Page 336 - Petroleum Geology

pressure on them measured. From this the proper mudweight for further
drilling could be determined.
At the first kick, or when satisfied that the top of abnormal pressures had
been reached, casing was set and drilling proceeded with a properly weighted
mud, but as light as possible, in greater safety and with greatly reduced risk
of wall-sticking. The same techniques were used to decide when to increase
the mudweight, and by how much. By drilling for kicks, the rare combina-
tion of increased safety at reduced cost was ach-ieved.
These experiences indicated that at some depth the pore-fluid pressures
departed from the normal hydrostatic and approached the overburden pres-
sures in what came to be called the transition zone. A few very high pressures
equal to, or slightly in excess of, the overburden pressures were reported, but
the common limit seemed to be about 90% of the overburden pressures. Al-
most all the sequences with abnormal pressures were Tertiary in age in the
early years, but later such pressures were also found in sequences of Mesozoic
and even late Palaeozoic age, although these were rarely of great severity.
For many years the cause was thought to lie in the depth, because there
was a remarkable constancy of depth to top of abnormal pressures in the U.S.
Gulf Coast (about 3 km). It was thought that the loss of permeability with
compaction during burial sooner or later was such that the pore fluids could no
longer be expelled. The pore fluids then bore some of the load. It was tacitly
assumed that once the top of abnormal pressures had been reached, they
would continue indefinitely to greater depths. It was George Dickinson who,
with a geological approach, demonstrated in a classic paper that should still
be read, that the cause of abnormal pressures is stratigraphic (Dickinson,
1951,1953).

INTERPRETATION OF OBSERVATIONS

The essence of Dickinson’s geological observations in the Louisiana Gulf
Coast is that the age of the sediments in which the top of abnormal pressures
occurs is younger the father south towards the Gulf, and that the lithology is
mudstone below the massive regressive sands (Fig. 14-2). That the top of ab-
normal pressures tended to occur at depths of about 3 km (10,000 ft) was a
coincidence of the geology of the area. Dickinson noted that high pressures
existed in sand lenses that were isolated by mudstones, and sandstones that
were isolated by faults. The permeability of these could lead to massive water
or petroleum influx into the borehole. He attributed these abnormal pressures
to the compaction of the mudstones under the gravitational load of the over-
burden. Numerous measurements indicated that the degree of abnormality
increased with depth on a gradient (AplAz) greater than that of the overbur-
den (Dickinson, 1953, p. 420, fig. 6).
Subsequently, the main development in the topic was in recognizing ab-