The host rocks of shale gas accumulations act as source, seal and reservoir. They are characterized by complex pore systems with ultra-low to low interparticle permeability and low to moderate porosity. The word "shale" is used in the sense of a geological formation rather than a lithology, so shale gas reservoirs can show marked variations in rock type from claystones, marlstones and mudstones to sandstone and carbonate lithological "sweet spots". The pore space includes both intergranular and intrakerogen porosity. The density of natural fractures varies markedly, and pore throat connectivity is relatively ineffective. Moreover, in-situ gas pore volume has to take account of both free and adsorbed gas, an evaluation exercise that is complicated by pronounced variations in water salinity. All these characteristics present major challenges to the process of petrophysical evaluation. The petrophysical responses to these issues are several fold. First, a broader calibrating database of core measurements is required at key wells, especially as regards mineralogy, porosity and permeability data, tight rock analyses, total organic carbon, gas desorption isotherms, and the analysis of extracted formation waters. Second, at least in the key wells, an extended suite of logs should include an elemental analysis log, magnetic resonance imager, electrical micro-imager, and a dipole sonic log. These databases lead to a populating electrofacies scheme that takes better account of dynamic properties and fracturability. They also allow reservoir partitioning based on exclusivity of empirical interpretive algorithms, e.g. quartz content vs. producibility. These responses comprise key elements of a functional petrophysical system that encompasses fit-for-purpose interpretation methods such as a pseudo-Archie approach. This system is presented as a workflow for application in shale gas reservoirs. The benefits are especially strong in reserves reporting of these unconventional gas reservoirs.
In contemporary petrophysical parlance, there are two types of reservoir: those that conform to the implicit assumptions underpinning the work of Archie (1942) and those that do not. The second category includes most of the World's reservoirs. It can be subdivided further into non-Archie conventional reservoirs and unconventional reservoirs (Worthington 2011a). Non-Archie conventional reservoirs include those with fresh formation waters, significant shale content, high capillarity, a bimodal pore system, or fractures. In other words, they infringe one or more of the Archie assumptions. Unconventional reservoirs include tight gas sands, coal seam gas reservoirs, gas hydrates, and shale gas reservoirs. Each of these infringes several of the Archie assumptions. At the limit, shale gas reservoirs infringe them all (Table 1). Yet, the interpretive challenges presented by shale gas reservoirs go even further, because gas-bearing shale deposits co-function as source, seal and reservoir. Therefore their character contains elements of all three. Thus, for example, shale gas deposits contain kerogen porosity, have very low effective permeability to gas, and yet can show a markedly variable pore character. To be successful, a petrophysical methodology for the evaluation of shale gas deposits has to be founded on approaches that sit outside the conventional range of thinking. This paper presents a synthesis of the technical challenges that face shale gas petrophysics and collates practical solutions based on what is currently known.