Structure-Oriented Mapping of the Subsalt Fractured Reservoir by Reflection Layer Tomography From a Perspective of the Zero-Offset Vertical Seismic Profiling

Abstract

The pre-/sub-salt fractured networks provide key clues to understanding the tectonic history and the subsurface reservoir evolution. Yet, they are among the most complicated targets for geophysical investigations. We develop a structure-oriented mapping technique to delineate the high-resolution dipping layers across the borehole region using the zero-offset vertical-seismic-profiling (ZVSP) survey, which is conventionally used to provide one-dimensional (1-D) wave propagation information. The key information for the single-shot VSP mapping, which is the structural dip across the borehole, is unreliable from the poor sub-salt surface seismic image. Alternatively, we obtain the structural dips via reflection layer tomography. Taking advantage of the accurate interval velocities and the high-fidelity identifications from the reflection events, we manage to invert for the geometry of the main reflectors identified across the wellbore. Based on such information, we propose an effective processing strategy and achieve a high-resolution image of the dipping structures around the borehole region from the ZVSP. The current result compares reasonably well with the corresponding surface seismic profile but supplies higher-resolution details of the dipping structures and fault networks below the thick evaporite caprock where the surface seismic image degrades sharply. The enhanced subsurface image encourages better structural evaluation, geologic interpretation, and future 2-D/3-D VSP survey design.