Page 225 - Origin and Prediction of Abnormal Formation Pressures

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

(3) Plastic, pelitic beds predominate.
(4) Gas and connate salt water are always present.
(5) The plastic beds are overlain by more competent deposits.
(6) Broad synclines are separated by sharp anticlines in which the deeper plastic
sediments push upward.
(7) Most eruptive centers consist of several volcanic cones.
(8) Shallow and steep-sided cones can be present together.
(9) Increasing stress mobilizes the plastic clay in the core with salt water, gas, and, in
many cases, oil. The resulting mud is pressed upward in a magma-like fashion and,
if the equilibrium of the surface is disturbed, it erupts and forms a mud volcano.
(10) The eruptions can be periodic, but commonly are irregular. Many large eruptions
have occurred after long periods of quiescence.
(11) Small and large rock fragments are commonly present with the mud, usually
originating from older formations.
(12) Diapiric zones with mud volcanoes generally coincide with areas of negative
gravity anomaly.
(13) The life of an individual eruptive center is usually short.
Yakubov et al. (1973) illustrated the magnitude of forces, sometimes associated with
erupting mud volcanoes. "After 13 years of no activity, the largest mud volcano of
Loktaban erupted in Azerbaijan for almost 6 hours. During the eruption, the volcano
discharged 125,000 m 3 of breccia in the shape of two huge tongues, each about 200 m
long and 60 m wide. During the eruption, gas flames ( 1000-1200~ reached 500 m into
the sky."

PREDICTION OF TECTONICALLY CAUSED OVERPRESSURES BY USING RESISTIVITY AND
DENSITY MEASUREMENTS OF ASSOCIATED SHALES
Predicting abnormally high formation pressures (AHFP) in carbonates caused by
tectonic activity is a very challenging problem. In the presence of thick shale sequences,
low porosities, high resistivities, and high bulk densities of shales are characteristic fea-
tures. The reason is simple: the greater the overcompaction (due to tectonic movement),
the greater is the amount of water squeezed from shales into the associated reservoirs,
which results in overpressuring. One such example is presented here.

ORIGIN AND DISTRIBUTION OF OVERPRESSURES IN CARBONATE RESERVOIRS

Many exploration wells drilled in the Pripyatskiy Deep, located in the Byelorussia,
have encountered overpressured formations (Fig. 8-8). Zavgorodniy and Pakhol'chuk
(1985) have investigated the nature and both lateral and vertical distribution of formation
pressure in this area.
The change with depth of electric resistivity in the Buregskiy shales, porosity
variations, and the ratio of Pres (measured reservoir pressure) to Pahyd (assumed
hydrostatic pressure) for the intersalt and subsalt carbonates in the northern Pripyatskiy