PERMEABILITY OF CRYSTALLINE AND ARGILLACEOUS ROCKS

Readily available laboratory, in situ and inferred values of permeability, k, of crystalline and argillaceous rocks have been compared. For crystalline rocks, in situ k ranged from about 1 mu D to 100 Md: for argillaceous rocks it was about 0.01 to 1 mu D. No systematic decrease of k with depth was evident: over some interval at nearly every well, k was 1 to 100 Md: these highly conductive intervals were as deep as 2-3 km. In situ permeability has been inferred from earthquake precursors, anomalous pore pressure, leakage from aquifers or other large-scale phenomena. Where crystalline rocks are involved, k was about 0.1 to 10 Md, and thus about the same as the more permeable zones in wells: this is close to the permeability of many sandstones and is about 1000 times greater than laboratory measurements for intact crystalline rocks. For argillaceous rocks, laboratory, in situ, and inferred values all agreed within about a factor of 10. Laboratory study of artificial fractures suggest that in situ values for crystalline rocks are high because of natural fractures; fractures may be sealed or absent in shale. Based on observed variation in wells, k at particular sites in crystalline rock is not predictable within a factor of 100000. For crystalline rocks, laboratory values provide little more than the minimum in situ k; for argillaceous rocks they may provide a good estimate of in situ k. Because of the great sensitivity of k to the effective stress, measurement or estimation of k must be tailored to the particular stress state of the application. If, as tentatively suggested by in situ and inferred values of k, average crustal k is about 10 Md, pore pressure much greater than hydrostatic seems ruled out in terrains of outcropping crystalline rocks. Apart from hot pluton environments, anomalously high pore pressures seem to require everywhere a thick blanket of clay-rich rocks, as originally suggested for sedimentary basins. (a) (TRRL)

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  • Corporate Authors:

    Pergamon Press, Incorporated

    Headington Hill Hall
    Oxford OX30BW,   England 
  • Authors:
    • Brace, W F
  • Publication Date: 1980-10

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Filing Info

  • Accession Number: 00330297
  • Record Type: Publication
  • Source Agency: Transport Research Laboratory
  • Files: ITRD, TRIS
  • Created Date: May 21 1981 12:00AM