Shale and mud rich rocks often vary in colour as well as grain size. Colours may vary from dark brown or grey through to black shale shown in (a) which is typical of the Utica Formation in Quebec, or the shales of the Horn River Basin in British Columbia.
In some basins, there can be fined grained shale rocks interbedded with coarser grained siltstones as shown in (b) can be sometimes called a Hybrid Shale
In other scenarios, structural processes can create fractures, as shown in (c), creating natural pathways for natural gas or oil to flow.
References:
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“Understanding Shale Gas in Canada“, Canadian Society for Unconventional Gas.