TRACE METAL OCCURRENCES IN THE MARCELLUS SHALE (Invited Presentation)

The Marcellus Shale is an important natural gas reservoir that is naturally enriched in uranium and other redox-sensitive metals compared to ordinary, organic-poor shale equivalents. Chemical and physical extractions, as well as time-of-flight secondary ion mass spectrometry (ToF-SIMS) and scanning electron microscopy (SEM), were used 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.

To date 30 samples, including 6 outcrop and 24 cores, have been investigated. The total organic carbon (TOC) content ranges from 0.7 to 13.7 wt % with an average value of 6.0 wt %. Uranium concentrations range from 4.0 to 72 ppm with an average of 30 ppm. There are no statistical differences in the TOC or metal concentrations when core and outcrop samples are studied separately. Oxidation of the shale with H₂O₂ caused significant uranium removal from outcrop samples, but not from core samples, suggesting that some but not all of the uranium is associated with the TOC. The extractability of other metals was consistent with the behavior of uranium; more metal could be removed from outcrop samples compared to cores and this trend was true for several different chemical extractions. The resilience of metals associated with the shale following chemical treatment and the concentrations of metals measured on fine mineral separates suggest that phyllosilicates may be important sorbents of metals in the Marcellus Shale. ToF-SIMS analyses identified two dominant forms of uranium in these samples: a charged uranium species (U⁺) and uraninite (UO₂). In addition, SEM-EDS analyses identified that microscopic uraninite and brannerite grains are present, but that they exist as accessory phases only. Several generations of pyrite were noted using SEM and may be important sources of other metals in the shale.