Geology Reference
In-Depth Information
The second one is that AHFP was caused by subsequent paleo- and
neotectonic stress and motions of individual fault-blocks and basement.
Additional mechanisms may have been a continuous effect of the second-
ary cementation of pore space in the reservoirs by mineral new-forma-
tions, of the aquathermal and thermoelastic effects.
A number of facts based on drilling conditions in this interval support
this model. These facts are:
t
Shows in the process of drilling confirming a wide presence
of abnormality on the individual structures and in the areas
(including inter-structural zones).
t
High drilling rate with relatively small hook load.
t
Frequent loss of the drilling fluid.
t
Rapid circulation loss with quite heavy mud.
t
Gas cutting in drilling fluid.
t
Intense water-gas shows.
t
Dilution and pulverization of cuttings.
t
Absence, with rare exceptions, of swelling/blowout in
boreholes.
t
Drillpipe sticking.
The viability of such abnormal pressure formation model in the Jurassic-
Valanginian section is substantiated by the following:
t
Prevalent presence in the section of consolidated
hydro-micaceous-montmorillonite clays already devoid
at normal compaction of the capacity to release any sub-
stantial volumes of pore water and smectite dehydration
products.
t
Together with temperature growth, continuous increase
down the stratigraphic section of hydro-micaceous associa-
tions in the Mesozoic clay mineralogical composition. They
gradually replace the montmorillonite varieties and domi-
nate clay intervals in the Jurassic-Valanginian rock volume
(Abramovich, 1916. Agabekov, 1963).
t
Sufficiently high temperature in the Jurassic-Valanginian
sediments so the montmorillonite dehydration may be com-
pleted with the release of significant amounts of dehydration
water.
