Page 143 - Origin and Prediction of Abnormal Formation Pressures

120 L.A. BURYAKOVSKY, R.D. DJEVANSHIR, G.V. CHILINGAR, H.H. RIEKE III AND J.O. ROBERTSON, JR.

SUMMARY

For the South Caspian Basin the findings of Buryakovsky et al. (1995) can be
summarized as follows.
(1) Regionally developed abnormally high formation pressures were encountered
onshore of Azerbaijan and offshore of the South Caspian Basin. The pore pressure in
the argillaceous rocks is higher than that in the reservoir rocks.
(2) Paleogene to Neogene shales and argillaceous rocks, widespread in the geo-
logic section of Azerbaijan and the South Caspian Basin, consist of montmorillonite
(smectites), hydromica (illite) and mixed-layered minerals. Onshore, the Oligocene to
Miocene argillaceous rocks (shales, mudstones, etc.) are higher in volcanic ash content
owing to their proximity to the Lesser Caucasus, than the Pliocene argillaceous rocks
formed in the South Caspian Basin. The most characteristic feature of Tertiary argilla-
ceous rocks in Azerbaijan and the South Caspian Basin is their undercompaction and
the presence of pores of various sizes, measured by SEM. Their open porosity (effective
porosity as used in USA) ranges from 3 to 20%.
(3) The incomplete compaction of such argillaceous rocks, even at depths down to
6.5 km, is explained by the comparatively young age, a high sedimentation rate (up
to 1 km per one million years), their great thickness, and incomplete squeezing-out of
pore water. Such argillaceous rocks have high pore pressures, often higher by a factor
of 1.5 (and more) than hydrostatic pressure, and constitute good seals for oil and gas
accumulations.
(4) The montmorillonite content of the Baku Archipelago shales is constant down to
depths of 6.5 km because the formation of secondary montmorillonite from hydromicas
predominates over the transformation of primary montmorillonite. An increase in
temperature causes the dehydration of montmorillonite to hydromica, but abnormally
high pore pressures in shales hinder the dehydration and favor the transformation of
hydromicas to secondary montmorillonite, which produces heat. This is particularly
characteristic of young basins with rapidly accumulated thick series of argillaceous
sediments.
(5) A formula was proposed for the limiting depth at which montmorillonite can
occur for any specific thermobaric conditions and, particularly, when the actual pore
pressure differs from the normal hydrostatic one. In the stratigraphic section of the
South Caspian Basin, the predicted limiting depth ranges from 15 to 17 km.
(6) The sealing properties of argillaceous rocks at depths greater than 6.5 km
probably persist, because of the presence of large amounts of montmorillonite. If
accompanied by (1) good reservoir rock properties, (2) abnormally high pore pressures
in shales and sandstones, and (3) relatively low formation temperatures (which allow
hydrocarbons to persist), the writers suggest that the South Caspian Basin may contain
commercial oil and gas accumulations at depths of 9 km and deeper.
(7) Development of abnormally high pore pressures may lead to lateral rock density
variation and, under certain geologic conditions, to folding, clay diapirism, mud
volcanism, and earthquakes.