DISTRIBUTION CHARACTERISTICS OF FRACTURES IN LOW PERMEABLE RESERVOIRS IN AN EXTENSIONAL STRUCTURAL REGION

The dolomite reservoir of Sha 4 Member in Leijia Formation,Liaohe basin is taken as an example to study the distribution characteristics of fractures in low permeable reservoirs of an extensional structural region.Three distributive types of fractures are identified,i.e.internal,transverse and bedding fractures,of which internal fracture is strictly controlled by dolomites,perpendicular to and terminated onto the bedding plane;whereas transverse fracture is in a high-angle oblique to bedding plane.On a section,a mono-fracture appears planar,listric and ramp-flat;and multi-fractures may be in a combined pattern of grabenuhorst or domino,showing a consistent distribution with the growth normal faults in the region.Fractures and normal faults will constitute a complete structural style of an extensional structure within the region,ranging from macroscope to microscope in scale.The distribution of fractures in a reservoir is controlled by the factors of lithology,bed thickness and faults,etc.Transverse and bedding fractures are well developed in the argillaceous interlayers of dolomlte reservoirs,and are interconnected;together with fractures in the dolomites,they make up a 3-D fracture network.As a result,the argillaceous interlayers can not be taken as impermeable layers but para-reser-voirs instead,and thus the dolomite oil reservoirs are characteristic of block.Toward an extensional fault,the fractures in the lianging wall are more developed than those in the footwall with good zonation.