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Geological applications of acoustic logging

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The results from acoustic logs can shed light on two issues geologists typically address: erosion and uplift estimation, and organic richness/source-rock potential.

Estimates of erosion and uplift (exhumation)

The amount of erosion that has occurred in a region that has been uplifted can be estimated from the degree of shale compaction measured by acoustic travel time.[1][2][3] This technique assumes that shale compaction is irreversible and that the shale retains the degree of compaction it gained at its maximum burial depth. Uplift and erosion will result in lower porosities than expected for the current burial depth (i.e., a shale will appear to be overcompacted).[4][5][6][7][8][9][10][11][12][13][14][15]

Determination of organic richness and source-rock potential

Acoustic slowness, used alone or in conjunction with formation resistivity, can provide qualitative indications and quantitative determination of source-rock potential (when calibrated to laboratory data). The identification of potential petroleum-source rocks and characterizing the thermal maturity of these rocks is important for assessing petroleum potential (risking) and for basin modeling.

Studies of coals and organic-rich shales have demonstrated that acoustic velocity is reduced by the presence of organic material, that changes in velocity are proportional to the volume of organic material present, and that increases in thermal maturity (largely a function of burial depth and temperature) are accompanied by increases in acoustic velocity (decreases in transit time, Δt). Total organic carbon and the level of organic thermal maturation, expressed in terms of vitrinite reflectance, are two key parameters used for determining the potential of a formation to source hydrocarbons, and each can be mathematically related to Δt.[16][17][18] Because acoustic velocity is influenced by a number of factors in addition to organic carbon content, a combination of log measurements can provide improved results when other factors do not mask responses. In particular, acoustic-resistivity crossplot techniques (Fig. 1[19]) and log overlays (Fig. 2) have proved successful.[20][21][22]

  • Fig. 1 – Identification of source-rock potential using a sonic transit-time-resistivity crossplot – points plotted above the line are source rocks, while those below are not.[19] R75°F is well-log resistivity corrected to 75° (courtesy of AAPG).

    Fig. 1 – Identification of source-rock potential using a sonic transit-time-resistivity crossplot – points plotted above the line are source rocks, while those below are not.[19] R75°F is well-log resistivity corrected to 75° (courtesy of AAPG).

  • Fig. 2 – Source-rock identification and assessment using an acoustic/resistivity overlay technique. Separation of the acoustic and resistivity curves (Track 2), labeled Δ log R, indicates organic-rich intervals as shown by the core analyses in Tracks 3 and 4. The amount of separation is directly related to the amount of total organic carbon and is a function of thermal maturity[21] (courtesy of AAPG).

    Fig. 2 – Source-rock identification and assessment using an acoustic/resistivity overlay technique. Separation of the acoustic and resistivity curves (Track 2), labeled Δ log R, indicates organic-rich intervals as shown by the core analyses in Tracks 3 and 4. The amount of separation is directly related to the amount of total organic carbon and is a function of thermal maturity[21] (courtesy of AAPG).

References

  1. ↑ Magara, K. 1976. Thickness of Removed Sedimentary Rocks, Paleopore Pressure, and Paleotemperature, Southwestern Part of Western Canada Basin. AAPG Bulletin 60 (4): 554–565.
  2. ↑ Magara, K. 1986. Thickness of Erosion. In Geological Models of Petroleum Entrapment, Ch. 7, 129-151. London: Elsevier Applied Science Publishers.
  3. ↑ Bulat, J., and Stoker, J.S. 1987. Uplift Determination from Interval Velocity Studies, UK Southern North Sea. In Petroleum Geology of North West Europe, Brooks, J. and Glennie, K. eds., Vol. 1, 293-305. London: Graham and Trottman.
  4. ↑ Storvoll, V., Bjorlykke, K., and Mondol, N.M. 2005. Velocity-Depth Trends in Mesozoic and Cenozoic Sediments from the Norwegian Shelf. AAPG Bulletin 89 (3): 359–381.
  5. ↑ Hillis, R.R. 1993. Quantifying Erosion in Sedimentary Basins from Sonic Velocities in Shales and Sandstones. Exploration Geophysics 24: 561–566.
  6. ↑ Hillis, R.R., Thomson, K., and Underhill, J.R. 1994. Quantification of Tertiary Erosion in the Inner Moray Firth Using Velocity Data from the Chalk and the Kimmeridge Clay. Marine and Petroleum Geology 11 (3): 283–293.
  7. ↑ Hansen, S. 1996. A Compaction Trend for Cretaceous and Tertiary Shales on the Norwegian Shelf Based on Sonic-Transit Times. Petroleum Geoscience 2 (1): 59–68.
  8. ↑ Heasler, H.P. and Kharitonova, N.A. 1996. Analysis of Sonic Well Logs Applied to Erosion Estimates in the Bighorn Basin, Wyoming. AAPG Bulletin 80 (5): 630–646.
  9. ↑ Evans, D.J. 1997. Estimates of the Eroded Overburden and the Permian-Quaternary Subsidence History of the Area West of Orkney. Scottish J. of Geology 33 (Part 2): 169–181.
  10. ↑ Japsen, P. 1998. Regional Velocity-Depth Anomalies, North Sea Chalk—A Record of Overpressure and Neogene Uplift and Erosion. AAPG Bulletin 82 (11): 2,031–2,074.
  11. ↑ Densley, M.R., Hillis, R.R., and Redfearn, J.E.P. 2000. Quantification of Uplift in the Carnarvon Basin Based on Interval Velocities. Australian J. of Earth Sciences 47 (1): 111–122.
  12. ↑ Fuh, S.-C. 2000. Magnitude of Cenozoic Erosion from Mean Sonic Transit Time, Offshore, Taiwan. Marine and Petroleum Geology 17 (9): 1,011–1,028.
  13. ↑ Al-Chalabi, M. 2001. The Use of Instantaneous Velocity in Uplift Investigation. Geophysical Prospecting 49: 645–655.
  14. ↑ Ware, P.D. and Turner, J.P. 2002. Sonic Velocity Analysis of the Tertiary Denudation of the Irish Sea Basin. In Exhumation of The North Atlantic Margin; Timing, Mechanisms and Implications for Petroleum Exploration, Dore, A.G., Cartwright, J.A., Stoker, M.S., Turner, J.P., and White, N.J. eds., 355-370. London: Special Publication 196, Geological Society.
  15. ↑ Mavromatidis, A., and Hillis, R. 2005. Quantification of Exhumation in the Eromanga Basin and Its Implications for Hydrocarbon Exploration. Petroleum Geoscience 11 (1): 79–92.
  16. ↑ Stocks, A.E. and Lawrence, S.R. 1990. Identification of Source Rocks from Wireline Logs. In Geological Applications of Wireline Logs, A. Hurst, M.A. Lovell, and A.C. Morton eds., 241–252. Geological Soc. of London Special Publication No. 48.
  17. ↑ Herron, S.L. 1991. In Situ Evaluation of Potential Source Rocks by Wireline Logs. In Source and Migration Processes and Evaluation Techniques, Treatise of Petroleum Geology--Handbook of Petroleum Geology, R.K. Merrill ed. Ch. 13, 127-134. Tulsa, Oklahoma: AAPG.
  18. ↑ Lang, W.H. 1994. The Determination of Thermal Maturity in Potential Source Rocks Using Interval Transit Time/Interval Velocity. The Log Analyst 35 (6): 47–59.
  19. ↑ 19.0 19.1 Meyer, B.L., and Nederlof, M.H. 1984. Identification of Source Rocks on Wireline Logs by Density/Resistivity and Sonic Transit Time/Resistivity Crossplots. AAPG Bulletin 68 (2): 121–129.
  20. ↑ Meyer, B.L., and Nederlof, M.H. 1984. Identification of Source Rocks on Wireline Logs by Density/Resistivity and Sonic Transit Time/Resistivity Crossplots. AAPG Bulletin 68 (2): 121–129.
  21. ↑ 21.0 21.1 Passey, Q.R. et al. 1990. A Practical Model for Organic Richness from Porosity and Resistivity Logs. AAPG Bulletin 74 (12): 1,777–1,794.
  22. ↑ Huang, Z. et al. 1996. Cyclicity in the Egret Member (Kimmeridgian) Oil Source Rock, Jeanne d’Arc Basin, Offshore Eastern Canada. Marine and Petroleum Geology 13 (1): 91–105.

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See also

Reservoir geology

Acoustic logging

Acoustic logging tools

PEH:Acoustic_Logging

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