THE GEOCHEMICAL FINGERPRINT OF PRODUCED WATERS AND NATURAL GAS FROM HYDRAULICALLY FRACTURED BLACK SHALES: A NOBLE GAS PERSPECTIVE

Organic-rich black shales, which were previously thought to be non-economically viable, are now major non-conventional natural gas reservoirs throughout North America and the world. The increase in shale gas production has been accompanied by the production of significant volumes of formational brine waters. Thus, production waters now present significant economic and environmental challenges for natural gas producers because of the need for the treatment and disposal of these contaminant-rich formational fluids. Additionally, accurately understanding the formations that source flow back and produced waters is critical to shale gas production strategies. Therefore, there is significant interest in understanding the source of natural gas and formation brines, and the extent to which these fluids migrate within the fractured matrix of gas-bearing black shales.

Here, we present a method to determine the genetic fingerprint of hydrocarbon gases and produced fluids using the atmospheric and radiogenic noble gas composition of these geofluids. Although the stable isotopes of carbon are a staple of gas geochemistry and traditionally have been applied for these purposes, these proxies can be altered by microbial activity and fluid migration. By comparison, the elemental and isotopic composition of noble gases provide inert geochemical tracers that are unaffected by chemical reactions or microbial activity. Their inert behavior eliminates the need to make assumptions regarding the sources, original concentration, and/or isotopic compositions of C₁, C₂, C₃₊ hydrocarbons. The atmospheric, mantle, and/or crustal components are characterized by unique noble gas elemental and isotopic signatures and preserve information about the source, migrational process, and residence time of crustal fluids. Thus, noble gas geochemistry provides a novel, unique, and externally defined variable capable of distinguishing the genetic fingerprint of hydrocarbon fluids and formational brines sourced from fractured black shales in the Earth’s crust.