PHOSPHATE MINERALS IN MUDROCK: A POSSIBLE SOURCE OF TRACE METALS IN FLOWBACK WATER

Understanding physical, chemical, and mineralogical properties of the Utica-Point Pleasant formations is important for predicting how the subsurface environment will respond during hydraulic fracturing. This study focuses on characterizing phosphate mineral assemblages and measuring trace element concentrations in Ca-phosphates as compared to other mineral assemblages in the fabric of the mudrock. Although phosphate phases constitute only a few percent of the total rock, they are of general interest geochemically and for hydraulic fracturing in particular because 1) they are commonly porous and thus potentially accessible to input fluids; 2) they are relatively soluble and reactive and may represent areas of increased porosity from water-rock interaction; 3) they contain elevated concentrations of trace metals that can be released to flowback fluids; and 4) they represent a nutrient source for subsurface microbes.

A deep core sample (approximately 10,930 feet) of the Utica-Point Pleasant from Washington County, Pennsylvania has been analyzed by SEM (backscattered electron (BSE) imaging, energy dispersive X-ray spectrometry (EDXS), and laser ablation ICP-MS. Several distinct calcium phosphate phases, are observed. Of particular interest are low BSE intensity calcium phosphate grains (presumably collophane) with relatively high organic matter (OM) content that also yield elevated characteristic X-ray intensities for Na, Cl, and Sr, as observed in EDXS spot analyses, and may represent a significant source of these salts in the flowback fluids. Preliminary laser ablation analysis of this phase yields elevated signals for U, Th, La, and Ce relative to the rock matrix (20, 100, 200, 500 ppm, respectively). Ca-phosphate with a brighter BSE signal intensity, more typical of detrital apatite, is also common, although these grains generally lack pores (as observable in the SEM), and are devoid of Na, Cl, and Sr in the EDS spectra. Elongate Ca-phosphate (apparently biogenic) grains with porous granular textures oriented roughly parallel to bedding are common. Some of these are intercalated with organic material, others contain inclusions of sulfides and/or sulfates. A fourth calcium phosphate phase occurs as fine grained porous material with micron-sized spherical pores within phosphate matrix.