Study On The Origin And Distribution Of Saline Sediments Of E₁f₄ In Zhaoji Sag, Hongze Depression, Northern Jiangsu Basin

Large sets of saline mineral layers which are thick and pure and have broad exploration prospects develop in the Member 4 of Funing Formation of Paleogene（E₁f₄） in Zhaoji Sag of Hongze Depression, North Jiangsu Basin. In order to find out the origin of saline sediments of E₁f₄, the saline sediments’ characteristics were studied by means of mud logging, well logging, paleontological and geochemical data.The E₁f₄ saline formation mainly develops halilyte rock, sulfate rock, and a small amount of carbonate rock. The halilyte and sulfate mineral layers are relatively thick, pure and distribute mainly in the Upper Salt Bed Sub-member and the Lower Salt Bed Sub-member in a lenticular shape, and compared with the sulfate mineral layers, the halilyte mineral layers distribute less stably, especially the thin halilyte mineral layers. On the plane, saline rock mainly distribute in the deep sag or the slope in the form of rings, and the halilyte rock distribute in the sedimentary center, sulfate rock, anhydrite rock and mudstone distribute outside in turn.Halite mineral deposit harder, and distribute mainly in the deep sag, and display in the single centers-lenticular’s form, and halite is easy to dissolve,which makes the distribution become complex, therefore, the halite mineral layers also display in the multiple centers-lenticular’s form and zero center-wedge’s form. Sulfate mineral deposit easier, and distribute in the outside of the halite mineral layers, mainly in the the deep sag and slope, generally display in the multiple centers-lenticular’s form, and because sulfate mineral is hard to dissolve and so distribute more stably.Through analyzing the tectonic characteristics, geomorphology, paleoclimate and lithofacies paleogeographic environment and the source of salt substance during the salt-forming period of E₁f₄ in Zhaoji Sag, it is found that Zhaoji Sag is the semi-closed deep basin with high mountains during the sedimentary period of E₁f₄, much saline substance of Pukou formation of Late Cretaceous（K2p） in Huaian Uplift was dissolved and transported into the deep water of Zhaoji Sag, and then made the saline substance reach saturation, finally resedimented to form the saline formation of E₁f₄.The salt-forming period of E₁f₄ in Zhaoji Sag can be divided into two stages: the Upper Salt Bed Sub-member and the Lower Salt Bed Sub-member. Through comparative study on the Upper Salt Bed Sub-member and the Lower Salt Bed Sub-member, it is found that there is a big difference on the types and characteristics of the saline sediment and sedimentary sequences, therefore, the salt-forming models could be trying to establish according to the differences in the rate and composition of salt supplies. The salt-forming model of the Upper Salt Bed Sub-member is “the resedimented model :Halite---Thenardite---Halite” in the saline substance supplied quickly, near source and deep water sedimentary environment; and the Lower Salt Bed Sub-member is “the resedimented model : Halite---Glauberite---Halite” in the saline substance supplied quickly, near source and deep water sedimentary environment.