MAPPING CRITICAL MINERAL DISTRIBUTIONS IN MARCELLUS SHALE

The middle Devonian Marcellus shales are widely distributed in eastern North America, and they form important unconventional shale gas reservoirs. Certain intervals of Marcellus shale are known for their unusually high metal contents (Rigali and Krumhansl, 2018). To quantify the contents of critical minerals in representative oil and gas shale reservoirs in the United States, we present element maps of some critical metals in a vein-rich interval from the Marcellus Formation. The studied shale (7461ft) contains a cm-sized vein of calcite, barite, and ankerite, imbedded inside the calcite-rich shale matrix. It was cut into a rectangle shape, and subsequently polished for SEM and electron-probe analyses. Finally, its spatial trace metal compositions were determined using laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) following the protocol described in Cruz-Uribe et al. (2021).

Large element composition variations are observed on the ablated sample surface. As expected, element composition variations show clear mineral control. For example, barite has up to several tens of thousand ppm Sr, compared to several hundred ppm in other phases. Mn is enriched in ankerite (up to 3,000 ppm). In contrast, the shale surrounding the vein is enriched in many critical minerals, such as Sc (up to 10 ppm, compared to <1 ppm in minerals), V (up to 100 ppm, compared to several ppm in minerals), Cr (up to 30 ppm, compared to several ppm in minerals), Co (up to 20 ppm, compared to <1 ppm in minerals), Cu (up to 200 ppm, compared to several ppm in minerals), and Ga (up to 10 ppm, compared to <2 ppm in minerals). Hydrothermal alteration process is likely responsible for the enrichment.

References

Cruz-Uribe, AM, Page FZ, Lozier E, Feineman, MD, Zack, T, Mertz-Kraus, R, Jacob, DE and Kitajima K. 2021. Trace element and isotopic zoning of garnetite veins in amphibolitized eclogite, Franciscan Complex, Calofornia, USA. Contributions to Mineralogy and Petrology, 176:41.

Rigali, MJ and Krumhansl, J. 2019. Metals in Oil and Gas‐Bearing Shales: Are They Potential (Future) Ore Deposits? In Shale: Subsurface Science and Engineering, Geophysical Monograph 245.

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