Factors controlling Li concentration and isotopic composition in formation waters and host rocks of Marcellus Shale, Appalachian Basin

In this study, water and whole rock samples from hydraulically fractured wells in the Marcellus Shale (Middle Devonian), and water from conventional wells producing from Upper Devonian sandstones were analyzed for lithium concentrations and isotope ratios (delta Li-7). The distribution of lithium concentrations in different mineral groups was determined using sequential extraction. Structurally bound Li, predominantly in clays, accounted for 75-91 wt.% of total Li, whereas exchangeable sites and carbonate cement contain negligible Li (<3%). Up to 20% of the Li is present in the oxidizable fraction (organic matter and sulfides). The delta Li-7 values for whole rock shale in Greene Co., Pennsylvania, and Tioga Co., NewYork, ranged from -2.3 to +4.3 parts per thousand, similar to values reported for other shales in the literature. The delta Li-7 values in shale rocks with stratigraphic depth record progressive weathering of the source region; the most weathered and clay-rich strata with isotopically light Li are found closest to the top of the stratigraphic section. Diagenetic illite-smectite transition could also have partially affected the bulk Li content and isotope ratios of the Marcellus Shale. In Greene Co., southwest Pennsylvania, the Upper Devonian sandstone formation waters have delta Li-7 values of +14.6 +/- 1.2 (2SD, n = 25), and are distinct from Marcellus Shale formation waters which have d7Li of +10.0 +/- 0.8 (2SD, n = 12). These two formation waters also maintain distinctive Sr-87/Sr-86 ratios, suggesting hydrologic separation between these units. Applying a temperature-dependent illitilization model to Marcellus Shale, we found that Li concentration in clay minerals increased with Li concentration in pore fluid during diagenetic illite-smectite transition. Samples from north-central Pennsylvania show a much smaller range in both delta Li-7 and Sr-87/Sr-86 than those in southwest Pennsylvania. Spatial variations in Li and delta Li-7 values show that Marcellus formation waters are not homogeneous across the Appalachian Basin. Marcellus formation waters in the northeastern Pennsylvania portion of the basin show a much smaller range in both delta Li-7 and Sr-87/Sr-86, suggesting long term, cross-formational fluid migration in this region. Assessing the impact of potential mixing of fresh water with deep formation water requires establishment of a geochemical and isotopic baseline in the shallow, fresh water aquifers, and site specific characterization of formation water, followed by long-term monitoring, particularly in regions of future shale gas development. (C) 2015 Elsevier B.V. All rights reserved.