Fracture Distribution Characteristics And Comprehensive Evaluation Of Shale Reservoir In Southern Songliao Basin

Qingshankou Formation and Nenjiang Formation in southern Songliao Basin are fractured shale reservoirs with obvious natural fractures.The distribution of natural fractures controls the enrichment and high yield of shale oil,determines the fracture development characteristics and main controlling factors of shale reservoirs in the study area,and has very practical and important significance for guiding shale oil exploration.Based on field geological outcrop,core data,imaging logging and thin section data,this paper describes in detail the origin,types and distribution characteristics of natural fractures in mudstone shale in the study area,and analyses their formation mechanism and main controlling factors.On this basis,fractal prediction theory and finite element numerical simulation technology are used to quantitatively predict and comprehensively evaluate the fracture distribution in shale reservoirs.The results show that there are three types of fractures in the rocks of the study area,namely,structural fracture,overpressure fracture and diagenetic fracture,in which high-angle structural fracture accounts for the largest proportion.According to the spatial configuration relationship between structural fracture and rock mechanics interface,it can be divided into stratabound fracture,cross-layer fracture and small fault.The fracture development degree in the study area is relatively high.The distance between the outcrop cracks is mainly between 5and 20 centimeters,and the density of the cracks is mainly between 5 and 17.5 meters.The cracks near the fault zone are especially developed.he fracture linear density of core is mainly located in 0.5-2.0 strips/m,and the maximum value is 2.1 strips/m.The filling capacity of the cracks is weak,and more than 88%of the cracks are effective cracks.The formation and distribution of structural fractures are mainly influenced by organic carbon content,mineral composition,rock brittleness index,abnormal high pressure and structural faults.Fractal geometry theory and finite element numerical simulation technology are used to quantitatively predict fracture distribution.By comparing the results of fracture prediction with the measured density of single well,it is found that there is a positive relationship between them,which indicates that the results of fracture prediction are relatively accurate.On this basis,after comprehensive evaluation of fracture distribution,it is divided into three levels of fracture development units.The fissures in the development unit of grade I are well developed.The density of fissures in the development unit of grade II is more than 1.5 m/m~2.The density of fissures in the development unit of grade II is 0.5-1.5 m/m~2.The fissures in the development unit of grade III are less developed,and the density is mainly 0.0-0.5 m/m2.High productivity wells are located in the fracture development units of Class I and II.When the fracture density is less than 0.5m/m~2,the productivity is poor.