Development Pattern Of Deep Fractured Reservoirs In The Foreland Thrust Belt On The Southern Margin Of Junggar Basin,China

The Jurassic in the foreland thrust belt on the southern margin of the Junggar Basin is abundant in oil and gas resources.The burial depth of the reservoir is usually greater than 4500 m,which is the typical deep tight sandstone reservoirs.Natural fractures are developed in these reservoir,and effective fracture distribution is of great significance to oil and gas enrichment and productivity.Clarifying the development rule and effectiveness of natural fractures in these reservoirs and establishing the development pattern of deep fractured reservoirs can provide a geological basis for deep oil and gas exploration and development.In this study,field outcrops,cores,and thin sections were used to analyze the genetic types,development characteristics and major influcing factors of natural fractures in the deep Jurassic in the southern Junggar Basin.Using outcrop observation and numerical simulation methods,the distribution patterns,differences and control mechanisms of fracture domains in typical structures of fault-bending folds and compressed anticlines in the foreland thrust belt are studied.Based on conventional logging identification methods and reservoir geomechanics methods,the longitudinal and planar distribution of natural fractures are predicted.Combined with static sealing pressure calculation and permeability test under overburden conditions,the fracture effectiveness and their influencing factors are evaluated.Finally,comprehensive tectonics,stratum burial history and reservoir diagenesis evolution,the mechanism of tectonics diagenesis and its control on the reservoir were analyzed,and the development pattern of deep fractured reservoirs in the foreland thrust belt was established.The deep Jurassic reservoirs in the foreland thrust belt on the southern margin of the Junggar Basin have developed four types of natural fractures: tectonic,diagenesis,tectonic-diagenesis,and abnormal high-pressure origin fractures.Their development degree is mainly determined by three key factors: lithology,rock mechanics,and structures.In siltstone and fine sandstone,fractures are more developed.Tectonic fractures are dominated by intra-layer fractures,and the line density of fractures decreases with the increase of rock mechanics layer thickness.The development degree of fractures in the hanging wall of the fault is relatively high.As the distance from the fault plane increases,the degree of development of the fractures gradually decreases.Vertically,the natural fractures in the Badaowan Formation have the highest degree of development,followed by the Toutunhe and the Xishanyao formations.On the plane,from the east to the west and from the front of the Tianshan Mountains to the inside of the basin in the thrust belt,the development degree of fractures is gradually decreasing.The fault-bending fold in the foreland thrust belt is divided into six fracture domains,and the compression anticline is divided into five fracture domains.The distribution patterns of the fracture domains of these two structures have similarities and differences.The similarity is that fractures in the front wing of the two structural folds are more developed than the rear wing,and a set of extension fractures parallel to the axis are developed in the core of the fold.The main difference is that fractures in the two wings of the fault-bending fold are more developed than the core,while the compression anticline is opposite.Fractures in the fault-bending folds are more developed and more complex in types and systems than compression anticlines.The control mechanism leading to the difference in the fracture distribution of the two typical structural fracture domains is that the fracture development of the fault-bending fold is greatly affected by the fault,while the fractures in the compression anticline are mostly intra-layer fractures,which are mainly affected by structural compression.The opening and permeability of natural fractures under the confining pressure of the layers are controlled by their static sealing pressure,and are affected by factors such as burial depth,fracture occurrence,direction and size of situ stress,and pore fluid pressure.In the target layer of the study area,the NNE-SSW high-angle fractures that intersect with the maximum principal stress direction at a small angle have the smallest static sealing pressure and the largest opening,which is the easiest to form a network of interconnected fractures and increase the seepage capacity of the reservoir.Experiments on fracture permeability under pressure show that as the pressure increases,the fracture permeability decreases in a power function.The larger the fracture dip angle,the higher the permeability value,the stronger the sensitivity to pressure,and the easier it is to stabilize.In deep reservoirs with a buried depth of more than 4500 m,natural fractures still maintain good opening and permeability,and their permeability is greater than 15 m D,which is 3 orders of magnitude higher than the matrix permeability,showing good effectiveness.Tectonic diagenesis has a significant impact on the pore structure,fractures and physical properties of deep tight sandstone reservoirs in the foreland thrust belt,and is a key geological factor that controls the formation,evolution and quality of fractured reservoirs.The formation and evolution of deep reservoirs in the foreland thrust belt on the southern margin of the Junggar Basin can be divided into three stages: compaction and reduction of porosity,reservoir densification,and reservoir transformation(sweet reservoir formation).Tectonic diagenesis can control tectonic subsidence,structure deformation strength,diagenesis occurrence time and diagenesis strength and affect the development of pores and fractures in different stages of reservoirs,determining the distribution of high-quality reservoirs.Based on the mechanism of tectonic diagenesis,the development pattern of deep fractured reservoirs in the foreland thrust belt of the southern margin of Junggar Basin was established.The development patterns of reservoirs in different structural belts show obvious differences,which better reflect the coupling relationship between the interaction of structure and diagenesis and the evolution of reservoir physical properties.