T204 Geochemical and Isotopic Evolution of Sedimentary and Crystalline Formation Brines

Tuesday, 2 November 2010
1:30 PM - 5:30 PM
Room 402

INORGANIC GEOCHEMISTRY OF FORMATION WATERS FROM DEVONIAN STRATA IN THE APPALACHIAN BASIN – PRELIMINARY OBSERVATIONS FROM PENNSYLVANIA, NEW YORK, AND WEST VIRGINIA

Paper number: T204-2
Program: Technical Sessions
Time: 3:50 PM – 4:05 PM

Authors

Abstract

A recent surge in drilling for natural gas in the Middle Devonian Marcellus Shale in the Appalachian basin has been focused in Pennsylvania, New York, and West Virginia. Following the common practice of hydraulic fracturing, both gas and varying amounts of water are produced from the wells. The salinity of the co-produced water increases with time, from nearly fresh following the hydraulic fracturing, to ranges that are typical of the natural formation waters: 100,000–300,000 ppm TDS. Similarly high salinities have been reported in formation water samples throughout much of the Paleozoic stratigraphic section (Breit, 2002). In a preliminary effort to characterize the inorganic geochemistry of these waters, we examined available data for the Marcellus shale from the published literature and other publically available sources. We also compared the Marcellus shale data with formation water analyses from adjacent Devonian strata, including sandstone in the overlying Bradford Group, and the underlying Onondaga Limestone and Oriskany Sandstone.

Formation waters from the Marcellus Shale are Na-Ca-Cl dominant, with low HCO3 and SO4 concentrations. The Na/Br and Cl/Br ratios indicate mixtures of brines, with a major component derived from evaporatively concentrated seawater. The Marcellus Shale is known to be enriched in uranium, and the formation is identifiable on geophysical logs by its anomalously high gamma-ray response. Of the Marcellus formation water analyses that included radiochemistry, most showed elevated total radium concentrations (hundreds to thousands of pCi/L). Formation water analyses from the overlying and underlying strata appear to indicate somewhat lower radium concentrations. In water samples from the Marcellus, the presence of radium and the virtual absence of uranium in solution likely reflects equilibration of saline formation waters with uraniferous shale in a reducing environment.