Imaging of Steep Salt Face and Surrounding Sediments Using Vertical Seismic Profile (VSP) Converted Waves

Li, Yingping, Xiaomin Zhao, Fran Doherty, VSFusion, A Baker Hughes-CGG Company, Houston, TX

In the Gulf of Mexico, North Sea, and Western Africa, important oil and gas reservoirs are often trapped in updip sediments around or beneath salt bodies. Accurately mapping subsurface salt flank and sub-salt structures are critical tasks for exploration and reservoir characterization. The steep sides of a salt dome with irregularly shapes are hard to image with adequate accuracy by using surface seismic alone. Thus, VSP surveys with 3C receivers in a wellbore are usually requested for improving images of reservoir structures.

In this paper, an offset VSP (OVSP) and a refraction salt proximity (SP), simultaneous­ly acquired in South Louisiana Salt Basin, USA, were 3C processed to imaging a steep salt boundary and updip sediment reflectors. Both transmitted P-P and P-S converted waves from the SP survey were used to calculate 3D salt exit points which delineate the steep salt face. Furthermore, we developed a robust method to estimate Vp/Vs ratios in the sedimen­tary rocks flanking the salt dome. Reflected waves PP and PS conversion, from the steep salt interface and updip sediment reflectors, were separated from 3C OVSP data and depth migrated to obtain their images, using a velocity model from a check shot and the estimat­ed Vp/Vs ratio. The migrated images using both PP and PS converted waves provide a pre­cise definition of the steep salt face and reservoir sands flanking the salt dome. This study also indicates that both reflection and reflection surveys can provide a consistent location of the steep salt flank, demonstrating the strength of combining these VSP techniques.