Mudstone or shale intervals deposited in deep-lake environments in large down-warped lacustrine basins in China—such as the Upper Cretaceous first member of the Qingshankou Formation in the Songliao Basin, and the Middle to Upper Triassic third sub-member of the seventh member of the Yanchang Formation in the Ordos Basin—are source rocks and shale oil “sweet-spot intervals.” However, the influence of lithofacies and depositional processes for mudstone intervals in deep-lake environments on the development of shale oil sweet-spot intervals is unclear. Based on observations and descriptions of cores, thin-sections, X-ray fluorescence (XRF) images, scanning electron microscopy (SEM) images, and analyses of X-ray diffraction (XRD) and trace element data, combined with data and images of the quantitative evaluation of minerals by scanning electron microscope (QEMSCAN), nine mudstone lithofacies (ML) are recognized in the mudstone intervals. They are organic-rich horizontally laminated mudstone (ML1), organic-rich horizontally laminated tuffaceous mudstone (ML2), organic-rich massive mudstone (ML3), laminated silty mudstone with silty clasts/fragments (ML4), convolute bedded silty mudstone (ML5), silty mudstone with lenticular and irregular silty clasts/fragments aligned along bedding (ML6), ripple-cross laminated silty mudstone (ML7), ripple laminated silty mudstone (ML8), and graded bedded silty mudstone (ML9). A warm, wet climate is indicated, together with regional flooding and anoxic events, lake-level rises and falls, tectonic phases and volcanic eruptions, all of which helped to form the MLs in deep-ake environments. ML1 and ML2 were deposited by suspension settling, turbulent flows, or hyperpycnal currents in deep water. ML3 and ML9 were deposited by muddy turbulent flows that settled at a rapid or moderate sedimentation rate. ML4 was deposited by cohesive mudflows. ML5 and ML6 were deposited by a typical “freezing” depositional mechanism involving a thin silty mud layer slide or cohesive fine-grained debris flows. ML7 and ML8 were deposited by lacustrine bottom-currents and fine-grained transitional flows respectively, which resulted from turbulent flows linked to monsoon rainfall. Fine-grained gravity flow sediments were deposited in distal lobes, levees in silty channel-levee-lobe systems, or in distal sub-lacustrine fans. Slack-water fallout sediments were developed in quiet, deep-water lakes, in interchannels, and interlobes in channel-levee-lobe systems, or between sub-lacustrine fans in deep-lake environments. ML4–ML9, intercalated with lithofacies ML1 to ML3 with high total organic carbon (TOC) contents, constituting good source-reservoir assemblages, are favorable “sweet-spot intervals” for shale oil development.