GEOCHEMICAL AND STRONTIUM ISOTOPE STUDY OF SEQUENTIALLY EXTRACTED METALS FROM MARCELLUS SHALE DRILL CORE

The middle Devonian Marcellus black shale is an unconventional reservoir for natural gas. High total dissolved solids are found in produced waters associated with hydrofracturing techniques used to liberate the gas, and an understanding of their sources is important for gas exploration and water monitoring and reclamation. Dry-drilled cuttings from a well in Tioga County, NY, including four Marcellus shale samples between 1338 and 1390 m depth, were sequentially extracted with (1) ultrapure water to target soluble salts and sulfates; (2) 1N ammonium acetate buffered to pH 8 to target exchangeable cations; (3) 8% acetic acid to target carbonate minerals; and (4) 0.1N hydrochloric acid to target other acid-soluble phases. The fluid:rock ratio for the leaches was approximately 100:1.

The total amount of Na mobilized by the combined extractions of Marcellus shale material was 680-925 µg/g, the bulk of which was released in the water leach. Barium (240-1100 µg/g total) was relatively immobile during the water leach, but significantly solubilized (64-73% of the total extracted) by the subsequent ammonium acetate leach. In contrast, the bulk of the Ca, Mg, and Fe was extracted by acetic acid, suggesting they were held primarily as carbonate. The combined water and ammonium acetate leaches extracted 39-83% of the strontium. The leachate data suggest that fluid:rock ratios <<1 during hydrofracturing would be required to explain the extreme Na, Ba and Sr concentrations frequently observed in Marcellus flowback waters. Preliminary ⁸⁷Sr/⁸⁶Sr data from the water leaches of the Marcellus shale cuttings (0.7101- 0.7108) fall within the range of measured values for Marcellus produced waters we have analyzed to date, consistent with a dominant water-soluble source for the TDS in produced water. Leachate ⁸⁷Sr/⁸⁶Sr decrease with each subsequent extraction from water to ammonium acetate to acetic acid, and are generally higher than those expected for middle Devonian seawater, even when corrected for ⁸⁷Rb decay. The water and acetic acid leaches of Marcellus shale show a positive correlation of ⁸⁷Sr/⁸⁶Sr with Rb/Sr ratio, which we interpret to reflect mixing of a component rich in Rb and radiogenic ⁸⁷Sr with a seawater-derived component.