PHASE ASSOCIATION OF RARE EARTH ELEMENTS IN MARCELLUS AND HAYNESVILLE SHALE: IMPLICATIONS FOR RESOURCE RECOVERY

Hydrocarbon-rich shales have been a major natural gas supplier in the US over the last decade. These organic-rich shales can also potentially serve as a source of some rare earth elements (REY). However, the mode of occurrence and the geochemical processes that lead to REY enrichment in these shales are still poorly understood. In this study, we investigated the whole rock REY concentration and associations of REY in the different phases of Marcellus and Haynesville Shale. A traditional sequential extraction procedure was adopted to understand the association of REY in - i) exchangeable, ii) acid-soluble, iii) pyritic, iv) organic matter and v) silicate fraction. To assess the extraction efficiency, the mineralogies of the pre and post-sequential extraction samples were determined using XRD.

Our results indicate that the whole rock REY content in all the samples was > 290 ppm, with the Haynesville samples having higher concentration than the Marcellus sample. However, the average total REY extraction efficiency was just approximately 20%. The yield was relatively higher in the Haynesville samples that contained lower total organic carbon (TOC) and higher carbonate content. About 95% of the extracted REY was obtained from the acid-soluble fraction, i.e., from carbonates, phosphates, and oxide fractions. The XRD results indicate that the poor REY yield is due to a high amount of the refractory aluminosilicate/clay fraction in these samples. We postulate that most of the REY’s in these shale samples were associated with the clay fraction. However, these adsorbed REY’s were not released during extraction due to their low solubility. Our results indicate that traditional sequential extraction procedures may not be effective in extracting REY’s from high organic- high aluminosilicate shale. Our research group is designing new strategies to increase the extraction efficiency of REY’s from black shales.