A set of mixed clastic-carbonate deposits are developed in the Lower Cretaceous Erlian Basin, and abundant hydrocarbon shows exist in the dolomitic rocks. According to core observation, microscopic identification, physical analysis and rock thermolysis, the dolomitic rocks is a kind of low permeable-tight reservoir with high porosity and low permeability, which has developed "sweet points" with mold holes and structural corroded fissures generated by dissolution or tectonism. It is concluded that source rocks with high carbonate content are located near the low permeable-tight dolomitic rock reservoirs and lack hydrocarbon-generating ablxormal high pressure due to the lower maturity; nevertheless, the high-quality dolomite mudstone (TOC〉2%) is conducive to the formation of dissolution pores in the adjacent reservoir and the reduction of oil filling resistance, providing good conditions for oil accumulation of the proximal source type.