The quality of seismic image of underlying strata is negatively affected by shallow gas. Therefore, it is difficult to predict the depth of target zone accurately. Thus the drilling risk of horizontal wells is greatly raised as well. In order to improve the accuracy of depth prediction, we use theoretical model analysis to quantize the gas influence including the range and extent. Furthermore, we build many models to achieve pre-stack forward modeling and acquire a series of correlation templates which can be applied to quantitatively depth correlation. According to our research, we successfully explain why there exits so large depth difference between two drilled wells in B oilfield. Furthermore, we compute correlation templates to achieve a high-precision depth prediction for underlying reservoir of shallow gas. Therefore we efficiently guide the drilling of follow-up development wells. We believe our research production will be well used in other offshore oilfields containing shallow gas as well, especially under the condition of sparse well patterns offshore.