Page 164 - Origin and Prediction of Abnormal Formation Pressures

140 G.V. CHILINGAR, V.A. SEREBRYAKOV, S.A. KATZ AND J.O. ROBERTSON JR.

Another advantage of the compressional curves method is that it enables the
estimation of thickness of eroded deposits and also the detection of unconformities
(Serebryakov and Chilingar, 1994).

RADIOACTIVITY STUDY OF ZONES WITH ABNORMALLY HIGH FORMATION PRESSURE
Zoeller (1984) and Starostin (1985) discovered a gamma-ray phenomenon (de-
creasing radioactivity) in zones of abnormally high pressure. They attributed this
phenomenon to a high porosity in zones of abnormally high pressure. Upon extensive
research, however, Serebryakov et al. (1995) noted that the decrease in radioactivity is
not related to the change in porosity, because this phenomenon can be found only in
basins with nonequilibrium compaction and only in overpressured zones.
Radioactivity of sedimentary rocks primarily depends upon the presence of uranium,
thorium and potassium. Starostin (1985) examined 166 core samples of shale both in the
zones of abnormally high pressure and zones of normal hydrostatic pressure. He found
that the contents of uranium and thorium are not different in the normally pressured and
abnormally high pressured zones. In addition, U and Th are not sufficiently soluble.
In the opinion of Serebryakov et al. (1995), the most important indicator is the
radioactive isotope of K: (4~ The potassium ion has a negative hydration in water
(Blokh, 1969), i.e., water molecules become more mobile in the vicinity of potassium
ions than they do in pure water.
According to the principle of Le Chatelier, there is a mobile balance between intersti-
tial solutions and the solid phase. An increase in pore pressure leads to the disruption of
mobile balance and, as a result, ions which can decrease the pressure (potassium ions)
of the solution move into filtrating water. Water molecules become more mobile in the
presence of potassium ions than in pure water (negative hydration) and migrate more
easily, removing potassium ions. This leads to a decrease in concentration of 4~ ions in
the shales of abnormally high pressured zones.
It is interesting to note that mud (drilling fluid) engineers are quite familiar with
potassium-based muds which inhibit clay swelling and hydration and, consequently,
prevent heaving and sloughing of shales (Chilingarian and Vorabutr, 1981). The removal
of potassium ions from abnormally pressured zones prevents the transformation of
montmorillonites to illites, which requires potassium ions to complete the reaction. In
addition, there is a conversion of illites to montmorillonites (reverse reaction). Both
phenomena (in addition to overpressure) contribute to the greater potential of shales
to swell because montmorillonites swell more than illites (Rieke and Chilingarian,
1974).
The results of radioactivity studies of natural shales in the Kharasavey oilfield
(northwestern Siberia) are presented in Table 5-4 and Fig. 5-10. The total radioactivity
and the radioactivity of 4~ were obtained from gamma-ray logs and by measuring the
radioactivity of 4~ in the core samples.
Thus, it appears possible to use the natural shale radioactivity to locate the abnormally
high pressured zones in basins with nonequilibrium compaction (origin of abnormal
pressure) and where pore pressure is abnormal.