Velocity Model Building Challenges and Solutions in a SE Asian Basin

Most of offshore and part of onshore Brunei is covered with modern seismic surveys. These seismic data sets are extensively used for Exploration and Development purposes. While conventionally smooth compaction-driven velocity models are used for imaging in Brunei with satisfactory results, certain geological features require advanced velocity model building strategies: Shallow channels, corals and gas bags. Furthermore, as the subsurface is heavily faulted, fault imaging and positioning is crucial.

Traditionally, velocity model building in Brunei is done by means of isotropic reflection tomography. With the acquisition of more modern broadband, long-offset datasets and multi-azimuth coverage, more sophisticated algorithms and workflows can and must be used for proper imaging and positioning.

In many parts of offshore Brunei, the near-seabed contains channels characterized by low velocities. When not incorporated into the velocity model, these channels will cause dim zones, as well as push-downs, in the resulting image. If sufficiently far below the seabed, detailed tomography may resolve them. However, for the first 200 m below the seafloor, guided-wave inversion can be used to derive the shallow part of the velocity model.

Another common overburden feature are gas bags: Gas accumulations which manifest themselves as wipeout zones with a chaotic image underneath. Introducing ultra-low velocities may improve the imaging. Full-waveform inversion is able to identify and delineate these ultra-low velocity zones automatically from diving waves.

As most of the subsurface consists of a sequence of sand-shale layers, anisotropy is expected. Including tilted transverse isotropy into the velocity models leads to improved positioning of dipping faults. Anisotropic parameters can be constrained by depth markers from wells as well as higher-order moveout. Anisotropic imaging improves fault positioning, which is crucial for proper well placement.

Production from very shallow offshore Brunei is mostly done through fishhook wells: Wells drilled from onshore targeting tilted fault blocks offshore. In 2014, a DAS acquisition with 6 instrumented wells was executed. Joint inversion of surface seismic first arrivals and DAS first arrivals gave rise to improvements in the velocity model. These improvements again lead to better fault positioning.

The amount and quality of seismic data available in Brunei provides great opportunities to go beyond classical reflection tomography for velocity model building. Methods such as guided-wave inversion and full-waveform inversion, the use of well-based data such as DAS, and incorporating anisotropy in the velocity models improves imaging as well as fault positioning, with direct business impact.