Dipping structures revealed by the zero-offset VSP: a fractured-reservoir case in the Tarim Basin, China

Complex fault networks and highly-deformed structures associated with the deep fractured-reservoirs bring great challenges to seismic exploration and reservoir characterization in the foreland basins in Tarim Basin, China (Kao et al., 2001; Jia and Wei, 2002; Guilbaud et al., 2017). The study area features high-dipping angle at a great burial depth and is accompanied with a significant near-surface complexity. The high-impedance seal rocks (including evaporates and carbonates) block a great amount seismic energy generated at surface from penetrating through. Hence, the surface seismic surveys suffer from degraded signal-to-noise ratio (SNR), strong multiple interference, narrow frequency spectrum band, and limited illimitation coverage such that more constrains of the subsurface parameters are urged to support better development and drilling decisions. Vertical seismic profiling (VSP) is often employed to provide high-resolution seismic image and rock properties around the borehole as an enhancement to the surface seismic profile (Stewart, 1984; Gaiser, 2016). However, a 2D or 3D VSP acquisition can be expensive for deep targets especially for offshore exploration or when the surface condition is unfavourable. For instance, the study area is located at the Kunlun foreland belt in southwest Tarim Basin where the surface topography is significant. Only a zero-offset VSP (ZVSP) was acquired at the early exploration stage after the surface seismic survey. ZVSP has been used to obtain 1D subsurface information along the borehole including time-depth table, velocity profile, Q-factors, and corridor stack to enhance the subsurface evaluation. While in areas with highly-dipping structures, reflections away from the borehole are recorded (Conn and Buck, 1987). To take advantage of the extra illimitation and to fully exploit the potential of the ZVSP data, we process and image the three-component (3C) ZVSP dataset (Figure 1). The 2D VSP-CDP image around the borehole compares well with the surface seismic image, but supplies higher-resolution details of the complex and steeply-dipping structures in better image quality. The current results encourage us to make full use of the 2D/3D information recorded in the ZVSP data in areas of steep dips if a 2D/3D VSP acquisition isnt available.