Establishment Of Sequence Stratigraphic Framework And Prediction On Distribution Of Lithologic Trap Zone In The Middle Of The Third Member Of The Shahejie Formation, For The North Salt-Lake Basin Of Dongpu Depression

After 30 years’ exploration and development, Dongpu depression has entered a high mature stage of exploration. As the conventional structural traps become fewer and fewer, lithologic traps will be the main target for future exploration. However, Dongpu depression has more complex structure and thick gypsum halite. At present, all the negative factors, which include larger arguments on the provenance direction and type, characters and distribution of the depositional system, the development rule of halite, as well as unclear relationships between the rule and the lithologic traps, are the barriers for the development to find the traps.In this paper, I did the sequence stratigraphic research on the middle section of the third member of Shahejie formation in the north of Dongpu depression, Based on the fruit of former researcher, analyzed drilling, well logging, seismic and testing materials, applied high resolution sequence stratigraphic theory and method. And then i established the stratigraphic framework, studied the regulation of gypsum halite distribution, clarified the type and distribution of depositional systems. Finally, we did the research on the lighologic traps, especially on traps related to halite, and established its mode, pointed out the favorable zone for exploration. Then several points were achieved as follows:Firstly, the middle section of the third member of Shahejie formation might be divided into one long-term base-level cycle, or three intermediate base-level cycles, or six short half base-level cycles. Secondly, multi-source in Dongpu depression converged on the central uplift. The scope effected by the different stages of the source was different. The source direction, which in the west had entered into the lake diagonally and in the centre and north had gone along with the basin strike, was controlled by the transform tectonic zone. Thirdly, There were 14 gypsum halite rhythms in the middle of the third member of Shahejie formation. And halite mainly located in Hubuzhai, Weicheng and Wenbei, among which Weicheng and Hubuzhai were the main depocenters, which had migrated during different periods. In the vertical section, halite mainly developed in the upper of the middle set of the third member of shahejie formation, and the single halite rhythm near the flooding surface was thicker. Halite and sandstone behaved growth-decline relationship, and finger-like cross distribution. Fourthly, the main depositional systems were different kinds of delta fronts and lack of delta plain in the study area. In the east, the topography to the Lanliao fault zone was steep, the source was near, and fan delta deposited here. In the west, the topography was gentle, the source was far, and braided river delta developed. In the north and south, delta along the strike of basin distributed, turbidity channel, storm deposition system, beach, bar also existed. Fifthly, the main lithologic traps were sand pinch-out trap, sand lenticular trap, sand or conglomerate fan trap, isolated channel and fault-lithologic traps in the middle of third member of Shahejie formation in the north of Dongpu basin. The development of the favorable traps mainly were the central uplift and its both side, east-west sags and around sags, where had have abundant oil and gas source, and where had been a good place for developing traps.