PROCESSES THAT CONTROL ELEMENT AND MINERAL ABUNDANCES IN BLACK SHALES

Over the past decade, application of hydrofracturing (“fracking”) has dramatically increased U.S. production of natural gas and oil from black shales and afforded the U.S. greater energy independence. However, because black shales contain high concentrations of organic carbon and potentially toxic trace elements these activities have prompted concern about their environmental impact. Improved understanding of black shale mineralogy and geochemistry is needed in order to alleviate these environmental issues.

We examined more than 500 source documents and compiled information about the most important chemical and physical processes that affect black shale mineralogy and geochemistry. In addition, quantitative information about elemental and mineral abundances were tabulated. These data were used to construct enrichment diagrams, distribution diagrams, and correlation plots, which highlight how mineral and element abundances evolve as shales pass through the stages of deposition, early diagenesis, burial diagenesis, uplift, and weathering. Our literature survey provides evidence for significant changes caused by microbial processes during early diagenesis, uplift, and weathering. Organic acids produced by the microbes and by thermal degradation of kerogen during burial diagenesis are responsible for significant amounts of mineral dissolution and element redistribution.

We hope that our efforts will inspire others to thoughtfully evaluate the validity of the different processes proposed and that this compilation will aid in the understanding of the environmental release, fate and transport of trace elements from black shales.