The first high-end OBC imaging for fractured basement in Bohai Bay, China

Abstract

The gas and oil-bearing fractured basement rocks in Bohai Bay, offshore China, form important and complicated hydrocarbon reservoirs. It has proved a challenging area for seismic imaging due to the presence of substantial velocity heterogeneity and strong azimuthal anisotropy in both the overburden and reservoir layers. The legacy narrow-azimuth streamer acquisition and processing were not able to resolve the geologic complexity, thus the resulting velocity model and seismic images are inadequate to support exploration and field development in the fractured basement. We present the first high-end ocean bottom cable (OBC) imaging case in Bohai Bay. Full-waveform inversion (FWI) is the natural choice for velocity updates using OBC data. We demonstrate how the new OBC acquisition and FWI together produce a high-resolution velocity model that effectively solves the imaging challenges in this area. The wide-azimuth (WAZ) illumination and better signal-to-noise ratio (S/N) from the OBC data not only improve the stacking power, but also provide the chance for detailed orthorhombic (ORT) anisotropy modeling, a key to successful FWI application in this area. With the azimuthal anisotropy correctly handled by ORT modeling, FWI reveals the detailed anomalies related to deep basement fractures. The new results improve the image of the complex overburden, achieve more detailed and accurate fracture mapping inside the basement, and justify the value of WAZ OBC data. The improved images match well with existing drilling results, enhancing the exploration potential of this area.