1887
Volume 49, Issue 2
  • E-ISSN: 1365-2478

Abstract

This is the final paper in a series on the 3D multicomponent seismic experiment in Oman. In this experiment a 3D data set was acquired using three‐component geophones and with three source orientations. The data set will subsequently be referred to as the Natih 9C3D data set. We present, for the first time, evidence demonstrating that shear waves are sensitive to fluid type in fractured media. Two observations are examined from the Natih 9C3D data where regions of gas are characterized by slow shear‐wave velocities. One is that the shear‐wave splitting map of the Natih reservoir exhibits much larger splitting values over the gas cap on the reservoir. This increase in splitting results from a decrease in the slow shear‐wave velocity which senses both the fractures and the fracture‐filling fluid. Using a new effective‐medium model, it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect. Secondly, an anomaly was encountered on the shear‐wave data directly above the reservoir. The thick Fiqa shale overburden exhibits a low shear‐wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content. No such effects are evident in the conventional P‐wave data. This feature is interpreted as a gas chimney above the reservoir, a conclusion supported by both effective‐medium modelling and the geology.

With this new effective‐medium model, we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves (S2), polarized perpendicular to the fracture orientation, whilst leaving the vertical compressional‐wave velocity largely unaffected. This conclusion has direct implications for seismic methods in exploration, appraisal and development of fractured reservoirs and suggests that here we should be utilizing S‐wave data, as well as the conventional P‐wave data, as a direct hydrocarbon indicator.

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2001-12-21
2024-03-29
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References

  1. BudianskyB. & O'ConnellR.J.1976. Elastic moduli of a cracked solid. International Journal of Solids and Structures12, 81–97.
    [Google Scholar]
  2. CrampinS.1984. Effective anisotropic elastic constants for wave propagation through cracked solids. Geophysical Journal of the Royal Astronomical Society76, 133–145.
    [Google Scholar]
  3. EshelbyJ.D.1957. The determination of an ellipsoidal inclusion and related problems. Proceedings of the Royal Society of LondonA241, 376–396.
    [Google Scholar]
  4. GassmannF.1951. Über die Elastizität poröser Medien. Vierteljahresschrift der Naturforschenden Gesellschaft in Zürich96, 1–23.
    [Google Scholar]
  5. GregoryA.R.1977.Rock Physics in Seismic Interpretation. AAPG Memoir 26, ‘Seismic Stratigraphy – Applications to Hydrocarbon Exploration’.
  6. HakeJ.H., GeversE.C.A., Van Der KolkC.M., TichelaarB.W.1998. The 3D shear experiment over the Natih field in Oman. II. Pilot seismic and borehole data. Geophysical Prospecting46, 617–646.
    [Google Scholar]
  7. HillR.1965. A self‐consistent mechanics of composite materials. Journal of the Mechanics and Physics of Solids13, 213–222.
    [Google Scholar]
  8. HitchingsV.H & PottersH.1999. Production and geological implications of the Natih 9C3D seismic survey. GeoArabia5, 511–524.
    [Google Scholar]
  9. HudsonJ.A.1980. Overall properties of a cracked solid. Mathematical Proceedings of the Cambridge Philosophical Society88, 371–384.
    [Google Scholar]
  10. HudsonJ.A.1981. Wave speeds and attenuation of elastic waves in material containing cracks. Geophysical Journal of the Royal Astronomical Society64, 133–150.
    [Google Scholar]
  11. HudsonJ.A., LiuE., CrampinS.1996. The mechanical properties of materials with interconnected cracks and pores. Geophysical Journal International124, 105–112.
    [Google Scholar]
  12. MacBethC., JakubowiczH., KirkW., Xiang‐YangLi, OhlsenF.1999. Fracture‐related amplitude variations with offset and azimuth in marine seismic data. First Break17, 13–26.
    [Google Scholar]
  13. MallickS., CraftK.L., MeisterL.J., ChambersR.E.1998. Determination of the principal directions of azimuthal anisotropy from P‐wave seismic data. Geophysics63, 692–706.
    [Google Scholar]
  14. MercadierC.G. & MäkelG.H.1991.Fracture patterns of Natih formation outcrops and their application for reservoir modelling of the Natih field, North Oman. SPE Paper 21337.
  15. NurA. & SimmonsG.1969. The effect of saturation on velocity in low porosity rocks. Earth and Planetary Science Letters7, 183–193.
    [Google Scholar]
  16. O'ConnellR.J. & BudianskyB.1977. Viscoelastic properties of fluid‐saturated cracked solids. Journal of Geophysical Research82, 5719–5735.
    [Google Scholar]
  17. PottersJ.H.H.M., GroenendaalH.J.J., OatesS.J., HakeJ.H., KaldenA.B.1999. The 3D shear experiment over the Natih field in Oman. Reservoir geology, data acquisition and anisotropy analysis. Geophysical Prospecting47, 637–662.DOI: 10.1046/j.1365-2478.1999.00155.x
    [Google Scholar]
  18. SayersC.M. & KachanovM.1991. A simple technique for finding effective elastic constants of cracked solids for arbitrary crack orientation statistics. International Journal of Solids and Structures27, 671–680.
    [Google Scholar]
  19. SchoenbergM. & DoumaJ.1988. Elastic wave propagation in media with parallel fractures and aligned cracks. Geophysical Prospecting36, 571–590.
    [Google Scholar]
  20. SchoenbergM. & MuirF.1989. A calculus for finely layered anisotropic media. Geophysics54, 581–589.
    [Google Scholar]
  21. WalpoleL.J.1969. On the overall elastic moduli of composite materials. Journal of the Mechanics and Physics of Solids17, 235–251.
    [Google Scholar]
  22. WalshJ.B.1965. The effect of cracks on the compressibility of rock. Journal of Geophysical Research70, 381–389.
    [Google Scholar]
  23. WuT.T.1966. The effect of inclusion shape on the elastic moduli of a two‐phase material. International Journal of Solids and Structures2, 1–8.
    [Google Scholar]
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