**4. Results**

Eleven types of lithofacies related to organic-rich clasts are recognized within the Carboniferous-Permian coal-bearing strata (Table 1). These have been divided into 5 types of facies associations standing for different sedimentary processes and settings, which have been identified in cores in conjunction with the interpretation of logging curves. These facies associations are mainly interpreted as channels (fluvial channels and delta distributary channels), crevasse splays, sand flats, tidal channels, and subaqueous debris flow deposits. The occurrence of organic-rich clasts will be described in detail below, representing the variation of water-flow energy during sediment transport.


**Table 1.** Main lithofacies of the Upper Paleozoic Benxi, Taiyuan and Shanxi Formations in the northeastern margin of the Ordos Basin.


**Table 1.** *Cont.*

The organic-rich clasts are widely distributed in the coal-bearing tight sandstones of Benxi Formation, Taiyuan Formation and Shanxi Formation in the study area. The lithology of ORCs identified in the core includes carbonaceous clasts, carbonaceous mudstone clasts, dark mudstone clasts, shale clasts, and plant fragments (Figure 3A–D). In terms of color, the organic-rich clasts are mostly black or grayish black, and easy to dye the hosting sandstone. Moreover, the associated pyrite, commonly identified in the core hand specimens, is a characteristic mineral formed from organic-rich clasts in a local reducing microenvironment during diagenesis (Figure 3E). The shapes of ORCs are varied, including blades, discs, plate strips, tearing chips and irregular forms. The grain size is generally gravel size, with a maximum of more than 10 cm. Although strongly deformed, the long axes of most ORCs are parallel or sub-parallel to the stratum (Figure 3A,B).

**Figure 3.** Typical photographs of core (**A**–**E**), thin sections (**F**–**H**) and polished blocks (**I**–**N**) of ORC in the studied coal-bearing sandstones. Abbreviations: Cc = Carbonaceous clasts, Cm = Mudstone clasts, Cs = Shale clasts, Sdp = Secondary dissolution pores, PPL = plane-polarized light, RL = reflected light. (**A**) Scattered coaly clasts with plastic deformation (LX-32, 1890.20 m). (**B**) Cobble dark mudstone clasts in the conglomerate (SM-18, 1871.10 m). (**C**) Shale clasts showing clear bedding (SM-21, 1694.11 m). (**D**) Plant leaves with clear outlines (SM-18, 1961.29 m). (**E**) Associated pyrite nodules of ORC (LX-8, 1969 m). (**F**) Thin-bedded ORC containing fine-grained sediments (PPL, LX-44, 2063.12 m). (**G**) Secondary dissolution pores of feldspars around the ORC (PPL, SM-4, 2134.1 m). (**H**) A strip of ORC with secondary dissolution pores (PPL, LX-103, 1721.5 m). (**I**) Homogenous collotelinite (RL, SM-19, 2116.52 m). (**J**) Cementing collodetrinite (RL, LX-35, 1922.54 m). (**K**) Blocky collotelinite and vitrodetrinite (RL, SM-20, 2072.98 m). (**L**) Scattered vitrodetrinite in the matrix (RL, SM-19, 2083.1 m). (**M**) Clumpy pyro-fusinite and scattered vitrinite (RL, LX-33, 1732.1 m). (**N**) Cutinite with yellow fluorescence (fluorescence-mode, SM-7, 1872.80 m).

Under the microscope, ORCs with small particle size can also be observed widely, which usually show a characteristic of compressional deformation (Figure 3F–H). The results of microscopic observation show that these ORCs are mainly composed of organic matter (OM), fine-grained sediments (Figure 3F–N), and some authigenic inorganic minerals (such as pyrite). As for the organic macerals, there are mainly vitrinite, inertinite and liptinite (Figure 3I–N). Vitrinite (type III kerogen, formed by the gelification of plant remains) is the most abundant organic component, with light gray to gray under the reflected light (Figure 3I–M). Most of these Vitrinite components are collotelinite (Figure 3I,K), collodetrinite (Figure 3J) and vitrodetrinite (Figure 3I,K–M). Vitrodetrinite, especially for the muddy ORCs, is present in the form of small discrete particles in the argillaceous matrix (Figure 3K,L). Inertinite (type-IV kerogen, derived from terrestrial plant tissues) consists of semifusinite, fusinite (Figure 3M), micrinite and inertodetrinite, and has bright white color under the reflected light. Liptinite (type-II kerogen, transformed from plant organs) is relatively rare in the analyzed samples. Only some cutinite (Figure 3N) and bituminite with obvious fluorescence characteristics are seen. Thus, it can be seen that the organic matter found in ORCs is mainly derived from terrestrial higher plants.

#### *4.1. Facies Association I*
