Abstract: Based on thin section analysis and X-Ray diffraction bulkrock analysis, large amount of authigenic analcite is discovered for the first time in fine-grained sedimentary rocks and tight sandstone interbeds of the Second Member of Kongdian Fromation (Ek₂) in Cangdong sag, Huanghua Depression. Under microscope, analcite is observed mainly in the form of concentrated laminations, locally as dispersed micro-crystals, and occasionally as silt to coarse sized cement, filled in inter- and intro-granular pores, as well as in fractures and liquefied veins. This occurrence of analcite suggests an early diagenetic origin. Macroscopically, it is closely related to clay minerals and the mineral contents display an interactive relationship, which indicates a non-volcanic precursor, consistent with the lack of volcanic detritus in the fine-grained deposits in this block. As an authigenic mineral precipitate under alkaline environment, its early diagenetic origin indicates that the sedimentary water body was moderately saline and alkaline. Moreover, its early precipitation could inhibit compaction and partially preserve pore spaces, and in deeper burial diagenesis, secondary pores are fromed due to local dissolution of analcite when react with organic acid. In this way, analcite plays an important role in improving properties of tight reservoirs.