Recent analyses of the potentially vast unconventional shale-gas resource in the Canning Basin, onshore Western Australia, estimate nearly 800 Tcf in the Goldwyer Formation alone. This and other Paleozoic marine shales share numerous characteristics with successful US shale-gas plays and commonly source conventional petroleum accumulations, although none has yet been produced directly. In order to further constrain volumetric estimates in the Canning Basin, we examined two formations that we identify as highly prospective, the Goldwyer and Laurel. Our assessment includes all data currently available for the basin and applies a combined volumetric modeling approach (USGS and PMRS). We compiled data regarding kerogen type, thermal maturity, hydrocarbon generation potential, rock mineralogy, and fluid analyses, in addition to data on porosity, permeability, and pressure and temperature variation, in every well that intersected these shales. Analysis showed maximum total organic content (TOC) of 6.4% (Goldwyer Formation), maximum vitrinite reflectance of nearly 2.0%, (Laurel Formation), and average Hydrogen Index of 0.13 gHC/gTOC (maximum of >1 in both formations). We then calculated total gas-in-place for the rock volumes corresponding to gas-prone sections of each shale (the Goldwyer III and Upper Laurel) by estimating total generation potential, original TOC, primary and secondary cracking of kerogen, and retained oil. Probabilistic analysis of the distribution of key parameters allowed estimation of total hydrocarbon in place, by applying a Monte Carlo simulation based on P90, P50, and P10. This resulted in the third independent estimate of Canning Basin shale-gas volumes and the first ever for the Goldwyer III and the Upper Laurel. Our work thus greatly improves confidence in estimates of the size of shale-gas accumulations in the basin, significantly increases the amount of data utilized in such estimations and provides the first reported volumes for previously unexamined shale layers.