Description

The facies association I is composed of Gc, Sm, St, Sp, Sr, Fl, Si, Fr, and C (Figure 4A). The sandstones are characterized by large thickness, intercalated with thin layers of siltstone and mudstone. Sedimentary structures such as basal erosional surface, massive bedding, cross-bedding, parallel bedding, and wavy bedding are well developed. Overall, the sandstones of facies association I have a fining-upward trend and a bell-shaped or jugged box-shaped logging curve (Figure 4A). On the erosion surfaces, the organic-rich clasts are most concentrated, which are poorly sorted, angular to sub-angular, and chaotic (Figure 4B,E–G,K). In the middle parts of the sand body, the floating ORC is scattered with long-axis tending towards bed-parallel orientations, mostly the shape of slender and deformed (Figure 4C,J,L). The occurrence of small granule ORC is in the form of aggregates as discontinuous or continuous laminations in the upper parts (Figure 4D,H,I). These clasts are flaky, good sorted and rounded, coexisting with high fine-grained sediment content (Figure 4C,J,M).

**Figure 4.** Sedimentary characteristics of the organic-rich clasts in the fluvial channel or distributary channel (log LX-21). (**A**) Schematic sedimentary succession with natural gamma-ray curve in the channel environment, showing the occurrence of ORCs in different sedimentary locations. (**B**–**M**) Typical core photographs of various ORCs occurring in different lithofacies. (**C**) The position of logA in the schematic diagram, showing the cause of ORC formation in channel: bank collapse and basal erosion.
