Fracture prediction based on walkaround 3D3C vertical seismic profiling data: A case study from the Tarim Basin in China

Abstract

Natural fractures are well developed in the Sangtamu carbonate formation, which is the primary oil and gas production unit in the Tarim Basin, China. The analysis of converted waves via S-wave splitting (SWS) is an effective tool for predicting natural fractures in carbonate units. Compared with surface seismic data measurements, vertical seismic profiling (VSP) is more advantageous for acquiring and imaging converted waves. At present, there is a lack of robust methods on using 3D3C VSP data for fracture prediction. We propose an innovative workflow for predicting the spatial fracture distribution with the first ever application of SWS analysis to a 3D3C VSP data set. Instead of traditional Cartesian coordinates, we generate image bins based on a polar coordinate system to obtain accurate P-to-S converted waves of different azimuth angles. This is followed by a two-step SWS analysis. First, we conduct a multidirectional SWS analysis to estimate the fracture-induced anisotropy in the upper layers. Then, the wavefield of the target layer is corrected using time compensation and coordinate rotation. Finally, we apply SWS analysis again to obtain the azimuth and spatial intensity of the fractures in the target layer. We find that there is an overall good agreement in the fracture densities derived from the VSP waveforms and well-log data. The areas with high fracture development, as indicated by the SWS and ant-tracking analyses, also are consistent. Our study indicates that azimuth processing of walkaround 3D3C VSP combined with SWS analysis can serve as a quantitative diagnostic tool for fractures in a carbonate reservoir.