Geochemical and spatial investigation of uranium in the Marcellus Shale
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The Marcellus Shale, deposited 389.5-388 million years ago, is an important gas reservoir containing 500 trillion cubic feet (Tcf) of natural gas, of which 10% is technically recoverable. The Marcellus Shale is abundant in organic matter and naturally enriched in uranium and other redox-sensitive metals. The goal of this study is to determine the chemical and physical associations of uranium with the mineralogy of the Marcellus Shale and to determine if these relationships differ between core and outcrop samples. 14 core and 14 outcrop samples have been collected form WV, NY, VA and PA. Main mineral phases consist of quartz, clays and calcite. Accessory minerals consist of dolomite (including Fe-dolomite), pyrite, ankerite, siderite, barite and feldspar (including microcline and albite). The total organic carbon (TOC) content of the studied samples varies between 0.06 wt% and 14.0 wt%. Uranium concentrations range from 4.0 ppm to 67.8 ppm. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and scanning electron microscopy (SEM) were used to locate uranium in 6 core and 4 outcrop Marcellus Shale samples. ToF-SIMS identified two dominant forms of uranium in core and outcrop samples: a charged uranium species (U + ) and uraninite (UO 2 ). By ToF-SIMS analysis, the charged uranium species was found in all 10 samples and uraninite was identified in 9 of 10 samples. Region of Interest (ROI) analysis indicates that both forms of uranium are adsorbed to inorganic material present in all core and outcrop samples. SEM-EDS analyses also identified two different forms of uranium in one Marcellus Shale sample only; uranium is present as uraninite and as brannerite ((U, Ca)(Ti, Fe) 2 O 6 ). Uraninite grains were <1 micrometeter in size and brannerite grains were 5-10 micrometers in size. Adsorption and reduction by organic matter, clay, and silicate minerals are possible causes for enrichment of uranium from seawater at the time of sediment deposition.