Comparison between whole rock lithogeochemistry and fine lithogeochemistry of Marcellus shale
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The Marcellus Shale is a geological formation well known not only for being rich in natural gas but also for its high concentrations of uranium and other heavy metals. The goal of this project is to identify the concentrations of heavy metals in different size and mineral fractions of the Marcellus Shale, due to different sorption capabilities of organic matter, clay and detrital minerals. To reach this goal, all samples were crushed and sieved to less than 125 μm. Then, whole rock lithogeochemistry, TOC and mineralogy were analyzed by Instrumental Neutron Activation Analyses (INAA), an Infrared Analyzer and X-ray Diffraction (XRD) analyses respectively. Successively, through centrifugation in a 0.005 M NaCl solution, fine sediments (< 2 μm) were separated from the rest of the sediments and analyzed by INAA. Comparing the concentrations of As, Cr, Mo, U, Zn and Ba between whole rock and fine sediments, I found that not all heavy metals are influenced in the same way by the size and mineral fraction of Marcellus Shale. In fact Mo is enriched in the fine fraction and As and Cr are present in greater concentrations in whole rock sediments. U, Zn and Ba show no statistical difference between fine and bulk sediments. Looking at outcrop and core samples separately, the results are the same except As is not statistically enriched in the whole rock fraction in core samples and Mo is not enriched in the fine fraction in outcrop samples. In regard to TOC and mineralogy, some elements, such as As, Mo and U, are correlated with TOC and pyrite content, suggesting a likely relationship with organic matter and reducing conditions. These results are in agreement with syndepositional enrichment of these metals in the Marcellus Shale.