Outstanding subsalt images in the Gulf of Mexico reduce drilling risk
Final results from IBALT and DEUX, the first and second surveys of CGG’s StagSeis™ multi-client program of 871 offshore blocks in select locations of Garden Banks, Keathley Canyon, Walker Ridge and Green Canyon, are now available and widely recognised as the clearest subsalt images in this complex area of the Gulf of Mexico. Final results from the third survey, TROIS, are due next year. The original IBALT survey commenced acquisition in 2012 but planning began three years earlier. At the time, wide-azimuth (WAZ) surveys with advanced processing techniques provided the best subsurface images in the Gulf of Mexico, but some features below more complex salt structures remained difficult to illuminate and image fully.
Building on their experience in WAZ capabilities, CGG’s subsurface imaging and marine technology experts worked closely with clients to define key parameters to deliver better subsalt imaging than was previously possible, within the clients’ time frame. These included better sampling of offset and azimuth along with increased bandwidth.
The resulting patented StagSeis solution uses multiple vessels with a staggered geometry and orthogonal shooting, delivering consistent regular fold and azimuth distribution and providing full azimuthal coverage to 9 km with ultra-long inline offsets to 18 km (on 4 azimuths). BroadSeis™ is also employed to provide bandwidth down to 2.5 Hz along with CGG’s proprietary deghosting and imaging technology. Once modelling by both CGG and clients confirmed that StagSeis would deliver the desired results, acquisition began.
StagSeis deploys the largest areal spread of any available acquisition configuration (equivalent to the area of Manhattan Island). Operating such a large spread safely and efficiently requires strict adherence to robust HSE procedures, and CGG maintained an exceptional safety and environmental record during more than seven vessel-years of operation.
Fast-Trax results from a priority area were delivered in the first three months. Initial Full Azimuth Fast-Trax RTM images from DEUX were of such high quality that a client included a data image in an investor presentation, stating that StagSeis provided step-change improvements in subsalt imaging, boosting confidence in appraisal well locations and accelerating maturation of off-setting prospects.
Final results from IBALT and DEUX show staggering improvements of subsalt images over previous best images obtained from WAZ data, particularly in terms of continuity, illumination and fault definition, as shown in the image comparison.
StagSeis also delivered new and valuable data to imaging. Full azimuths provided better sampling of anisotropy, and the ultra-low frequencies and long offsets of StagSeis allowed Full Waveform Inversion (FWI) to build high-resolution velocity models. The enormous full-azimuth, ultra-long offset, broadband datasets acquired with StagSeis also brought processing challenges. A series of innovations from CGG Subsurface Imaging included development of true 3D deghosting and source designature to accommodate much larger inline and cross-line offsets. CGG also optimised demultiple, velocity model building, and imaging modules to handle larger datasets.
CGG achieved a very high level of pre-commitment for the StagSeis surveys, with nine underwriters signing up for the program. The StagSeis solution provided better geological definition of the subsurface, illuminating complex salt bodies and previously obscured subsalt reservoirs. Superior images reduce financial and HSE risk from both reservoir and drilling perspectives. CGG believes that this tailor-made, collaborative approach, from project design through end results, is the way of the future.