Imaging of steep salt face and surrounding sediments using vertical seismic profile (VSP) converted waves

Abstract

In many deep-water oil and gas reserves around the world, significant reservoirs are often trapped in the vicinity of salt bodies. Accurate mapping of the salt flank and sub-salt structures is a critical task for exploring and characterizing the reservoirs. The steep sides of an irregularly-shaped salt dome are difficult to image accurately using surface seismic alone. Thus, threecomponent (3C) VSP surveys are usually acquired to improve the image of reservoir structures.We demonstrate a VSP imaging application in the South Louisiana Salt Basin, USA. An offset VSP (OVSP) and a refraction salt proximity (SP) survey were simultaneously acquired and processed to image a steep salt boundary and updip sediment reflectors. We use both transmitted P-P and P-S converted waves from the SP survey to calculate the 3D salt exit points that delineate the steep salt face. Furthermore, we develop a robust method for estimating the Vp/Vs ratio of the sedimentary rocks flanking the salt dome. The 3C OVSP data was processed to extract the reflection-converted PP and PS waves from the steep salt interface and updip sediment reflectors. Using a velocity model derived from the check shot data and the estimated Vp/Vs ratio, we migrated the extracted reflection data to create depth images of the salt face and the nearby structures. The migrated images using both PP and PS converted waves provide a precise definition of the steep salt face and the reservoir sands flanking the salt dome. This study also indicates that both reflection and refraction surveys can provide a consistent location of the steep salt flank, demonstrating the strength of combining these VSP techniques.

Imaging of offset VSP data acquired in complex areas with modified reverse‐time migration

“…Imaging of salt body boundaries near a well is a very important task. The most effective methods of solving this problem are based on offset vertical seismic profiling data migration (Whitmore and Lines 1986; Li et al 2002; Brandsberg‐Dahl et al 2003; Zhao et al 2006; Hornby and Yu 2006). Most of these approaches use the waves reflected from the target's flanks.…”