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Pioneering the Global Subsalt/Presalt Play: The World Beyond Mahogany Field*

 

Dwight "Clint" Moore1

 

Search and Discovery Article #10285 (2010)

Posted December 17, 2010

 

1ION Geophysical Corp., Houston, TX ([email protected])

 

*Adapted from presentation at: AAPG Annual Conference and Exhibition, New Orleans, Louisiana, April 11-14, 2010, AAPG International Conference, Rio de Janeiro, Brazil, November, 15-18, 2009, Houston Geological Society General Meeting, March, 2010, New Orleans Geological Society, General Meeting, July, 2010, Oklahoma Geological Foundation, Cronin Lecture, November, 2009.

 

Abstract

Ten years into the 21st century, the Subsalt play that began in the U.S. offshore Gulf of Mexico during the 1980’s, has evolved into a growing global Subsalt/Presalt play, of likely historic impact. Today, we are at the dawn of major reserve and production additions to the world oil & gas supply, as global Subsalt/Presalt petroleum exploration yields major results, not only in the Gulf of Mexico, but also off Brazil and West Africa. In the years ahead, these new discoveries will fuel further exploration & production below complex salt layers worldwide.

Since its inception, the greatest challenge for the Subsalt/Presalt play concept has been explorers’ difficulty in using seismic data to accurately image the subsurface below and around salt, in order to identify structures to drill. As a result of recent major advances in seismic processing algorithms & computer processing speeds, explorers’ can now see Subsalt/Presalt images much more clearly. The most progressive are applying these latest technologies to more salt basins globally.

Reverse Time Migration (RTM) represents the most recent and significant advance in seismic imaging below salt layers. Propelled by advances in workflows, computing power, and data management, RTM now provides the most accurate view of Subsalt/Presalt prospects, discoveries, and fields. In addition, improved seismic acquisition technology utilizing longer seismic cable lengths and denser, larger data volume collection programs, such as wide-azimuth (WAZ) and multi-azimuth (MAZ) geometries, provide extensive data volumes for the application of advanced RTM technology. Without accurate seismic imaging technology, the drilling and development of prospects is much riskier and more expensive than desired.

Discovery and development of Subsalt/Presalt fields found in past decades, using less-advanced pre-stack depth imaging applied to short-offset 2-D and narrow azimuth 3-D seismic surveys, has discovered significant reserves and production, but represents a fraction of the potential that will likely be globally discovered using new RTM technology. As was learned in the early years of exploring subsalt in the US Gulf of Mexico, we must accurately image below the salt layers, in order to have sufficient success rates to justify future economic investment. There are now fewer limits on the future global oil & gas potential below salt, and the likely discovery of substantial oil & gas reserves and production for the world of tomorrow.

 

Copyright © AAPG. Serial rights given by author. For all other rights contact author directly.

 

From the Past into the Future
Pioneering: Discovery Thinking

“Several times in the past we have thought we were running out of oil whereas actually we were only running out of ideas.” Geology Professor Parke A. Dickey, 1958.

“Discovery consists of seeing what everybody has seen, and thinking what nobody else has thought.” Albert Szent-Gyorgyi (1893-1986) Nobel Prize in Medicine (1937) -Discoverer of Vitamin C

Subsalt vs. Presalt
(Figure 1)

Subsalt –exploring beneath an allochthonous salt layer that overlies stratigraphically younger rock. Presalt–exploring beneath an autochthonous salt layer that overlies stratigraphically older rock.

Worldwide there are more than 100 salt tectonic basins (Figure 2); paleogeographic settings during the Mesozoic are instructive for considering those basins in the Western Hemisphere and sub-Saharan Africa (Figures 3, 4, 5, and 6).

Figure 1. Subsalt vs. Presalt (graphic by Mike Hudec, Bureau of Economic Geology, The University of Texas at Austin; annotation by author).

Figure 2. Global map of salt basins (Hudec and Jackson, 2007).

Figure 3. North Atlantic Triassic salt basins - circa 210 - 200 Ma (Atlantic Geoscience Society; Fensome and Williams, 2001; Geological Society of Canada, a).

Figure 4. Gulf of Mexico Jurassic salt basins- circa 170-165 Ma. (Atlantic Geoscience Society; Fensome and Williams; 2001;Geological Society of Canada, a).

Figure 5. South Atlantic Cretaceous salt basins - circa 120-110 Ma (Atlantic Geoscience Society; Fensome and Williams, 2001; Geological Society of Canada, a).

Figure 6. Five major salt provinces--subsalt presalt potential (Hudec and Jackson, 2007).

Gulf of Mexico Offshore Salt Basins

The Gulf of Mexico region has two large salt provinces and several other relatively small provinces (Figures 7, 8, and 9). The northern Gulf of Mexico, of course, contains a multitude of oil and gas fields, including subsalt discoveries (Figures 10, 11, 12, and 13).

Figure 7. Global setting of Gulf of Mexico offshore salt basins (Hudec and Jackson, 2007).

Figure 8. Gulf of Mexico basin.

Figure 9. Salt provinces in Gulf of Mexico basin (from Bird, Burke, Hall, and Casey, 2005).

Figure 10. Location map of U.S. Gulf of Mexico oil and gas fields (from Earth Science Associates, 2009), with outline of subsalt play area.

Figure 11. Salt provinces in northern Gulf of Mexico (from Moore and Brooks, 2009).

Figure 12. Location map of U.S. Gulf of Mexico subsalt discoveries.

A Chapter in Discovery Thinking

Offshore Louisiana has been the active area in exploring for and developing subsalt oil and gas resources (Figures 13, 14, 15, 16, and 17).

Figure 13. Index map of part of U.S. Gulf of Mexico (offshore Louisiana) to show location of South Marsh Island-200 (SMI 200), along with subsalt features and wells (after Moore and Brooks, 2009).

Figure 14. Well log of MSI 200, drilled in 1985, with wet sand in section below salt sheet (after Moore and Brooks, 2009).

Figure 15. Location map of 1993-1998 shelf discoveries, offshore Louisiana (after Moore and Brooks, 2009).

Figure 16. Well log of Phillips OCS-G 12008 #1, discovery well (1996) of Mahogany, first GOM subsalt field (after Moore and Brooks, 2009).

Figure 17. Conceptual cross section, onshore to deepwater, deep shelf vs. deepwater subsalt, demonstrating ultra-deep subsalt play (from McMoran investors report).

U.S. Gulf of Mexico Subsalt Potential

Over 40+ Apparent Subsalt Fields Discovered To Date

  • Subsalt Play below Allochthonous Salt Canopies & Welds
  • Projected Potential Recoverable Reserves –7 to 18+ BBOE (per published reports)

2000-2009 Subsalt Fields Discovered

  • 75+ New Field Wildcats
  • 40+% apparent wildcat discovery rate
  • Water Depths = 1000 –8500 feet
  • Reservoir Depths = 8000 –32,000 feet
  • Latest discovery -“Lucius” –announced November 2009 by APC

Important Tool in Salt-body Resolution: Reverse Time Migration Imaging

Reverse Time Migration Depth Imaging enhances depth imaging of seismic data by processing the seismic wave equation forward in time for the source AND backwards in time for the receiver (Figures 18, 19, 202122,  and 23). It rapidly computes actual numerical solutions to the complete wave equation. Our RTM algorithm breakthrough significantly reduces processing time.

Figure 18. Diagram illustrating utility of Reverse Time Migration.

Figure 19. Chart illustrating features of pre-stack depth migration methods.

Figure 20. Seismic line after utilization of wave equation migration (WEM).

Figure 21. Seismic line (same as in Figure 20, illustrating value of ION-GXT reverse time migration (RTM).

Figure 22. Isotropic, VTI (vertical transverse isotropy) and TTI (tilted transverse isotropy) in the RTM imaging flow.

Figure 23. TTI in the RTM imaging flow.

Brazil Offshore Salt Basins: Presalt Potential
(Figures 24, 25, and 26)

Over 20+ Apparent Presalt Fields Discovered To Date

  • Presalt Play below Autochthonous Salt Layer
  • Projected Recoverable Reserves = 15 BBOE to 50+ BBOE (per published reports)

2000-2009 Presalt Fields

  • 25+ New Field Pool Wildcats
  • 80+% apparent wildcat discovery rat
  • Water Depths –250–7000+ feet
  • Reservoir Depths –10,000–20,000+ feet

 

Figure 24. Location map of Brazilian salt basins in global setting (Hudec and Jackson, 2007).

Figure 25. Index map of Santos, Campos, and Espirito Santo basins, with locations of key subsalt wells.

Figure 26. Campos basin cross-section (from Anadarko investor relation presentation).

West Africa Offshore Salt Basins
(Figures 27, 28, and 29)

Figure 27. Index map of West African salt basins within global setting (Hudec and Jackson, 2007).

Figure 28. Location map of Angola subsalt area of most interest.

Figure 29. Map showing Angola subsalt field discoveries (after ION GXT BrasilSPAN; S. Henry, A. Danforth, et al., 2007).

East Canada Offshore Salt Basins
(Figures 30, 31, 32, 33, 34, and 35)

Figure 30. Index map of Eastern Canada offshore salt basins within global setting (Hudec and Jackson, 2007).

Figure 31. Location map of area of most interest, Scotian basin, Eastern Canada.

Figure 32. Map of Scotian basin, showing carbonate bank and diapiric structures (Geological Society of Canada, b; Atlantic Geoscience Society).

Figure 33. Triassic (~210 Ma) paleogeographic map, with salt sub-basins, Scotian basin (Geological Society of Canada, a; Atlantic Geoscience Society; Fensome and Williams, 2001).

Figure 34. Late Jurassic (~150 Ma) post-salt paleogeographic map, Scotian Basin (Kidston et al, 2005: Geological Society of Canada, a; Atlantic Geoscience Society; Fensome and Williams, 2001).

Figure 35. Scotian basin subsalt potential (from Shimeld, 2004).

Madagascar Offshore Salt Basin
(Figures 36, 37, and 38)

Figure 36. Location map of Madagascar offshore salt basin, within a global setting (Hudec and Jackson, 2007).

Figure 37. Index map of Majunga basin, Madagascar, with location of area of interest.

Figure 38. Cross section, illustrating basinal structure, with subsalt and presalt sections (from Tari et al., 2004).

Concluding Comments

Of the 100+ salt tectonic basins, 35+ have known salt sheets (Figure 39).

Reverse Time Migration Technology: A key tool leading Discovery Thinkers to find tomorrow’s Giant Subsalt & Presalt Fields

“Ultimately, there are few limits on future global oil & gas potential below salt. Whether it’s subsalt or presalt, we just need to accurately image below it.” (Hart’s E&P Magazine, July 2009).

Figure 39. Global map showing at least 35 salt tectonic basins with salt sheets (Hudec and Jackson, 2007)).

Acknowledgement

The author extends his appreciation to ION Geophysical Corporation for their encouragement and support of this presentation.

Additionally, ION colleagues Ken Williamson, Nick Bernitsas, Peter Nuttall, Al Danforth, Sujata Venkatraman, Steve Henry, Barbara Radovich, Naresh Kumar, Menno Dinkelman, Ian Davison, Paul Farmer, David Jones, and Thomas Misek provided assistance and interpretation in the content of the slides contained in this talk.

Special thanks to Martin P.A. Jackson and Michael R. Hudec.

References

Bird, Dale E., Kevin Burke, Stuart A. Hall, and John F. Casey, 2005, Gulf of Mexico tectonic history: Hotspot tracks, crustal boundaries, and early salt distribution: AAPG Bulletin, v. 89, no. 3, p. 311–328.

Earth Science Associates, 2009 (internal database).

Geological Survey of Canada, a -- Geology of the Scotian Margin:Geologic overview: http://gsc.nrcan.gc.ca/marine/scotianmargin/go_e.php (accessed December 8, 2010).

Geological Survey of Canada, b -- Geology of the Scotian Margin: Stratigraphic overview: http://gsc.nrcan.gc.ca/marine/scotianmargin/so_e.php (accessed December 8, 2010).

Fensome, R.A and Williams, G.L., 2001, The last billion years – A geological history of the Maritime Provinces of Canada: Atlantic Geoscience Society, Nimbus Publishing, Halifax, 212p.)

Henry, S., A. Danforth, et al., 2007 (Presentation by Al Danforth at 2007 HGS-PESGB Cape Town, South Africa Conference).

Hudec, M., and M.P.A. Jackson, M.P.A., 2007, Terra infirma: Understanding salt tectonics: Earth-Science Reviews, v. 82, p. 1-28.

Kidston, A.G., D.E. Brown, B.M. Smith B.M., and B. Altheim, 2005, The Upper Jurassic Abenaki Formation offshore Nova Scotia: A seismic and geologic perspective: Canada-Nova Scotia Offshore Petroleum Board, Halifax, 165p.

Moore, Dwight “Clint”, and R.O. Brooks, 2009, The evolving exploration of the subsalt play in the offshore Gulf of Mexico: Search and Discovery Article #60021 (2009) (http://www.searchanddiscovery.com/documents/2009/09004moore02/figures/moore02_online.PDF)

Shimeld, J., 2004, A comparison of salt tectonic subprovinces beneath the Scotian slope and Laurentian Fan, in P.J. Post, D.L. Olson, K.T. Lyons, S.L. Palmes, P.F. Harrison, and N.C. Rosen, Salt-Sediment Interactions and Hydrocarbon Prospectivity: Concepts, Applications and Case Studies for the 21st Century: 24th Annual GCSSEPM Foundation Bob F. Perkins Research Conference, Houston, Texas, p.502-532 (CD-ROM).

Tari, G., et al., 2004, Salt tectonics and sedimentation in the offshore Majunga Basin in in Salt-Sediment Interactions and Hydrocarbon Prospectivity: GCS-SEPM 2004 Perkins Conference, p. 614-639.

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