Origin And Evaluation Of Effective Reservoir Of Sha3 And Sha4 Member In Northern Steep Slope Of Dongying Sag

Dongying Sag is located in the southeast of Jiyang Depression, it's very rich in oil and gas resources. There is a strong need to determine the effective reservoir in order to explore and develop natural gas in the area. The research object of this paper is Sha3 and Sha4 member in northern steep slope of Dongying Sag. Based on previous study, and guided by the theory of sedimentology and reservoir geology, author elaborated the depositional system, diagenesis and the cut-off of effective reservoir etc. in this paper, by identifying rock thin section, cathodoluminescence, SEM observation, mercury intrusion analysis, logging, database of core porosity and permeability. There are five primary views are achieved in this study as following:Firstly, in northern steep slope of Dongying Sag, the main types of reservoir sandbody of Sha3 and Sha4 member include alluvial fan, nearshore subaqueous fan, fan delta, turbidite fan.Secondly, the deposition in northern steep slope of Dongying Sag is near source coarse-grain, so it has the low textural and compositional maturity. Overall, in the studied area, the rigid stability component of rock is lesser than the plastic instability component. And it is mainly composed of lithic feldspathic sandstone, feldspathic lithic sandstone and lithic sandstone.Thirdly, the Paleogene of Dongying Sag has a higher degree of diagenetic evolution. As the Sha3 and Sha4 member exists deeply, the diagenesis is intensive. By identifying rock thin section and SEM observation, there is a discovery of dissolution of framework grains and cements. The feldspar and carbonate cements are dissolved obviously. Thus, although most primary pores of target layers are not developed(the porosity is generally not more than 5%), the secondary pores are more. That is a relatively basic condition for development of excellent reservoirs of deep layers. Fourthly, the porosity of reservoir in northern steep slope of Dongying Sag is from 10% to 25%, and its average value is 15.63%; The permeability is in a wide range, changing from 0.01×10-3μm2 to 1000×10-3μm2, but it mainly ranged 1~100×10-3μm2. The average value of permeability is 10.23×10-3μm2.Finally, the controlling factors of effective reservoir mainly include material composition, depositional environment, diagenetic evolution and tectonic fracture. Because of the existence of these factors, the cut-off of effective reservoir between different depth and different deposition system is various.