**3. Lithofacies of shales**

In the last few decades, the most of organic-rich shale-related plays, for example Barnett, Niobrara, Eagle Ford, Haynesville, and Bakken, have been labeled as "shale plays," and the terms "shale oil," "tight oil," and "resource play" are often used interchangeably in a public discourse [28–32]. The Barnett shale is a typical organic-rich siliceous shale (**Figure 2a**). The Eagle Ford shale is an organic-rich shale interbedded with thin fine-grained carbonate beds (**Figure 2b**). The lacustrine Green River shale consists of organic-rich shale with fine-grained ostracod rich carbonate (**Figure 2c**). The Niobrara shale play is dominated by a low permeability fine-grained chalk sourced by its adjacent marl source rock with continuous hydrocarbon accumulation [33]. The chalk and marl cannot be differentiated by naked eyes and appear like

**Figure 2.** Lithofacies of typical fine-grained "shales" in U.S. The Green River Fm is of lacustrine environment and the rest are of marine environment.

organic-rich argillaceous shale due to fine-grained nature and both containing organic matter (**Figure 2d** and **e**). The Bakken/Exshaw play (the USA and Canada) mostly consists of permeable fine-grained tight dolomite within upper and lower organic-rich shales (**Figure 2f**). The reason for which all these different lithofacies are called shale is the fine-grained nature of these deposits. Shale is defined as a fine-grained sedimentary rock that is a mix of clay minerals and quartz, calcite, and so on. The typical grain size range of shale is less than 1/256 of a millimeter in diameter. The ratio of clay to other minerals is variable. Shale used to be mistakenly considered as clay. For the organic-rich source rock interval, it usually has a mixed succession of interlayered fine-grained shales, sandstone, and/or carbonates. They are often called shale play due to their fine-grained grain size and to their close relationships with organic-rich shale. The shales representing different geologic settings in this study from different basins around the world refer to the fine-grained stratigraphic interval consisting of siliciclastic-dominated and carbonate-dominated lithofacies ranging from primary organic-rich shale to minor finegrained organic-lean carbonate or sandstone interbeds.
