This study for the first time demonstrates that some of the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales, which are rich in black organic matter (including oil and asphaltene), in the Chang 73 Submember of the Upper Triassic Yanchang Formation in the Ordos Basin of China are actually clay-sized tuffaceous rocks (including tuff, sedimentary tuff and tuffaceous sedimentary rock) with high hydrocarbon generation capacities. Thus, these rocks can be defined as clay-sized tuffaceous source rocks. Identification of this lithology has important theoretical and practical significance for the exploration and development of shale oil in the Chang 7 Member. Through the macroscopic observation of drill cores and outcrop profiles, microscopic observation of electron probe thin sections and whole-rock inorganic geochemical analysis (including major, trace and rare earth elements), this work demonstrates that the organic matter-rich clay-sized tuffaceous rocks, especially clay-sized tuffs, have the following characteristics. First, the clay-sized tuffaceous rocks with little black organic matter are mainly greyish white, yellowish brown and purplish grey, and mixed colors occur in areas with strong bentonite lithification. Second, the clay-sized tuffaceous rocks have experienced strong devitrification and recrystallization, forming abundant flaky aluminosilicate minerals with directional arrangement. In thin sections under a polarizing microscope, the interference colors generally show regular alternation between the lowest interference color of first-order yellow and the highest interference color of second-order blue-green. Third, the rock samples plot in the igneous rock field in the TiO₂-SiO₂ cross-plot and exhibit similar trace element and rare earth element patterns on spider diagrams, indicating that the samples are derived from the same source. The results prove that clay-sized tuffaceous rocks may be widespread in the Chang 73 Submember of the Upper Triassic Yanchang Formation in the Ordos Basin, China. 摘要 在鄂尔多斯盆地上三叠统延长组长 7₃ 亚段中, 发现一些能肉眼观察到的“泥级泥岩”(如泥级黑色泥岩、 泥级油页岩等), 实际为富含黑色石油、 沥青质等黑色有机质并具有较强生烃能力的泥级凝灰岩、 泥级沉凝灰岩、 泥级凝灰质沉积岩等泥级凝灰质岩, 可定义为泥级凝灰质烃源岩. 泥级凝灰质岩的识别对长 7 段页岩油的勘探开发具有重要意义. 通过钻井取心及野外露头剖面观察, 电子探针薄片的显微镜镜下观察, 全岩主量元素、 微量元素、 稀土元素等无机地球化学分析, 认为富含黑色石油沥青质等黑色有机质的泥级凝灰质岩, 特别是泥级凝灰岩主要具有以下识别标志: 仅少量黑色有机质覆盖的部位主要呈灰白色、 土黄褐色和紫灰色, 斑脱岩化作用强烈时颜色杂乱; 脱玻化-重结晶作用强烈时, 生成大量呈片状定向分布的铝硅酸盐矿物, 在电子探针薄片的偏光显微镜正交光下插入石膏试板、 旋转载物台, 干涉色整体呈现最低干涉色一级黄、 最高干涉色二级蓝绿的规律变化; 在 TiO₂−SiO₂ 图解上落在火成岩范围, 微量元素和稀土元素蜘蛛图特征大体相似, 反映同源特征. 结果表明, 鄂尔多斯盆地上三叠统延长组长 7₃ 亚段可能发育很多泥级凝灰质岩.