Geology Reference
In-Depth Information
in the Tersk-Caspian Trough within the local structures: average for all
waterhead complexes excess of the formation pressure over the hydrostatic
is 22.8 MPa with the abnormality factor of 1.57.
3.2.2.4
he Zagros Foredeep
This is a region where intense AHPP are encountered in the process of
drilling through regional seals, i.e., the evaporite-marl-clay Fars Series
(Middle-Upper Miocene), gypsum-anhydrite Hith Formation (Upper
Jurassic, Titonian) and AHFP in the underlying fractured limestone pro-
ductive reservoirs of the Asmari Formation (Upper Oligocene-Lower
Miocene), Bangestan Series (Upper Cretaceous, Cenomanian-Turonian)
and Arab (Upper Jurassic, Kimmeridgian-Oxfordian).
The environment of AHPP manifestation includes:
t
Evaporite seals.
t
Regional omnipresence.
t
On a number of occasions the intensity of the super-static
level requiring constant application of weighted drilling
mud (sometimes 2.57-2.64 and even 2.71 g/cm
3
(Agha-
Djari field, Lower Fars Formation) (Lane, 1949).
t
Type of complications in drilling (Anikeyev, 1964).
t
Nappe contact of the plastic Fars Sries with the underlying
competent rocks.
These and other features determine mostly syngenetic nature of AHFP
predominantly caused by the drastic outflow restriction of abnormally
paleo-compressed pore fluid from the salt-clay sequence and by its pres-
ent-day compression through vertically and horizontally applied neotec-
tonic stress.
At the same time, maximum AHPP values are recorded directly over
the underlying hydrocarbon accumulations. This indicates a local (“halo”
model, Anikeyev, 1964) “burst” of the pore pressure abnormality (it occurs
through invasion of high-pressure fluids from the underlying accumula-
tions of a great height, up to 2,000 m or even greater). It causes significant
excess formation pressure in the crestal zones and gives reason to believe
that within productive local highs syngenetic AHPP is superposed by the
epigenetic component.
Combined effect of both of these mechanisms is generation in the seals
of AHPP with high level pore pressure abnormality, especially on high-
amplitude productive structures.
