Multi-Wave Joint Characterization Technique of Channel Sand Body in Shaximiao Formation, Northwestern Sichuan Basin

Abstract The channel sand bodies in the first member of Middle Jurassic Shaximiao Formation (Sha 1 Member) in the northwest Sichuan basin ZT area shows a vertically multi-period superimposed and horizontally contemporaneous crisscrossed. Meanwhile, it is highly non-homogeneous, with significant differences in sand thickness, porosity and gas content at different locations in the channel, resulting in complex and variable seismic reflection characteristics, which makes it difficult to accurately characterize the channel sand body distribution through a single amplitude 'bright spot' in the conventional longitudinal wave (PP-wave) post-stack seismic profile. In this study, the newly collected three-dimensional three-component transverse wave (PS-wave) seismic data in this area are used, combined with pre-stack and post-stack PP-wave seismic data, and based on the forward modeling, the multi-wave seismic response characteristics of the channel sand body are analyzed and summarized with the variation of sand body thickness and porosity, and a multi-wave joint channel sand body characterization technique is proposed for Sha1 Member of Saximiao Formation. The results of the study show that: 1. PP-wave seismic response of the top interface of the sand body gradually shifts from a strong wave peak to a strong wave through with increasing porosity, whereas the sand body response of the PS-wave does not reverse with porosity changes. The stability of the top interface of the sand body corresponding to the peak reflection of PS-wave, hence the lithological inscription based on the PS-wave amplitude attribute can effectively identify different types of river sand bodies. 2. Compared with the pre-stack P-G attribute, which reflects the PS-wave impedance changing rates, the PS-wave reflection amplitude shows better energy focus, and the boundary of channel sand body is also clearer based on PS-wave amplitude ‘bright spot’ compared with P-G attribute. While the P-G attribute based on the pre-stack seismic data has higher longitudinal resolution, thus it is more beneficial to distinguish the superimposed sand body and determine the phase of the sand body. In summary, the multi-wave joint sand body characterization technique makes full use of the advantages of multiple seismic data in sand body characterization, with the high-resolution P-G attribute helping to determine the longitudinal phase of the sand body, and the PS-wave used to map the transverse boundary of the sand body. The composite results can be used to provide a fine-scale characterization of the sand body in the main channel of Sha 1 Member. The application is highly effective and provides favorable support for well deployment and reserve upgrading in the Zitong area.