| Great breakthroughs have been made in shale oil exploration of Fengcheng Formation in Mahu sag,Junggar basin,but the reservoir characteristics and internal shale oil occurrence characteristics of fine-grained sedimentary rocks of Fengcheng Formation need to be further studied.Based on the core observation of MY1 well and combined with macro-micro analysis means,the reservoir and oil-bearing characteristics of various types of fine-grained rocks are fully revealed,and the following conclusions are obtained:The fine-grained sedimentary rocks of Fengcheng Formation in well MY1 are mainly deposited in semi deep lake to deep lake,with thin-layer event deposits such as gravity flow and hydrothermal solution.X-ray diffraction data shows that the fine-grained rocks of Fengcheng Formation are mainly composed of felsic and carbonate minerals,and contain a small amount of clay minerals dominated by illite and Yimeng mixed layer.Based on the classification scheme of felsic-carbonate-clay minerals,it is determined that siliceous rock,mixed siliceous mudstone,siliceous mudstone containing carbonate and siliceous carbonate mudstone are the main fine-grained rock types and research units,accounting for more than 95%of the stratum.Combined with identification under microscope argon ion polishing scanning electron microscope observation,intergranular pores,intergranular pores and dissolution pores are generally developed in siliceous rocks,mixed siliceous mudstones and siliceous mudstones containing carbonate,a large number of dissolution pores and more intergranular pores are developed in siliceous carbonate mudstones,and each fine-grained rock contains a small amount of organic pores and fractures.Based on the combined analysis of low-temperature nitrogen adsorption-high pressure mercury injection-CT-FIB-SEM and other means,the pore diameter of siliceous rock,carbonic acid containing siliceous mudstone and carbonic acid containing siliceous mudstone is mainly 5～60nm,and the nano pore of mixed siliceous mudstone is concentrated in 2～20nm.The micron sized pores of siliceous rock,carbonic acid containing siliceous mudstone,carbonic acid containing siliceous mudstone and mixed siliceous mudstone are mainly between 0.10～0.50μm、0.03～0.22μm、0.006～0.10μm、0.01～0.10μm.The pores of siliceous rock and carbonic acid bearing siliceous mudstone are in a focused mass and nearly ellipsoidal shape,and most of the isolated pores are connected with the small throat and converge on the medium and large throat.The micro pore throat system of siliceous carbonate mudstone and mixed siliceous mudstone is basically not connected,and the throat is mainly nano.According to the fractal theory of nitrogen adsorption and high-pressure mercury injection,the pore structure of siliceous rock is the simplest,which is followed by siliceous carbonate mudstone,carbonate siliceous mudstone and mixed siliceous mudstone.S1,TOC and OSI parameters of fine-grained rocks show that Fengcheng Formation has good oil bearing property.The oil bearing property of siliceous rock is the best,the oil bearing property of siliceous carbonate mudstone is poor,and the oil bearing property of mixed siliceous mudstone and carbonate siliceous mudstone is medium.Based on the constraints of extraction experiment and correlation pore size analysis,it is estimated that the lower limit of shale oil occurrence in Fengcheng Formation is about20nm.In general,siliceous rock is the most dominant shale oil reservoir,followed by carbonic acid containing siliceous mudstone,siliceous carbonate mudstone is relatively poor,and mixed siliceous mudstone is the worst.This achievement will provide a target for further exploration and evaluation of shale oil in Fengcheng Formation. |
