Clays are major costituents of shales and, as such, deserve attention aspotential contributors to gas storage capacity, especially when shales areorganic-lean and clay-abundant (Zhang et al.
, 2017). Clay minerals arephyllosilicates with significant surface areas and degrees of microporosities, asopposed to biogenic quartz, which has avery low microporosity. Hence, they are able to give signification contributionto shale surface areas, micropore volumes, total porosities and, ultimately,gas sorption capacities (Ross & Bustin, 2009). Cambrian clay-rich shales have a surface area(31.
57 m2/g) eight times greater than Niobrara clay-poor shales.Clay-rich shales in the Western Canadian Sedimentary basin showed higher totalporosities (5.4-6.1 %) than clay-poor shales (0.5-4.2 %), with pore sizedistributions skewed towards smaller pores (Ross & Bustin, 2009).
The typesof clay have an impact on methane sorption capacity. Montmorillonite, forexample, has the highest internal surface area, when compared to kaolinite,chlorite and illite. Hence, since CH4 sorption capacities onclay-rich rocks are controlled by specific surface areas, montmorillonite hadthe highest sorption capacity (0.48 mmol/g), compared to illite-smectite mixedlayer (0.41 mmol/g), kaolinite (0.
12 mmol/g), chlorite (0.10 mmol/g) and illite(0.08 mmol/g) (Ji et al., 2012). Nevertheless, we need to consider that surfaceareas, within the same clay minerals, depend on source, depositionalenvironment and diagenesis.
Montmorillonite, for example, is transformed toillite during diagenesis and this phenomenon will significantly reduce surfacearea and gas sorption capacity (Aringhieri, 2004; Wang et al., 2013). Clay-richshales from the Lower Cambrian Niutitang formation (China) showed betterproperties (e.g. pore volume, surface area, correlation with TOC) thanclay-poor shales, even if the latter were characterized by organic matterabundance (Wu et al., 2017). On the other hand, studies by Chen et al.
(2016b)and Pan et al. (2015) contradict theseresults, as negative or inconclusive correlations were found between claycontent and TOC or porosities.