Seal capacity of different rock types

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Predicting the occurrence of oil and gas traps
Chapter Evaluating top and fault seal
Author Grant M. Skerlec
Link Web page
Store AAPG Store

Range of capacities[edit]

Figure 1 Range of seal capacities of different rock types.

Figure 1 shows the range of seal capacities of different rock types. This figure was compiled from published displacement pressures based upon mercury capillary curves. Column heights were calculated using a 35°API oil at near-surface conditions with a density of 0.85 g/cm3, an interfacial tension of 21 dynes/cm, and a brine density of 1.05 g/cm3. Data were compiled from Smith[1] Thomas et al.[2] Schowalter[3] Wells and Amaefule[4] Melas and Friedman[5] Vavra et al.[6] Boult[7] Khrushin (1993), and Shea et al..[8]


These data show the following:

  • Good shales can trap thousands of feet of hydrocarbon column.
  • Most good sands can trap only length::50 ft or less of oil column.
  • Poor sands and siltstones can trap 50–400 ft of oil column.

Shale seals[edit]

Shales have high displacement pressures and can trap large columns of oil as large as depth::1830 m (6000 ft). Nonsmectite shales have pore throat radii of less than 12 nm and can trap gas columns of more than depth::1000 m (3,000 ft) (Krushin, 1993). Shales in the Cretaceous section of the Powder River basin have displacement pressures of 1000–4000 psi and can trap gas columns of 460–1830 m (1500–6,000 ft) (Jiao et al., 1993). The shortest oil columns among the shale data include some true shales as well as siltstones, silty mudstones, and interbedded sand/shale cores.

Sand seals[edit]

Sands commonly have low displacement pressures and can trap only small oil columns. Three-quarters of the sands, most of which are Gulf Coast reservoirs, are capable of trapping less than length::50 ft of oil. Sands can have sufficiently high displacement pressures to trap hundreds of feet of oil. Oil column heights between 50–400 ft are from sands with diage-netic pore fillings, tight gas sands, and very fine-grained sands that probably include siltstones.

Carbonate seals[edit]

Carbonates have a wide range of displacement pressures. Some carbonates can seal as much as 1500–6000 ft of oil. These better seals are argillaceous limestones and shelf carbonates. In the Gulf Coast basin, shorter oil columns are sealed by grainstones, mud-stones, and wackestones of the Smackover Formation and chalk.

See also[edit]


  1. Smith, D., A., 1966, Theoretical considerations of sealing and non-sealing faults: AAPG Bulletin, vol. 50, no. 2, p. 363–374.
  2. Thomas, L., K., Katz, D., L., Ted, M., R., 1968, Threshold pressure phenomena in porous media: Transactions of SPE, vol. 243, p. 174–184.
  3. Schowalter, T., T., 1979, Mechanics of secondary hydrocarbon migration and entrapment: AAPG Bulletin, vol. 63, no. 5, p. 723–760.
  4. Wells, J., D., Amafuele, J., O., 1985, Capillary pressure and permeability relationships in tight gas sands: SPE/DOE paper 13879.
  5. Melas, F., F., Friedman, G., M., 1992, Petrophysical characteristics of the Jurassic Smackover Formation, Jay field, Conecuh Embayment, Alabama and Florida: AAPG Bulletin, vol. 76, no. 1, p. 81–100.
  6. Vavra, C., L., Kaldi, J., G., Sneider, R., M., 1992, Geological applications of capillary pressure: a review: AAPG Bulletin, vol. 76, no. 6, p. 840–850.
  7. Boult, P., J., 1993, Membrane seal and tertiary migration pathways in the Bodalla South oilfield, Eronmanga Basin, Australia: Marine and Petroleum Geology, vol. 10, no. 1, p. 3–13., 10., 1016/0264-8172(93)90095-A
  8. Shea, W., T., Schwalbach, J., R., Allard, D., M., 1993, Integrated rock-log evaluation of fluvio-lacustrine seals, in Ebanks, J., Kaldi, J., Vavra, C., eds., Seals and Traps: A Multidisciplinary Approach: AAPG Hedberg Research conference, unpublished abstract.

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