Hydrocarbon reservoirs are recognized as spatially heterogeneous entities in terms of pore fluid content, pore fluid saturation, porosity, permeability, lithology and structural control. Knowledge of these reservoir parameters and their spatial variation is critical in the evaluation of the total volume of hydrocarbon reserves in place, in understanding and predicting physical processes in the reservoir and in projecting and monitoring reservoir fluid production and recovery as a function of time. Variation in fluid saturation, pressure and temperature would normally result in seismic responses. These changes are the main aim of 4D seismic or seismic monitoring. This method can evaluate the internal changes of the reservoir monitoring temporal changes in seismic responses, resulting in better productio output. Relationship of seismic responses to changing of reservoir condition causes problems in possibility of accomplishing these projects. Therefore estimation of the changing values and it’s uses in observing probability of reservoir’s changing, before investing in 4D seismic probes, is essential. In this thesis, we study the porous carbonate reservoir rock of one of the south-western oilfield to evaluate 4D seismic feasibility of reservoir conditions monitoring. In this manner, we estimate P and S wave velocities of core samples using Gassman’s equation with 4 percent relative error. Results of the changes in seismic velocities and travel times and reflection coefficients as a result of reservoir’s change’s indicate that by verifying effective pressure, relative changes in seismic velocities ( %12 in V P and %22 in V S ) and reflection coefficient (0.06) are monitorable. But these changes by verifying the saturation and temperature ( less tha %2 ) is negligible in 4D seismic studies. So, according to the special characteristic of this oilfield and present conditions, we concluded that there is low probability successful associated with monitoring studie of reservoir internal changes. Due to the large extension of reservoir of the studied oilfield, It is however advisable to put forward same extra complementary studies using suitably located wells for further monitoring examination.