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The U.S. Gulf of Mexico (GoM) has long been the seedbed of innovative technologies and developments for the oil & gas industry. This drive is partially due to the industry's size in the region, the complexity of the geology, and the water depths currently being explored.
In the 1980s and 1990s, seismic companies acquired multiple phases of 2D seismic, creating a seamless grid of data across the GoM region. When the industry moved onto the use of 3D seismic, these companies were well-positioned to apply their geological understanding of the GoM to target core areas and enhance their offering to clients. The same happened as exploration focus moved to subsalt imaging when TGS responded by upgrading to Wide azimuth (WAZ) and multi- wide-azimuth (M-WAZ) acquisition in the same core areas.
Technology Agnostic Advantage
With the freedom to utilize any available seismic acquisition solution, TGS recognized that ocean bottom node (OBN) technology was the ideal solution for the geology in the GoM. OBN has existed since the late 1990s, and its use grew during the 2000s for small-scale projects. As the technology became more reliable and less costly, TGS decided in 2019 to undertake the world's first and largest ultra-long offset deepwater multiclient OBN survey - named Amendment - in the US Gulf of Mexico. The survey was optimized to acquire a minimum of 40 km ultra-long offsets - another industry first for an area of this size. Over four months, Amendment Phase 1 was acquired in water depths of up to 2,070 meters and covered 118 OCS blocks (2,765 square kilometers). The survey was acquired by Magseis-Fairfield and in partnership with WesternGeco. TGS processed the data.
Key to the survey was the acquisition of data using Dynamic Matching Full-Waveform Inversion (DM-FWI) – a cutting-edge technique which focuses on solving kinematic difference between input data and synthetic data. With the data residual calculated in localized windows in time and space, DM-FWI provides a robust velocity-model update using the total energy in the data, including both diving wave and reflections.
One of the most significant objectives of the Amendment project was to improve the imaging of targets, particularly in subsalt basins. This was achieved through a very dense and large offset shot grid that allowed ray paths to travel, reflect and refract deeper. This combination of ocean bottom node data and FWI has demonstrated the ability to gain a significant uplift to the velocity model, which in turn was used to amend the existing Wide azimuth WAZ and M-WAZ data in the area.
Comparison of Amendment survey results before and after OBN aquisition with Dynamic Matching FWI application. Survey acquiredin partnership with WesternGeco.
The exceptional results seen in the final data from Amendment were proof positive of the approach taken. Following the successful acquisition of Amendment, TGS, in collaboration with Schlumberger, undertook and completed the acquisition of a second phase of the ultra-long offset node project in the U.S. Gulf of Mexico, named Engagement. Starting in September 2020, this new phase extends the footprint of ultra-long offset data in the Northern Green Canyon protraction area. Following the completion of the acquisition of Engagement, final data is expected in late 2021. The Amendment and Engagement projects represent a new generation of survey design, with significant areas in the Gulf of Mexico and elsewhere that will benefit.
A Future for OBN?
The downturn in the oil & gas sector in 2020 undoubtedly impacted activities in the GoM, though less so than onshore projects in the US. It is now clear that for detailed investigation of the subsalt, OBN and FWI technology are the real future of exploration in the GoM, and the outlook is good for the region – and particularly for TGS, which has been at the forefront of its design and acquisition.
Opportunities for multiple additional projects are presently being explored, and discussions being held with interested parties. Just as it has always been, what proves successful in GoM can roll out to other regions worldwide. Hopes are particularly high for exploration of the complex deepwater areas in the pre-salt offshore Brazil and West Africa.