Innovate challenge sub basalt exploration
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Innovate challenge:Sub-basalt explorationIntroduction
Peter Hanssen
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Motivation
• Discovering new hydrocarbon reservoirs has become increasingly challenging
• Most of the easy-to-find resources are already discovered
• Exploration now moves into more complex geological settings
• These settings are harder to reach and more difficult to image
• The ultimate challenge areas are covered with cooled lava
• These basalts often absorb and scatter the seismic energy one needs to image beneath them
Do you have an innovative idea to solve this challenge?Think out of the box!
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Seismic exploration
• To image the subsurface, geophysicists usually employ active seismic methods
• With these methods, artificially produced seismic waves travel into the subsurface
• These waves get reflected at interfaces and travel up to the surface again
• The waves are then recorded with hundreds of receivers and processed into an image of the sub-surface
• This image shows us the geology for several kilometres in depth
• Finally, it gets analysed before the drilling proceeds to target promising structures
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What is basalt rock?
• Igneous - an extrusive volcanic rock formed from lava
• Covers 70% of the Earth’s surface
• Extreme variations due to eruption type and depositional environment:
– subaerial: scoria, ash or cinder, tuff, breccia, lava flows, columnar basalts
– submarine: pillow shape, littoral cones (tuffaceous)
– subsurface: sills
• Most excessive range of velocities from 1000-8000 m/s
• In rare cases the basalt may act as a reservoir or cap rock
• Each flow not deposited over centuries but in several days chaotic structure
• Basalts weather relatively quickly compared to other rocks
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What is a basalt flow?
• Multiple episodes of extrusions with weathering or sedimentation in between
• Flows can be smooth (less gas) or chaotic (rich in gas), both forming undulating surfaces
• Each flow cools down from the interfaces towards the centre, producing a velocity gradient
• Multiple large volcanic eruptions producing tholeiitic basaltic magma on a continental scale:
– Siberian Traps: 1 500 000 km2
– Lake superior: 12 km high
• Large Igneous Provinces (LIP) may cover several hydrocarbon-producing basins
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Large igneous provinces
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Problems for us?
Worse than sub-salt case due to:
• Mostly continuous basalt sequence without “mini basins” or holes:– No undershooting, so the wave-field has to pass twice
• Multiple layering of high (6km/s) and low (2km/s) velocity basalts and sediments:
– Scattering of all high frequencies above 20Hz– Very strong sea-surface and inter-bed related multiples
• Chaotic internal flow patterns in the basalt sequence causing scattering
• Basalt interfaces rugosity acts as an additional scatterer
• Gradient layers due to different speed of cooling
• Sub-basalt sills (8km/s) worsen problems
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What does the problem look like?
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LOW FREQUENCIES
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Tested improvements so far
• Low frequency response <30Hz from sub-basalt sequence
• Low-frequency setup of airguns and streamers
• Long offsets to record refracted and diving waves
• Ultra-long offsets (two boat or seabed receivers) to record the step back of the diving wave
• Downward datuming and extensive demultiple schemes
• Integrated analysis together with gravity, magnetic and electro-magnetic measurements
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Any solutions?
• Do we need new sources to extend the bandwidth further down below 3Hz?
• Do we need stronger high frequencies, more fold or only spectral enhancements?
• How can receivers on the seabed help to improve the image?
• Can we utilise converted waves for targets below the basalt sequence?
• How about electro-magnetic measurements or passive methods?
• Do long offsets help to resolve the main features?
• Does joint inversion of gravity, EM and seismic work over basalts?
• What is the best demultiple method and how to handle internal multiples
Be creative! Think out of the box!
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Thank you.
Innovative Challenge:Sub-Basalt ImagingIntroduction
Peter Hanssen Principal [email protected]