Significant advances in imaging beneath seismic-disrupting basalt are opening up opportunities in the Atlantic Margin of northwest Europe for geoscience companies, said TGS.
Significant advances in imaging beneath seismic-disrupting basalt are opening up opportunities in the Atlantic Margin of northwest Europe for geoscience companies, said TGS.
Volcanic provinces are generated during the continental break-up, marking the boundaries of both subduction zones and spreading ridges. They provide a mechanism necessary to release pressure and energy derived from tectonic movement. The material produced through vulcanism pours into growing basins while interbedding with, and covering, other sediment. Hydrocarbons are eventually produced and expelled from source rocks within the basin and find their way into ideal reservoirs. Many of these optimal reservoirs are sealed or interbedded with volcanic facies. ‘Unfortunately, for explorationists this volcanic material has presented a substantial challenge to image through seismic surveying, making it very difficult to accurately prospect for hydrocarbons,’ said TGS in its latest Insights article.
Basins in the Caribbean, offshore Argentina, Brazil, Greenland, Angola, India and northwest Europe have potential hydrocarbon reserves that exist beneath volcanic facies, particularly basalt flows. The Atlantic Margin of northwest Europe is a prime example of an area where basalt flows have hindered imaging of reservoirs. The Rosebank field in the Faroe-Shetland Basin was discovered in 2004 and was one of the first attempts to develop and derisk intra- and sub-volcanic reservoirs. The operators spent the next decade trying to derisk the prospects by drilling appraisal wells, some of which showed potential.
In 2010 Chevron drilled the Lagavulin well in the UK Atlantic Margin to prospect a sub-basalt target and but the well was dry. Then, in 2012, offshore the Faroe Islands, Equinor’s Brugdan II well targeted sub-basalt, but was also commercially unsuccessful. Like Rosebank, both prospects ran into seismic imaging challenges and technical drilling difficulties. Exploration in this region faltered during the 2015 economic downturn.
Over the past two decades, seismic service companies have developed workflows that are capable of producing detailed imaging of both intra-volcanic and sub-basalt reservoirs. In 2018 TGS developed a focused research programme called SIR (Sub-Basalt Imaging and Research). The focus of this research was to adapt workflows from seismic programmes in the Gulf of Mexico and Brazil where methods developed to image around and below salt have solved issues related to modelling velocity inversions, signal-to-noise (S/N) improvement in 3D shot migrations, and accurate removal of multiples.
Traditional narrow-azimuth surveys with short streamer lengths and limited source potential have been replaced by deploying multiple advanced sources in full-azimuth surveys acquired with ocean-bottom nodes (OBN) or multi-azimuth surveys with long streamer lengths. These advances have made it possible to acquire densely spaced data with a wide spectrum of low-to-high frequencies necessary to accomplish detailed imaging in a structurally complex environment. Advances in computing have made it cost- and time-effective to run additional de-multiple sequences with 3D Surface Relate Multiple Elimination (SRME) and advance velocity modelling methods. The improved computing efficiency when using wave equation-based techniques for velocity model updating has made
Example from EWW18 survey showing uplift after processing using DWFWI, COR, and DIS. Successful imaging features supra-basalt, intra-basalt and sub-basalt. Noticeably, in lower right the velocity inversion below the basalt indicates non-volcanic sediments.2 4
it possible to image and map intra- and sub-basalt events.
The key technologies in the processing sequence advancing intra- and sub-basalt imaging in the North Sea include image guided tomography with Common-Offset Reverse Time Migration (COR), RTM with Directional Image Stacking (DIS), Least Squares RTM (LSRTM) and Dynamic-Matching Full Waveform Inversion (DMFWI). COR can handle inadequate ray-coverage, produce a higher signal-to-noise (S/N) ratio and image more coherent events. DIS provides a weighted method to stack the RTM image and take into account source direction, survey geometry and other stacking criteria to achieve better separation of S/N and produce a more accurate final image. LSRTM improves S/N by suppressing migration artifacts and broadens the bandwidth of the data.
With the acquisition of modern OBN surveys and longer streamer lengths in multi-azimuth surveys, more long offset data has become available and has made it possible to use full waveform inversion wave-equations like DM FWI to further refine the intra and sub-salt image. DMFWI is an inversion method based on finite-difference modelling that reduces the differences observed in seismic and synthetic data through updates of the velocity model. Recent research by the SIB group at TGS has shown the DW FWI to create velocity model updates that are less sensitive to cycle skipping, are able to use all information in the data including reflections and refractions, can be fully automated for detailed high contrast delineations and create a structurally conformable update. When long offsets and robust data are available DM FWI in a processing sequence with COR and DIS are able to improve event continuity beneath the basalt without smearing faults or losing detail and improve imagining of intra-basalt reflectors.
Intra and sub-basalt plays show potential for untapped future growth and with recent advances in engineering and geophysical technologies these reservoirs can be derisked and developed economically.
‘The benefit of advancing wave-equations and seismic processing workflows to enhance imaging of volcanic facies is showing an additional benefit of providing techniques to progress other energy and climate initiatives,’ said TGS. ‘Carbon Capture Utilization and Storage (CCUS) and geothermal exploration are two areas where accurate mapping of volcanic facies and their surrounding sediments can provide the data needed to develop these new clean energy technologies. It is becoming possible to image once hidden sediments and structures that provide valuable insight into geological processes, hydrocarbon generation, and new avenues for clean energy generation and storage.’
