Page 117 - Origin and Prediction of Abnormal Formation Pressures

Chapter 4

SMECTITE-ILLITE TRANSFORMATIONS DURING DIAGENESIS AND
CATAGENESIS AS RELATED TO OVERPRESSURES

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

INTRODUCTION

In general, the properties of argillaceous rocks and the fluids contained in them
are important indicators of future trends in the processes of postsedimentary transfor-
mations. The clay minerals, which compose argillaceous rocks, are sensitive to the
formation pressure and temperature (thermobaric factors). The interstitial fluids (wa-
ter, oil and gas) in shales also influence the degree and character of diagenetic 1 and
catagenetic 2 transformations of clays.
Of special interest are the young sedimentary basins, which are characterized by
the presence of thick, rapidly accumulated sand/shale sequences. A vivid example
is the South Caspian Basin (Buryakovsky et al., 2001), which is distinguished by a
diverse and rather unique association of parameters: (1) an exceptionally high rate
of sediment accumulation (up to 1.3 km m -1 yr-1); (2) a very thick (up to 25 km)
accumulation of sediments with those of Quaternary-Pliocene age accounting for up to
10 km (sand-silt-shale); (3) abnormally high pore pressure in shales [average factor
of abnormality ranges up to 1.8 (abnormality factor Ka = Pa/Pn, where Pa is the
abnormally high pressure and Pn is the normal hydrostatic pressure)]; (4) low heat
flow and low formation temperature (at depths in the order of 6 km the temperature
is approximately 105~176 (5) an inverted character of the hydrochemical profile
(the chemistry of water changes with depth from calcium chloride and magnesium
chloride to sodium bicarbonate type, i.e., freshening of water with depth); and (6) wide
development of mud volcanism. Argillaceous rocks make up 50-95% of the section
and play a key role in determining the mineralogical, lithologic, geochemical, and
thermobaric characteristics of the basin (Buryakovsky, 1974, 1993a,b,c,d).
Cenozoic shales are widespread in the Azerbaijan and South Caspian Basin. Pa-
leogene and Neogene argillaceous rock cores were recovered from deep onshore and
offshore wells, exploratory and hydrocarbon producing wells, as well as the well-
logging and field development data, in order to appraise the origin and history of
abnormally high formation pressure in this area.

1Diagenesis includes all physical, chemical and biochemical processes, which occur in sediments after
sedimentation and through lithification at near-surface temperature and pressure.
2 Catagenesis comprises all physical and chemical processes which occur in sedimentary rocks at high
temperatures and pressures after lithification and up to metamorphism.