APPLICATION OF SEISMIC VELOCITY TO THE EXPLORATION OF SHALLOW GAS

Commercial oil/gas reservoir determined by the well drilling and seismic channels across the well are used to prepare combined records which are plotted on the seismic profile;then using a recursion formula for the seismic velocity spectrum and interval velocity,the velocity values having the same time domain with the oil/gas geologic body are picked up and calculated.Based on the method of superposition of differential velocity proposed by Leder,an American scholar,the superposition of velocity values from the top and bottom of the geological body are made.The results show that the abnormal sounding zone with a low velocity would be a gas reservoir.The method is applied to a trial study in Xinchang gas field after a delineation in plane by means of tracing and closing in the seismic net;it is found that the gas pool of Penglai Zheng Formation,and gas reservoirs A&B of Shaximiao Formation from the upper Jurassic above 1600-2200m indicate a higher rate of coincidence with low velocity abnormaly,as high as 80%.Gas reservoir C of Shaximiao Formation of middle Jurassic below 2200m indicate a lower rate of coincidence,about 2O%-30%.