**1. Introduction**

The soft-sediment clasts (SSCs), as aggregations of the fine sediments formed at the syngenetic sedimentary stage, are often dispersedly preserved in the water-transported sandy hosting sediments [1,2]. SSCs are widely developed in a variety of modern and ancient sedimentary settings, including glacial, alluvial, fluvial, estuarine, coastal, shoreline and deep-water environments [3–12]. The terms used by the researchers to characterize sedimentary features are varied, for instance clay balls [3], armored or unarmored mud balls [4,7,13–18], mud pebbles [4], mud clasts [6,8,12,19,20], rip-up clasts [21–25], blocks [26–28], intraformational clasts [29]. SSCs are also commonly regarded

as the palaeoenvironmental indicators to obtain geological information on clasts transportation, deposition and deformation processes, and regional-scale palaeoenvironmental setting [2,30]. Therefore, numerous targeted researches have been carried out on the morphology, classification, formation and transportation mechanism of di fferent SSCs under particular settings [1,12]. In addition to the above types, a special kind of soft sediment clasts is developed in coal-bearing strata, consisting of coaly fragments, carbonaceous mud clasts and coalified plant remains, etc., typically marked by enriching in organic matter. Here, the term "organic-rich clasts (ORCs)" is used for describing this outstanding feature.

As early as the beginning of the last century, the organic-rich clasts in coal-bearing strata had been observed [31]. Since then, the organic-rich clasts were widely found in the coal-bearing strata of the major coal-bearing basins in the world, mostly distributed in the sandstone either close to or far above the top of the coal seams [31–37]. Researchers often interpreted the significance of the organic-rich clasts from the perspective of coal petrology, including morphology, composition, degree of thermal evolution, time of coalification [37–39]. In recent years, coal-derived gas (especially tight sandstone gas) has been developed on a large commercial scale [40,41]. For example, in the Upper Paleozoic tight sandstone gas-bearing strata of the Ordos Basin in China, several tight sandstone gas fields with a reserve of over 1 × 10<sup>8</sup> m<sup>3</sup> have been discovered, like the Sulige gas field [42,43]. Meanwhile, the influence of organic-rich clasts in the coal-bearing strata on tight sandstone gas reservoirs has also received attention, involving diagenesis, property and gas accumulation [20,44,45]. Moreover, it must also be mentioned that organic-rich clasts in sandstone (mainly terrestrial higher plant) can supply abundant hydrocarbon [46,47]. Despite these geological implications, however, few studies have ever focused on the formation and transportation mechanisms of organic-rich clast in the coal-bearing sandstone strata.

Taking the Upper Paleozoic coal-bearing tight sandstone formation in the northeast margin of the Ordos Basin as an example, the present paper has three main objectives: (1) to identify the morphology and distribution of organic-rich clasts in di fferent sedimentary environments; (2) to investigate the classification and sedimentary sequence of organic-rich clasts in the coal-bearing strata; (3) to systematically analyze the formation mechanisms of organic-rich clasts in the coal-bearing tight sandstone. The results of this paper will be conducive to deepen the understanding of organic-rich clasts.

## **2. Geological Setting**

The Ordos Basin, located in the western part of the Sino-Korean plate (Figure 1A), covering an area of about 25 × 10<sup>4</sup> km2, is an intracratonic basin developed from the Archaean to Early Proterozoic [48,49]. As a large polycyclic superimposed basin, the Ordos Basin underwent a multi-stage tectonic movement during a long geological history. According to the current tectonic features of the Ordos Basin, six tectonic units were identified, which are composed of the Yimeng uplift in the north, the Weibei uplift in the south, the Western margin thrust belt and the Tianhuan depression in the west, the Jinxi fault-fold belt in the east, and the Shanbei slope in the central region [50]. The study area is located in the northeastern margin of the Ordos Basin, presenting a gentle monoclinal structure towards the west on the whole (Figure 1B). In addition, many small folds and faults have developed on it [51].

The Upper Paleozoic strata in the Ordos Basin consist of the Upper Carboniferous Benxi Formation, the Lower Permian Taiyuan Formation and the Shanxi Formation, the Middle Permian Lower and Upper Shihezi Formation, and the Upper Permian Shiqianfeng Formation (Figure 1C, D and Figure 2). Benxi Formation, Taiyuan Formation, and Shanxi Formation are coal-bearing strata, which are the interest intervals of this study. In the Late Carboniferous-Early Permian, a set of coal-bearing sedimentary formation deposited based on the Ordovician paleo crust of weathering with the Yinshan paleo-land supplying sources [52]. From the Benxi Formation to the Taiyuan Formation, a barrier coastal sedimentary system developed in the background of the epeiric sea, mainly including barrier islands, lagoons, tidal flats, and carbonate platforms [53]. Nevertheless, the fluvial-delta sedimentary system developed in the Shanxi Formation [54–56]. A warm and humid climate and lush vegetation promoted the development of organic-rich sediments composed of coal, carbonaceous mudstone and dark mudstone, with a cumulative thickness of > 200 m [57]. Moreover, the coal-bearing sequences are the main source rock characterized by a high content of thermally mature total organic carbon, providing a sufficient source of gas for the tight sandstone gas reservoirs in the Upper Paleozoic [58,59]. Besides, barrier island sandstone and channel sandstones constitute the favorable reservoirs adjacent to the source rocks [60].

**Figure 1.** (**A**) Location of the Ordos Basin in simplified tectonic map of China. (**B**) Tectonic division of the Ordos Basin (modified after Luo et al.) [61], showing the location of the study area. (**C**) Schematic geological map of the northeastern margin of the Ordos Basin showing the drilling sites. (**D**) Sketch cross section through the south of study area (modified from Liao et al.) [62], position located on (**B**).


**Figure 2.** Lithochronostratigraphical column of the Upper Paleozoic strata in the study area. Here, we described the development characteristics of organic-rich clasts in coal-bearing sandstones in detail through core observation, and the descriptions contain lithology, morphology, distribution, and deformation. Thirty-six typical hand specimens were selected from the cores to prepare thin sections; meanwhile, the microscopic feature information of ORC was captured by the ZEISS optical microscope. Twelve whole-rock polished blocks were prepared to carry out maceral analysis. Maceral analysis was performed under a Zeiss Axio imager microscope equipped with an oil immersion objective and a white incident and a blue light source where >800 points were considered for each sample. Maceral was classified by the ICCP (International Committee for Coal Petrology) System 1994 [63–65]. The lithofacies codes are named according to [66]; some codes are added and modified following [67]. Based on the facies data derived from core descriptions and logging interpretations, a series of detailed comparative analysis was made on the organic-rich clasts, in order to find out the origins and control factors of ORCs developed in different sedimentary environments.
