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Clays are major costituents of shales and, as such, deserve attention as
potential contributors to gas storage capacity, especially when shales are
organic-lean and clay-abundant (Zhang et al., 2017). Clay minerals are
phyllosilicates with significant surface areas and degrees of microporosities, as
opposed to  biogenic quartz, which has a
very low microporosity. Hence, they are able to give signification contribution
to 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 total
porosities (5.4-6.1 %) than clay-poor shales (0.5-4.2 %), with pore size
distributions skewed towards smaller pores (Ross & Bustin, 2009). The types
of clay have an impact on methane sorption capacity. Montmorillonite, for
example, has the highest internal surface area, when compared to kaolinite,
chlorite and illite. Hence, since CH4 sorption capacities on
clay-rich rocks are controlled by specific surface areas, montmorillonite had
the highest sorption capacity (0.48 mmol/g), compared to illite-smectite mixed
layer (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 surface
areas, within the same clay minerals, depend on source, depositional
environment and diagenesis. Montmorillonite, for example, is transformed to
illite during diagenesis and this phenomenon will significantly reduce surface
area and gas sorption capacity (Aringhieri, 2004; Wang et al., 2013).

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Clay-rich
shales from the Lower Cambrian Niutitang formation (China) showed better
properties (e.g. pore volume, surface area, correlation with TOC) than
clay-poor shales, even if the latter were characterized by organic matter
abundance (Wu et al., 2017). On the other hand, studies by Chen et al. (2016b)
and Pan et al. (2015)  contradict these
results, as negative or inconclusive correlations were found between clay
content and TOC or porosities.

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