Based on the sequence stratigraphy study, the relationship between dark mudstones content and sedimentary facies within different system tracts is observed and used to forecast the distribution of dark mudstones in the main formation combined with the seismic data and well log. However, not all dark mudstones can generate hydrocarbon, so, the source rock criterion is discussed by analyzing pyrolysis data of mudstones, and then the source rock quality is calculated by using logging data for the early-stage prediction of source rocks thickness. The results show that the favorable source rock in lacustrine progressive system tracts and the bottom of highstand system tracts of Xiagou Formation and Chijinpu Formation are related to a group of reflectors with medium-strong amplitude, medium-low frequency and medium to comparatively good lateral continuity. The source rock of Xiagou Formation with high organic content and wide-range distribution is the main hydrocarbon source in Ying’er Sag, while Chijinpu Formation with thick dark mudstones is the potential source rock and the target of the further exploration.