INSIGHTS INTO BARITE DIAGENESIS USING IN SITU S ISOTOPE MEASUREMENTS OF BARITE AND PYRITE IN MARCELLUS SHALE

The concentration of barium in flowback water from Marcellus Shale hydraulic fracturing is 38 to 6,800 times over EPA recommended levels. Determining the partitioning and redistribution of barium in the shale during diagenesis can identify a source for the elevated barium concentrations. Scanning electron microscopy of Devonian Marcellus Shale drill cores shows individual barite and coexisting barite-pyrite grains. Some bulk samples of the shale have barium but no visible barite and only pyrite, which replaced barite. We have recently discovered that a large fraction of barium in the shale resides in clays (Sharma et al 2013 AAPG). The coexisting barite-pyrite grains may represent a changing redox environment that could mobilize barium as barite is replaced by pyrite. We sought to examine the issue of how and when barium could be incorporated in clays. We obtained ­in-situ sulfur isotopes (δ³⁴S) of coexisting barite-pyrite grains in a drill core sample from Beaver Meadows, New York. The median δ³⁴S of the coexisting barite and pyrite are 63.82‰ and 1.69‰, respectively. The dissolution of biogenic barite, transport of its barium from below the zone of sulfate reduction, and precipitation of barite with heavy sulfate present directly above the sulfate-sulfide transition zone can explain the highly enriched coexisting barite. The median Δ_{sulfate-sulfide} = 62.13‰, which is much greater than the 2- 46‰ fractionation associated with bacterial sulfate reduction. We infer that an additional sulfur source with δ³⁴S= −16.4‰ must mix with the sulfate to produce the lighter coexisting pyrite and maintain the grain’s volume. Seventy-seven percent of the coexisting pyrite’s sulfur derives from this source with the remaining 23% coming from the replaced barite. The formation of the coexisting grains may represent the initial stage when barium from barite was mobilized just below the sulfate-sulfide transition zone. If so, barium was partitioned into extant smectitic clays whose subsequent illitization may have entombed it within the clay-structure.

Sharma M., Renock, D. and Landis, J. (2013) Reductive weathering of black shale during hydraulic fracturing and release of barium. Abstract American Association of Petroleum Geologists Meeting (May 2013)