Prediction of tight sandstone reservoirs by multi-wave joint prestack inversion technology: A case study of QL area in Sichuan Basin

The Shaximiao (SXM) Formation of the Middle Triassic in central Sichuan basin is a river-lake sedimentary system, with an average burial depth of 2800 m. The channel sand bodies of SXM Formation in QL area are developed with large lateral variation and high- and medium- porosity sand. This leads to great difference in seismic response, and difficulty in predicting sand bodies and reservoirs using compressional wave (P-wave) data. We carried out well log analysis for different types of sand bodies. No. 8 sand body of high porosity can be effectively identified by P-wave impedance ( PI). Whereas for No. 7 sand body of relatively lower porosity (mainly due to the higher shale content in the 7 No. 7 sand compared to No. 8 sand), sandstone and mudstone can be distinguished by using shear wave (S-wave) impedance ( SI), and reservoirs can be recognized by P-wave to S-wave velocity ratio ( V p /V s). Based on PP-wave and P-to-S converted wave (PS-wave) (after matching) angle stacked data, prestack inversion of multicomponent data is performed. Multiwave joint inversion results ( SI and V p /V s) have better accuracy and stronger stability than PP-wave inversion in practical application. Besides, the PI of joint inversion more clearly describes the distribution and boundaries of channel sand, and better matches the prediction and drilling data in Sand 8. The SI of joint inversion can identify subtle sand bodies (weak PI contrast) that are difficult to be detected by PP-wave inversion. The distribution of reservoir predicted by V p /V s of joint inversion are clearer (No. 7 sand body) and better than that by PP-wave inversion. This study demonstrates the advantage of the multiwave joint inversion technology in tight sand identification.