranium concentrations in the Marcellus shale are up to 100 ppm in some regions. The cause of enrichment is possibly due to sorption of uranium onto organic matter or precipitation of uranium oxides in reducing conditions. We set out to identify the physical association and complexation of uranium in outcrop and core samples of the shale from western New York and Pennsylvania. Major mineralogy, and major, minor, and trace element chemistry of 22 samples were measured using semi-quantitative X-ray Diffraction (XRD) and Instrumental Neutron Activation Analyses (INAA). Total organic carbon (TOC) was measured in each sample using a carbon analyzer. Chemical extractions were used to selectively dissolve certain mineral phases and quantify the associated uranium. Time-of-Flight Secondary Ion Mass Spectrometry (TOF SIMS) was used to map the uranium and organic matter on select core and outcrop samples.

Preliminary results indicate that the average and range of TOC and uranium concentrations of core and outcrop samples are not significantly different. However, major mineralogy of core and outcrop samples is different and the predominant form of uranium in each type of sample (core or outcrop) differs as well. TOF SIMS element maps indicate that uranium chemically bound to hydrocarbon is more abundant than uranium bound to just oxygen. Additionally, outcrop samples appear to contain fewer uranium oxides compared to the core samples. Extraction studies suggest that the hydrocarbon-bound uranium is more easily extracted from the outcrop samples compared to cores. Natural weathering of the samples may have mobilized the uranium within the shale formation. These data may indicate enhanced solubility and mobility of the uranium due too water-rock interactions over time. Ongoing studies will investigate the reaction between shale samples and drilling fluids used during gas extraction.