Shales TOC (total organic carbon) variabilities between several

Shales are the most abundant mudrocks found on Earth (Kuila & Prasad, 2013). They are characterized by a fine-grained texture, with a microstructure of <5 µm grain size (Lazar et al., 2015), and are formed by: (i) organic matter (kerogen), (ii) inorganic minerals and (iii) fluids (water, oil and gas) (Chong et al., 2010). Inorganic solids comprise variable quantities of clay (montmorillonite, smectite, illite, kaolinite, chlorite), quartz and calcite, and minor amounts of other minerals (e.g. chlorite, pyrite, dolomite, hematite etc.

) (Blatt & Tracy, 1996). Table 1 shows average mineralogical composition and TOC (total organic carbon) variabilities between several representative shales.TOC values may not appear extremely large, but in truth organic matter gives the highest contribution to the total shale pore volume (Kang et al., 2011). Porosity in shales is predominantly in the nano-scale range (Cole et al., 2010), comprising the following pores classified by their size: (i) macropores (widths greater than 50 nm), (ii) mesopores (widths between 2 and 50 nm) and (iii) micropores (widths less than 2 nm) (Sing et al.

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, 1985). Loucks et al. (2010) identified shale pores within the organic matter, interparticle pores between particles and crystals of the inorganic matrix, and intraparticle pores within inorganic particles. Shale pore systems are a complex function of the combination of mineralogy, sedimentology, diagenesis and thermal maturation (Mathia et al., 2016). The latter, specifically, determines the fluid available for retrieval (either gas or oil) and the amount of nanoporosity within the organic matter. Mineralogy is also crucial for ensuring an adequate quantity of brittle minerarls (e.

g. silica or carbonate), which, unlike ductile clays, facilitate fracking. (Scotchman, 2016).

Commercially exploitable shales need to have suitable maturity and thickness, TOC content greater than 2% and an amount of brittle minerals greater than 40% (Zou, 2013).