Abstract: Based on cores, geochemical and other data, in addition to regional sedimentary background, we studied the lithology and sedimentary characteristics of source rocks of Es₄ in the Bonan Depression. The source rock was formed in a wide deep basin and characterized by DBT/P value greater than 1, gammacerane index greater than 0.2, undeveloped dinosterane, lower developed triaromatic steranei, a sterane / hopane ratio of 0.10~0.28 and negative δ¹³C with relatively small variation. Carbonate content is rather high indicating a carbonate hydrocarbon. The high content of sulfur suggests a larger water depth.