TRACE ELEMENT GEOCHEMISTRY OF THE UTICA-POINT PLEASANT FORMATIONS

Understanding trace elements in rocks from the Utica-Point Pleasant Formations is important because elevated concentrations of trace elements in flowback fluids derived from hydraulic fracturing may be an environmental hazard. The goal of this study is to determine potential sources of trace elements from Utica-Point Pleasant rocks that could be released into hydraulic fracturing fluids with which they interact. Polished sections of core were analyzed to determine the concentrations and distribution of trace elements across selected mineralogical/textural regions of the samples.

Samples provided by Chesapeake Energy were observed with light and scanning electron microscopy (SEM) to identify areas of interest for analysis by inductively coupled plasma mass spectrometry laser ablation (ICP-MS-LA). For example, SEM backscattered electron images helped determine the spatial distribution of the sulfide and sulfate minerals of interest. Energy dispersive spectroscopy was used to obtain semi-quantitative spot chemical analyses of minerals. Images from the SEM show significant quantities of barite (BaSO₄), celestite (SrSO₄), pyrite (FeS₂), and sphalerite (ZnS). Identification of barite and celesite is important because they support previous reports of high levels of Ba and Sr in flowback fluids associated with hydraulic fracturing. Pyrite and sphalerite are known to be important host minerals for a wide range of minor and trace elements

Analysis from ICP-MS-LA line profiles across a pyrite/carbonate mineral assemblage and surrounding clay matrix identified major elements Mg, Ca, Sr, Ba, Al, Na, Ti and trace elements Mo, Mn, Ni, Zn, La, Ce, U, Th, Rb in low resolution and major elements Fe, S, P, Ti, Si in medium resolution. The pyrite assemblage had elevated levels of Mo (10 ppm) and Ni (up to 4000 ppm) compared to the clay, with Ni concentrations up to 3 orders of magnitude higher in the pyrite than the surrounding minerals. Sr concentrations ranged from 10 to 100 PPM in the pyrite but around 1000 PPM in the carbonate shells and the clay matrix.