Summary
Shale formations are developed during the period of late Cambrian system(WYP and GYT formation). The formations are favorable gas reservoir because of its high TOC content and gas shows. Well logging data shows that the low P-wave impedance (PI) is a key indicated characteristic for target shale, so we can identify gas reservoir by means of inverted PI. However, conventional pre-stack inversion method isn’t adaptable for this 2D seismic data because of severe noise contamination. Then in early period of exploration, post-stack inversion method is employed for higher S/N ratio, but at the same time decreasing the precision and resolution of inversion result due to AVO effects and NMO stretching. To solve these problems mentioned above, this paper applies a new efficient method to obtain zero-offset gathers (namely pure-P data) and its PI inverted from pre-stack data. In contrast with the post-stack seismic data and its inversion PI, pure-P data performs better in S/N and resolution because of the proper elimination of AVO effects and noise. At the same time, its inversion result agrees well with logging data. Finally, the preferable reservoir with low PI can be predicted.
Introduction
The study area named Xiuwu area locates in the Xiuwu synclinorium of Jiujiang depression and has developed unbroken shale layers in WYP and GYT formation. Figure 1 shows the characteristics of the WYP and GYT formations. These two formations show high weight of TOC and gas shows(Guo,2013;Chen et al,2012), especially WYP formation (marked by red rectangle). Meanwhile, the PI is relatively lower than the other formations. Thus, low PI value is a good indicator for higher TOC and gas content.
There is only one well in this study area, so seismic data and its inversion result play an important role in reservoir prediction and characterization. In early stage of exploration, due to poor quality of CRP gathers, post-stack inversion is superior to pre-stack inversion in calculating PI. However, the influence of AVO effects and NMO stretching can’t be ignored due to the large reflection angle (valid angle is up to 45 degrees) and complex AVO type, which result in low S/N and make a difference between post-stack data and zero-offset data. So a great challenge is presente+d: how to calculate the real zero-offset seismic data and precise inversion PI with high S/N and high resolution.
