Application of High Resolution Zero-Offset and Walkaway VSP in Reservoir Delineation

Abstract

Abstract To more finely delineate reservoirs and estimate more reliable oil and gas reserves, we have carried out one high-resolution zero-offset VSP(ZVSP) survey and two 2-D cross walkaway VSP (WVSP) surveys, based on available well log and 3-D surface seismic data, in L oil field. For each shot of VSP data acquisition, the depth of seismic source was selected carefully and the source was exploded with a small charge of dynamites in a single source well. Seismic signals were recorded in a denser 2.5m receiver spacing by eleven-level borehole geophones with clamping devices. The frequency bandwidth of recorded data is 8~120Hz. High resolution VSP data processing mainly consists of a first consistency processing, a subsequent estimate of absorption and attenuation coefficient Q from VSP and then the use of the Q value to increase the resolution of 3-D surface seismic data. The high-resolution-improved VSP data modify the depths of tops of three dominant reservoirs inferred from well log and add a new reservoir by a comparison of seismic events and geological layers. Finely delineating reserves by a joint use of WVSP images and the high-resolution-improved 3-D surface seismic data enhance the reliability of reservoir estimate of L oil field. 1 Introduction L oil field is a significant oil and gas developing area. Totally 49 wells have finished drilling and 22 of which have obtained industrial oil flowing. But the oil layers in the area are thin and the oil-water relations are complex, that results in high adventure for further developing. Although 3-D surface seismic survey has been carried out in the area to emphasize geological responds of core reservoirs and improve the identification ability of thin layers by frequency increasing processing, the processed 3-D surface data are a bit suspicious. Therefore one high resolution zero-offset VSP(ZVSP) survey and two 2-D cross walkaway VSP(WVSP) surveys for a well in the area have been carried out complementarily, in order to calibrate horizons of 3-D surface data more accurately, to describe lateral variation of reservoirs in the vicinity of borehole and to enhance the reliability and accuracy of processing and interpretation of surface seismic data though a bridge between well and surface 3-D seismic data constructed by VSP.