Page 340 - Petroleum Geology

310

U.S. Gulf Coast down a larger-than-normal geothermal gradient would lead
to a rate of water expansion that could be dissipated by a mudstone perme-
ability close to the smallest measured, and concluded that thermal expansion
could not be more than a minor component of abnormal pressures. Barker
and Horsfield (1982) disputed this conclusion, but did not convince Chapman
(1982).
At the time of writing, therefore, it cannot be said that a true cause of ab-
normal pressures has been identified. A vote would strongly favour the ther-
mal school - but science is not a matter of voting. The importance of correctly
understanding this topic is great because error in this leads to errors in the
related topics of primary migration and its timing in regressive sequences,
and so to error in exploration philosophy. We shall therefore examine the
components of this topic in some detail.

THE NATURE OF THE TRANSITION ZONE
The characteristics of the transition zone from normal to seriously abnor-
mal pore pressures are typically these: (a) the penetration rate of drilling in-
creases; (b) the pore pressures measured in sandstones or inferred for mud-
stones increases with depth, usually more rapidly than the overburden pres-
ures; (c) the geothermal gradient increases; and (d) the sonic log indicates
transit times in mudstones increasing with depth (Fig. 14-3):
- the resistivity logs show a decrease in resistivity in mudstones with depth
(Fig. 14-3); and
-logs that measure bulk density indicate a decrease in mudstone bulk
density with depth, and this is supported by cutting density measurements
(see Fertl, 1976, p. 150).
These are all reversals of the normal trend.
These features are not seen in every well, but most that penetrate the tran-
sition zone above depths of about 3 km (some deeper) in the U.S. Gulf Coast,
Niger delta, Mackenzie delta (Canadian Arctic), and south-east Asia show
them all. We are concerned here with the common observations, and must
accept that there will be wells and areas that do not conform to these obser-
vations.
In seeking a geological understanding of the transition zone, we must clearly
understand that the stratigraphic position of the top of abnormal pressures,
the top of the transition zone, means that there is probably a mudstone-facies
trend that may contribute to these features (see p. 219).
(a) The increase in drilling rate is the effect of drilling undercompacted
mudstones, and when heaving shales are encountered with the drilling break,
it is clearly reasonable to interpret them as such. However, there is an hydraulic
effect that is not related directly to undercompaction. Drilling rate is sensi-
tive to the relationship between mudweight and pore pressures: it has long