Recent investigation of positron lifetime in Barnett Shale samples has shown a small intensity of positronium (Ps) formation. The samples studied have XRF information on 35 elements and chemical information on total organic carbon (TOC). Previous research has shown that Ps is not formed in dry quartz-rich sandstone, calcite-rich limestone or dolomite-rich rocks, which contain minerals that also constitute a significant part of most shale samples. Recent study of twenty-five different common rock-forming minerals has helped to clarify the Ps contribution of different minerals commonly found in shale and the effect of hydration of some minerals on Ps formation. As a result, a better understanding of how Ps is formed in the shale samples is now available. It is suggested that the observed variation in Ps intensity in shale is due to variation in water content in the samples. While there is still no information about the possible Ps contribution of organic carbon in the sample, it is expected to be small. Development of a novel laboratory tool for the characterization of the amount of water in shale core using Positron Annihilation Lifetime Spectroscopy is discussed.