In order to clarify the origin of calcite minerals and the diagenetic characteristics of reservoirs in Dongying Sag，the calcite veins in Well Niuye1 are analyzed with thin section，X-ray diffraction and carbon-oxygen isotope. It is discovered that they can indicate top-bottom structure and have mechanical twin crystals characteristics，which provides the evidence that they are products of the in-situ stress generated by geological tectonic movement acting on the shale，and they provide carrier minerals for the study of in-situ stress in shale formation. Through comparative analysis of carbon and oxygen isotopes of micrite calcite and sparry calcite in shale of the study area，it is found that micrite calcite in shale mainly belongs to carbonate related to biogas generated by methane bacteria. The sparry calcite veins are likely to be the product of precipitation and recrystallization of the mudstone calcite after dissolution. The micro-fractures caused by hydrocarbon generation and pressurization are the prerequisite for the formation of the calcite veins，and the dissolution and reprecipitation of argillaceous calcite by organic acid is a necessary condition. Based on the four petrological characteristics and the carbon-oxygen isotope，the syngenetic evolution of the hydrocarbon-generating pressurization，micro-fracture formation，organic acid discharged from thermal evolution of organic matter，dissolution of micrite calcite，recrystallization of calcite，and formation of calcite veins are described. The source of carbon pool of shale and adjacent sandstone and the process of synergistic diagenesis of sandstone and mudstone are discussed as well.