Page 77 - Petroleum Geology

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Abnormal pressures imply that if the water is free to rise in a borehole it
would flow at the surface or, if we could insert a manometer into the forma-
tion, the water would rise in it to a level well above the surface of the ground
(Fig. 3-10). If we take sea level as the datum surface, the total head (seep. 165,
Fig. 8-10) may well be of the same order of magnitude as the depth - that
is, the pressure head may well be about twice the elevation head. Because
abnormal pressures in all but a few exceptional areas lie deeper than normal
hydrostatic pressures (shown in schematic but typical form in Fig. 3-11) we
infer that low permeability has retarded water flow to the normally pressured
part of the sequence.

Z
Fig. 3-10. If manometerscould be inserted into a sedimentary sequence, the pore pressures
would be indicated by the elevation of the water level in the manometer.
Fig. 3-11. Diagrammatic, but typical pressure-depth plot for pore fluids in a regressive
sequence.

Geologically, the great majority of abnormally pressured sequences have
the tops of abnormal pressures (as Dickinson, 1951 and 1953, noted for the
Louisiana Gulf Coast) near the top of the thick mudstone in a dominantly
regressive sequence of sedimentary rocks, and the normal hydrostatic pres-
sures occur in the sandier part of the sequence. It appears, therefore, that ab-
normal pressures do not occur where the permeability is sufficiently large to
drain the water of compaction. Harkins and Baugher (1969) found 5 to 10%
sand to be the regional indicator of impending high pressures when drilling in
the US. Gulf Coast region.