Characteristics and Genetic Mechanism of Tight Sandstone Reservoirs of Lower Crataceous in North Yellow Sea Basin

North Yellow Sea Basin is a typical Meso-Cenozoic superimposed fault basin, which has undergone few exploration and research in the offshore of China. The sandstone reservoirs of Lower Crataceous are the main exploration targets, but reservoirs tightness is the "bottleneck" which restricts the exploration activity in North Yellow Sea Basin. By core observation, thin section analysis, scanning scanning electron microscope observation, mercury penetration analysis, porosity and permeability statistics, the reservoirs characteristics of Lower Crataceous in eastern depression in North Yellow Sea Basin were researched in this paper. According to deposition, diagenesis, burial history, and so on, the origin mechanism of tightness of the reservoirs was analyzed. The results show that the sedimentary facies type of reservoirs is mainly fan delta. Reservoir physical properties are poor, belonging to extra-low porosity and extra-low permeability tight sandstone reservoirs. Main reservoir spaces are corrosion pores between grains or in grains and a few micro-fractures. The reservoir belongs to dual porosity reservoirs with pores and fractures, and the pore structure is complex. The reservoirs characteristics are controlled by sedimentary facies zone, diagenesis and burial history. The reservoir lithology is mainly feldspathic lithic sandstone and lithic feldspathic sandstone, which have medium textural maturity and compositional maturity, resulting in poor initial reservoir quality. The high content of plastic debris is not conducive to the preservation of primary porosity. The grain size of sediment affects the development of micro-fractures. The diagenesis predominates in the reservoirs tightness. The diagenetic process is decided by the burial history, which includes early deep burial, middle uplift denudation, and later deep burial. The reservoirs tightness is mainly caused by intense compaction in early phase and cementation in later phase, such as quartz increase, calcite cementation, and spontaneous mineral crystal. Feldspar corrosion and inverting kaolinite facilitate the formation of secondary porosity. The late of hydrocarbon injection causes the weak acid fluid activity, which inhibits dissolution. Fractures without displacement in later phase improve partly the reservoir property.