Formation Mechanism Of Tight Oil Reservoirs In Liangshang Member And Shayi Member Of Jurassic,Central Sichuan Basin

The Jurassic Liangshang Member and Shayi Member are the main tight oil exploration horizons,which are rich in tight oil resources.However,the exploration and development of tight oil in Central Sichuan Basin is greatly restricted due to unclear understanding of the formation mechanism of tight oil reservoir.Using reservoir engineering method,minimum flow pore throat radius method and other methods,the lower limit of tight oil reservoir is determined,and then the reservoir is classified and evaluated.On the basis of data obtained by means of optical microscope,cathodoluminescence,scanning electron microscopy,X-ray diffraction,high pressure mercury injection analysis,and physical property analysis,the characteristics of petrology,reservoir properties and diagenesis of tight oil reservoir are analyzed systematically.According to the vitrinite reflectance of mudstone,fluid inclusion temperature,X-ray diffraction analysis of clay minerals,oxygen isotope geothermometer and occurrence relationship of different authigenic minerals,the evolution characteristics of diagenesis and porosity are described chronologically.Based on the statistical analysis data of casting thin sections and constrained by diagenetic evolution sequence,the porosity evolution process in the geological history is described quantitatively and chronologically.According to the fluid inclusion homogenization temperatures and micro fluorescence analysis,and combined with the burial history and porosity evolution,the coupling relationship between the porosity evolution and main oil accumulation is described.Based on the above methods of petrology,mineralogy and geochemistry,the formation mechanism of tight oil reservoir is studied,the main control factors of tight oil reservoir development are summarized,and the formation models of three different types of tight oil reservoir are established,which provides theoretical basis for predicting the distribution of tight oil reservoir.The main achievements list as follows:The permeability and porosity of reservoir lower-limit of sandstones in Liangshang Member and Shayi Member of Middle Sichuan are 0.03 m D and 2% respectively.The permeability and porosity of effective lower-limit are 0.2m D and 2.8% respectively.According to physical properties,sandstones are classified into 4 types including I,II,III and IV,and type I,II and III are tight oil reservoirs.The tight oil reservoirs of Liangshang Member and Shayi Member in Central Sichuan Basin are mainly developed in fine sandstones and medium sandstones of lake beach bar,distributary channel microfacies of lake delta.Compaction is the most important factor leading to the densification of reservoir,and authigenic minerals related to dissolution is another important reason for the decline of physical properties in the middle to late stage.Dissolution of feldspars and magmatic rock fragments is the key factor to improve the physical properties of tight oil reservoir and to form tight oil reservoir.The pore-lining chlorite and hydrocarbon emplacement are the main reasons for primary porosity preservation,and are the important factors for secondary porosity preservation in the middle to late stage.Moreover,the pore-lining chlorite and hydrocarbon emplacement changed reservoir wettability to contribute to fluid migration which can further promote dissolution,and the widely developed fractures significantly improves the permeability of tight oil reservoir mainly formed by the late Yanshan Movement,the early to middle Himalayan Movement,which are the guarantees oil production in tight oil reservoir with low porosity.There are obvious differences in porosity evolution characteristics of different types of tight oil reservoirs.Up to 24.42% and 24.32% of primary porosity of hydrocarbon-emplacement type and fracture-dissolution type tight oil reservoirs in the Liangshang Member are destroyed respectively due to compaction.In addition to the filling of authigenic minerals,the present residual primary intergranular porosity is 2.87% and 1.11% respectively.The secondary porosity is mainly developed in stage A of Mesodiagenesis,and the dissolution porosity is 1.25% and 3.05% respectively.The loss porosity of pore-lining chlorite type,hydrocarbon-emplacement type and fracturedissolution type tight oil reservoirs in Shayi Member is 23.66%,24.79% and 23.0% respectively due to compaction,and the present residual primary intergranular porosity is 4.12%,3.70% and 1.45% respectively,and the dissolution porosity is 1.78%,2.33% and 2.81% respectively.The tight oil reservoirs of Liangshang Member and Shayi Member in Central Sichuan Basin mainly experienced three periods of hydrocarbon emplacement.The first period is characterised by a typical early hydrocarbon emplacement,and the second and the third periods are the main emplacement periods.The reservoir became tight(porosity <10%)during the second hydrocarbon emplacement,and the third hydrocarbon emplacement occurred in the context of tight reservoir,which is characterized by "synchronization of densification and oil accumulation".During hydrocarbon emplacement,the reservoir always has high porosity,which is an important factor for the tight oil formation and large-scale oil accumulation.The dissolution porosity of tight oil reservoirs in Liangshang Member and Shayi Member are mainly formed by organi acids in stage A of Mesodiagenesis.There are three secondary-porostiy zones from top to bottom,which is characterised by secondary-porosity zone and precipitation zone alternating.The secondary-porosity zones are mainly formed due to the episodic release of abnormal high pressure,diffusion and deep thermal convection which transported dissolution products out of the dissolution zones.According to the favorable diagenesis including pore-lining chlorites,dissolution,hydrocarbon emplacement and fractures,paleostructures and sedimentary microfacies,the comprehensive prediction of tight oil reservoir distribution has been done.