Association of Rare Earths in Different Phases of Marcellus and Haynesville Shale: Implications on Release and Recovery Strategies

Abstract

Hydrocarbon-rich shales have been a major natural gas source in the US over the last decade. These organic-rich shales can also potentially serve as a source of some rare earth elements (REYs). However, the mode of occurrence and the geochemical processes that led to REY enrichment in these shales are still poorly understood. In this study, we investigated the whole-rock REY content and associations of REYs in the different phases of Marcellus and Haynesville Shale samples. A traditional sequential extraction procedure was adopted to understand the association of REYs in (i) exchangeable, (ii) acid-soluble, (iii) pyritic, (iv) organic matter, and (v) silicate fractions. Extraction efficiency was assessed by comparing the mineralogy of the pre- and post-sequential extraction samples using XRD. Elemental ratios such as La/Lu, La/Sm, Gd/Lu, Y/Ho, and Ce and Eu anomalies were utilized to understand whole-rock-normalized REY distribution patterns. Further, the distribution pattern in each extracted phase was examined to account for the relative contribution of phases to REY enrichment. The economic potential of these samples was evaluated by calculating HREE/LREE ratios, outlook coefficients, and by comparing their REY levels with those of coal fly ash deposits. Our results indicate that whole-rock REY content in the analyzed shale samples ranged from 295 to 342 ppm, with Haynesville Shale having a higher concentration than the Marcellus Shale sample. All samples exhibited an MREE–HREE-enriched pattern, indicating that the REY content is primarily contributed by carbonate and siliciclastic inputs. However, the average total REY extraction efficiency was only approximately 20% from the Haynesville samples and 9% from the Marcellus sample. We postulate that the poor REY yield is due to a high amount of refractory aluminosilicate/clay fraction in these samples. We demonstrate that traditional sequential extraction procedures may not be effective for extracting REYs from high organic–high aluminosilicate shale.