Diagenesis stage division and petroleum geologic significances of Dainan Formation in Jinhu Sag, Northern Jiangsu Basin

Based on the analysis and testing data of microscopic slice, scanning electron microscope (SEMP), X-ray diffraction and fluid inclusion microthermometry, the diagenesis stage of clastic reservoirs of the Dainan Formation in the Jinhu Sag has been divided and confined according to vitrinite reflectance, buried history curve, and the variation of clay mineral as well as various diagenesis phenomena and marks. Most of the Dainan Formation is in A phase of the middle diagenesis stage, and less in B phase of the early diagenesis stage and B phase of the late diagenesis stage. Corresponding to diagenesis stage, pore types in the Dainan Formation also evolve from primary to secondary ones. At present, secondary pores account for a large amount in the Dainan Formation while primary pores are seldom found. It is predicted that the areas around wells Xinzhuang 1, Tian X77 and Guan X2 are favorable for reservoir formation. Based on the division of diagenesis stage, the organism in the underlying Funing Formation in most of the study area is predicted matured as the main hydrocarbon source rocks of the Dainan Formation. And it is inferred that the Dainan Formation in the areas around wells Qin X2, Tian X75 and Tian X92 has encountered strong structural uplift and erosion during the early buried stage.