4.7 Article

Shale transformations and physical properties-Implications for seismic expression of mobile shales

Journal

EARTH-SCIENCE REVIEWS
Volume 220, Issue -, Pages -

Publisher

ELSEVIER
DOI: 10.1016/j.earscirev.2021.103746

Keywords

Shale tectonics; Mobile shales; Clay transformations; Hydrocarbons; Seismic interpretation; Shale diapirs

Funding

  1. Anadarko
  2. Aramco Services
  3. BHP Billiton
  4. BP
  5. CGG
  6. Chevron
  7. Condor
  8. Ecopetrol
  9. EMGS
  10. ENI
  11. ExxonMobil
  12. Hess
  13. Ion-GXT
  14. Midland Valley
  15. Murphy
  16. Nexen USA
  17. Noble
  18. Petrobras
  19. Petronas
  20. PGS
  21. Repsol
  22. Rockfield
  23. Shell
  24. Spectrum
  25. Equinor
  26. Stone En-ergy
  27. TGS
  28. Total
  29. WesternGeco
  30. Woodside
  31. Applied Geodynamics Laboratory (AGL) Industrial Associates program

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Interpreting mobile shales using seismic data is challenging due to their unclear seismic expression and low impedance contrast with other sedimentary rocks. Their seismic properties are influenced by factors such as overpressure, fluid content, and diagenetic transformations during burial. Understanding the physical properties and evolution of mobile shales can help improve seismic interpretation and characterization.
Seismic interpretation of mobile shales is challenging, mostly because of their unclear seismic expression. Imaging of mobile shales is difficult because of their low seismic-impedance contrast with many sedimentary rocks, spatial variation of their seismic properties, complex geometries of mobile-shales structures, and their complex internal structures. Furthermore, their seismic properties depend strongly on both overpressure and fluid content, which are difficult to predict. To unravel this problem, we reviewed the physical properties of shales, merging data from in situ determinations of density and sonic velocities with experimental data and modeling results. We analyzed how diagenetic transformations during shale burial modify their physical properties and seismic characteristics. We reviewed conditions for smectite-illite transformation and gas generation (mostly methane) by oil cracking to evaluate how thermal gradient, shale composition, and hydrocarbon content modify the densities, sonic velocities, and seismic expressions of mobile shales. We then incorporated the amount and type of fluids released in shales during diagenesis into a study of seismic reflectivity of mobile shales. Results derived theoretically for various types of mobile-shale contacts are compared with high-quality seismic examples, including mud volcanoes and a variety of complex shale diapirs. Observed reflectivity and seismic fabrics are discussed to infer clay composition, fluid content and type, and temperature.

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