Page 333 - Petroleum Geology

PART 3. PETROLEUM GEOLOGY OF REGRESSIVE SEQUENCES

CHAPTER 14

ABNORMAL PRESSURES

SUMMARY

(1) Abnormal pressures are, for practical purposes, pore pressures that are
sufficiently greater than normal hydrostatic to have a noticeable effect when
drilling, and to require special precautions. They are stratigraphically related
to thick mudstones, usually but not invariably in regressive sequences, with
sand/shale ratios less than 10%.
(2) Abnormally high pore pressures are also related to growth structures
that are also stratigraphically related - that is, to those that occur in regressive
sequences. The top of abnormal pressures usually lies below the level of maxi-
mum growth-rate on growth faults, close to (above or below) the level at
which growth faulting began. These relationships appear to be causal.
(3) The cause of abnormal pressures is largely mechanical loading of a rela-
tively impermeable mudstone, and it seems likely that the pore pressures in
such mudstones have never been normal hydrostatic. In their turn, they cause
mechanical instability in such regressive sequences through the retention of
relatively large porosity, low bulk density and low equivalent viscosity while
the sandier part of the regressive sequence accumulates.
(4) Generation of petroleum in the source rock may also generate abnor-
mal pressures (and resistivities higher than normal).

OBSERVATIONS

There is a great deal of evidence from boreholes around the world that
pore-fluid pressures are, in general, normal hydrostatic - that is, the pressures
measured in water reservoirs and the water legs of oil reservoirs are sufficient
to support a column of that water to close to the land surface (or sea level
on the continental shelves). This implies that the pore water is in physical
continuity to the surface, however tortuous the paths may be, and that dur-
ing burial this continuity has been maintained. Were it not for the ever-in-
creasing depths of boreholes drilled for petroleum, it might reasonably have
been assumed that this was the universal rule.
The drilling of “gushers” in the cable-tool days does not constitute an