Abstract: In order to evaluate the hydraulic fracturing effect in the low permeability reservoir of Huagu 6 well， a microseismic monitoring system was established on the surface of the well. Based on microseismic data， the spatio-temporal extension process and fracture mechanism of hydraulic fractures are analyzed by means of crack extension path and moment tensor inversion. The results show that the spatial distribution characteristics of fractures changed with the liquid viscosity. The injection of high viscosity liquid formed a fracture network with low fracture density. The injection of low viscosity liquid formed a large-ranged dense fracture network with short extension path， and many crack branches. The injection of medium viscosity liquid formed some fractures that initiated from the branching fracture ends. The fracture type was mainly shear fracture(89.5%)， and was not affected by the viscosity of the liquid.

Key words: low permeability reservoir, hydraulic fracturing, microseismic monitoring, fracture extension, fracture mechanism