Velocity model building challenges and solutions in a SE Asian basin: beyond reflection tomography

Brunei Darussalam, on the South-East Asian island of Borneo, has a long history of oil and gas production. Oil was first discovered in the onshore Seria field in 1929. Other sizeable fields have been discovered since, such as Champion and Ampa, and a high level of exploration and development activities are continuing. To ensure the challenges that come with these activities can be addressed adequately, a large part of Brunei’s offshore has recently been covered by modern broadband acquisition. Remaining parts offshore are covered by older 3D seismic. The coastal strip of Brunei, encompassing among others the large Seria field, is covered by wide-azimuth 3D land and transition-zone data. Brunei’s subsurface geology poses some specific challenges for seismic imaging for hydrocarbon exploration and development purposes. Shallow gas accumulations – gas clouds or gas bags – absorb much of the seismic energy and cause a wipe-out zone below them. Many exploration and development targets lie in relatively shallow (less than 200m deep) water where the (sub-)seabed is characterized by corals and near-surface channels. This impacts the seismic image through the resulting variation of near-surface velocities as well as attenuation owing to scattering and negatively impacts the deeper structural imaging. An illustration of these challenges is shown in Figure 1. The subsurface offshore and onshore Brunei is also heavily faulted and large anticlines often have steep flanks, and suffer from crestal collapses. Drilling targets can be relatively narrow (few 100 m wide) fault blocks near the crestal collapses, which makes accurate fault imaging and positioning and thus an accurate seismic velocity model absolutely crucial for confident well placement. Figure 2 shows the importance of accurate imaging for well placement: The image from the final isotropic model indicates that the well trajectory is on the intended side of the fault, while the well results showed the trajectory had crossed the fault and needed to be sidetracked. The subsequently built tilted transverse isotropic (TTI) model reveals that the trajectory has indeed crossed the fault. This example stresses the importance of detailed and accurate anisotropic model building.