Innovate challenge: Sub-basalt exploration Introduction Peter Hanssen
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!
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
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
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 km 2 – Lake superior: 12 km high Large Igneous Provinces (LIP) may cover several hydrocarbon-producing basins
Large igneous provinces After After Coffin & Eldholm, 1994, Reviews of Geophysics, 32, 1.
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
What does the problem look like? After Martini, F., 2003, First Break 20, 6. LOW FREQUENCIES
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
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!
Thank you. Innovative Challenge: Sub-Basalt Imaging Introduction Peter Hanssen Principal Geophysicist PetHan@ Statoil.com www.statoil.com