CCS in Basalt: 20 Times More Difficult than Subsalt Imaging
For successful carbon storage in basalt, better intra and sub-basalt subsurface imaging is needed. In recent years, big 3D seismic data programs utilizing triple and Penta source technologies for better sampling have improved basalt imaging, but much progress is still to be made on the processing side: “Sub-basalt imaging is 20 times more challenging than sub-salt” (Zhiming Li, his statement in 2017 at the start of Sub-basalt Imaging Research).
In 2017 TGS undertook the challenge to investigate further the seismic imaging techniques that could be applied to solve the sub-basalt uncertainty. The first goal was to be able to interpret a horizon beneath the thick (1000-2000 m) basalts in the Møre Basin of the Norwegian Sea, Norway. A 625 km2 area was tested with different imaging algorithms such as image-guided tomography, Kirchhoff time and depth, Directional Image Stacking (DIS), Common Offset RTM (COR), and least squares RTM. A mappable horizon was achieved and further work on improving the result was spurred on by interested oil companies.
Above: Basalt and sub-basalt depth image with velocity overlay across the Vøring Escarpment in the outer Vøring Basin. The basalt is defined by the velocity overlay in warm colors (Light green, yellow and red) within a sedimentary basin (blue). The velocities were derived from FWI and Dynamic matching FWI processing. The different warm colors define separate volcanic facies (e.g., subaerial landward flows and coastal lava delta), where yellow is the slowest. The top basalt has an overburden of only a few hundred meters of mudstone.
From an Oil & Gas technology to CCS focus
This challenge set a vision to open up sub-basalt and volcanic areas for both oil and gas exploration, by identifying drillable prospects in these areas, and for CCS initiatives through the identification of single volcanic facies, creating the potential for determining compartments for carbon storage.
Through this work, TGS has partnered with a strategic oil company and Volcanic Basin Petroleum Research (VBPR) team. Investigations in transmission losses, mode conversion, post-critical reflections, wave scattering, multiples and peg-legs, attenuation of frequencies, and non-normal moveouts were identified as key challenges. Much research is still ongoing. The recent focus has been on multiples, sill transmission effects, and velocity building. Through the research to image the sub-volcanic geology, a better understanding of the basalts has emerged.
Where previously the volcanic rocks were treated as one unit in velocity model building, it is now possible to identify the different volcanic facies. This has been achieved by velocity model building techniques like Image Guided (IG) tomography and, lately, Dynamic Matching Full Waveform Inversion (DM FWI) that identify different compartments of velocities that correspond to features within the flow basalts. The example above is from the Vøring area in the Norwegian Sea. These are preliminary results, and better resolution is expected as the process continues. Similar results have been found in West of Shetland area.
Our research continues…