Page 219 - Origin and Prediction of Abnormal Formation Pressures

194 G.V. CHILINGAR, W. FERTL, H. RIEKE AND J.O. ROBERTSON JR.

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Fig. 8-3. Map of southern Louisiana showing the location of fields with abnormally high pressures.
Approximate depth to top of the abnormally high pressure formation (AHFP) are shown by the estimated
contours (depths are in ft). Stars - salt domes; encircled stars - salt domes with abnormally high
pressures; full dots = fields without salt domes. (Modified after Dickey et al., 1968, fig. 7, in Rieke and
Chilingarian, 1974, fig. 171, p. 321. Courtesy of Science, copyright 1968 by the American Association for
the Advancement of Science.)

stratigraphically in a basinward direction, being modified by growth faulting (Harkins
and Baugher, 1969).
In explaining the relationship between abnormally high fluid pressures and growth
faulting, Dickey et al. (1968) proposed that during compaction, pore fluids in the
marine sediments migrate vertically upward towards the seafloor at a constant rate. As
compaction progresses, the vertical permeability of the argillaceous sediments decreases
rapidly, forcing the interstitial fluids to travel parallel to the bedding planes. If growth
faulting occurs while abundant water is still present in the shales, the routes of up-dip
fluid migration parallel to the bedding would be shut off by the fault plane. Pressure
buildup tests in producing oil and gas wells have shown that faults, which cut reservoirs,
form pressure discontinuities and are seals to fluid movement. As a result of cutoff
because of faulting, the fluid has to sustain a heavier overburden load as sedimentation
proceeds. Whenever the growth faulting occurred after most of the water had been
expelled and the shales were already compacted, the abnormal pressures were observed
to be much lower, or pressures were normal.
Dickey et al. (1968) also pointed out that inasmuch as growth faults often have dip
angles of less than 50 ~ wells frequently cross fault planes. It is possible for a well to
encounter the abnormal-pressure zone and then, after crossing a fault plane, to enter a
different fault block where pressures are normal.