Lithologic Characteristics,Reservoir Quantification Model And Comprehensive Characterization With Geophysics Of Precenozoic Buried-hill In Lishui Sag,East China Sea Basin

With the gradual deepening of oil and gas exploration and the expansion of oil and gas exploration areas,basement buried hill reservoir as a special type of oil and gas reservoir has attracted more and more attention.Buried-hill reservoirs have complex lithology and are mostly found in igneous rocks,especially granite buried-hill reservoirs show good exploration potential.Previous studies have shown that the buried hill in Lishui sag has good reservoir forming conditions.Through the lithology revealed by drilling and comparing the lithology characteristics of the discovered buried hill oil and gas reservoir,it shows that the Xianqiao buried hill belt and Lingfeng buried hill belt in Lishui Sag have great exploration potential.Taking Lishui sag and its adjacent area as an example,this paper identifies and classifies the lithology of the study area according to the core,wall core,rock cuttings and logging data,and comprehensively explains the lithology and Geophysics in combination with waveform classification and magnetic anomaly,so as to clarify the plane distribution characteristics of the lithology of the basement buried hill in the two work areas of Lishui sag.According to the petrogenesis,fabric and mineral classification,a total of 3 types and19 kinds of lithology are identified.The results show that the lithology of buried hill is mainly regional metamorphic rock and intrusive rock,mainly granite and gneiss,which are significantly different in logging response and seismic facies characteristics.On the plane,the distribution of basement granite and gneiss is zonal.Lishui North work area is of medium and low magnetic lithology,mainly composed of medium magnetic granite and low magnetic granite,intercalated with medium magnetic gneiss and low magnetic gneiss.The western part is mainly low-magnetic granite,the eastern part is medium-magnetic granite,the middle part is low-magnetic gneiss between z13-6 and Z13-5 well,and the west side of Lingfeng fault in the northern part of the sag and the vicinity of Z20-1 well is medium-magnetic gneiss;Lishui South work area is medium and high magnetic rock,mainly including high magnetic granite,medium magnetic granite,high magnetic gneiss and medium magnetic gneiss.There are high magnetic granite in the west,high magnetic gneisses in the middle,medium magnetic granite in the southeast and medium magnetic gneisses in the northeast,among which the location of T1 well is high magnetic granodiorite.At the same time,the field outcrop and drilling data are used to identify the types of reservoir space and analyze the characteristics of reservoir physical properties.It is found that the buried hill in Lishui sag mainly develops five types of 9 kinds of reservoir space,mainly secondary pores and secondary fractures.Secondary pores include dissolution pores(intracrystalline micropores and matrix sponge dissolution pores)and inter breccia pores in fault breccia.Fractures include structural fractures(shear structural fractures and tensile structural fractures),weathering fractures(layer joints,spherical weathering fractures and micro fractures)and dissolution fractures.The porosity and permeability test results show that the physical properties of granite reservoir are better than gneiss.The reservoirs in the study area are mainly weathered reservoirs with obvious vertical zoning.The weathering degree gradually decreases from top to bottom,which can be divided into soil layer,weathering dissolution zone,fracture zone and bedrock zone in turn.The rock strength change rate ??? Permeability change rate ??? Magnetic susceptibility change rate ?? and CIA index are used to quantify the different zonation.The results show that rock strength,permeability and CIA index have better effect on reservoir zonation,but the magnetic susceptibility is not obvious.The crushing area in the fracture zone contributes more to the fracture,which is the sweet spot in the buried hill reservoir.According to the intersection of the magnetic susceptibility data measured in the field outcrop area and the calculation results of fracture surface density,they have a negative correlation.The lower the magnetic susceptibility,the higher the fracture surface density.Combined with the plane distribution characteristics of basement lithology,the reservoir prediction results based on magnetic anomaly are obtained.Through the simulation of relevant seismic attributes,the reservoir prediction results based on fracture strength,fracture development degree and fluid analysis are obtained.On this basis,the four prediction results are superimposed,and finally the reservoirs in the study area of Lishui sag are divided into four categories: I,II,III and IV.The results show that the area around the fault and the intersection of the fault are better.On the whole,the reservoir in Lishui North work area is better than that in Lishui South work area.Among them,the southwest of well F1 and the west of well Z20-1 are the areas with the widest distribution of class I reservoirs,which are the most favorable areas for reservoirs.The comprehensive prediction results of reservoirs in the study area can provide a basis for further exploration.