SEQUENTIAL EXTRACTION OF RARE EARTH ELEMENTS TO EVALUATE POST-DEPOSITIONAL GEOCHEMICAL EVOLUTION IN MARCELLUS SHALE

Knowledge of the diagenetic history of unconventional shale reservoirs can provide insight on the distribution and composition of organic matter within organic-rich shales. This information can be applied towards fracture treatment design for improved hydrocarbon recovery. Organic matter within the shale can change along with variations in geochemical conditions during diagenesis. Rare earth elements (REE) have been identified as a robust indicator of geochemical changes during post-depositional processes in shale. We focus on REE characterization in specific mineral fractions associated with shale and limestone samples from vertically-drilled cores recovered from the Marcellus Shale Energy and Environment Laboratory field site in Morgantown, WV. The multi-acid digestion and sequential extraction approach allows for an evaluation of different components within the shale that may be independently affected by post-depositional processes. The total REE concentrations of analyzed Marcellus Shale ranged from 135 to 277 mg/kg (n=26) which are within the previously reported range. While water soluble and exchangeable components contain negligible REE (<0.1%), a considerable amount of REE can be extracted from the dissolution of carbonate minerals (8% on average). REE patterns of carbonate extracted from an adjacent formation limestone samples are similar with seawater, whereas carbonate cement within the Marcellus Shale exhibits the enrichment of middle REE (MREE) similar to the oxidizable fraction (e.g., organic matter). The enrichment of MREE in carbonate cement likely reflects the pore water chemistry inherited from organic matter mineralization via either chemical or microbial metabolic pathways or both during diagenesis of Marcellus Shale. Relationships between the distribution of carbonate cements and organic material may be of interest for future design of chemically-based fracturing methods to increase connectivity of fracture networks generated to enhance gas recovery. Our observations may be further applied towards evaluation of the environmental behavior and beneficial use of Marcellus Shale drill cuttings, as the data collected to date suggest that 8% of REE can be recovered from acidic dissolution of carbonate minerals.