Geology Reference
In-Depth Information
it would have certainly appeared in the Akchagyl and Apsheron intervals
(Shuvalov, 1977).
Syngenetic AHFP intensity distribution in the Middle Pliocene sequence
clearly correlated with the depth to its surface. Most illustrative in this respect
is the Alyat Ridge (Kobystan). In its south portion (Koturdagh and Dashgil
prospects) manifestations of abnormal pressure in the process of drilling
reach their culmination and cause, despite the use of 2.00-2.35 g/cm
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drill-
ing fluids, significant gas- and water-shows, swelling, blowouts, etc. In the
north, however (Solakhay, Donguzdyk and Ayrantekyan prospects), the PT
surface is significantly shallower, its thickness is noticeably reduced and the
AHFP manifestations are much more moderate. Drilling there was less dif-
ficult and was conducted with 1.60-1.80 g/cm
3
mud.
Applying the reviewed syngenetic AHFP generation and preservation
mechanisms, it may be stated that the identified zoning of their manifes-
tation scale over the area is a result of more favorable abnormal pressure
discharge conditions in the areas where AHFP intervals are close to the
surface. Within the Lower Kura Depression, Baku Archipelago, Kobystan
and Gograndagh-Chikishlyar zones, syngenetic AHFP are recorded in the
entire PT-KT section. At the same time on the Apsheron Peninsula and
immediately adjacent South Apsheron shelf their manifestation interval is
only in its contact zone with the underlying clay sediments of the Ponthian
Stage
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(upper portion of the Upper Miocene) and of the Miocene-
Oligocene (Durmishyan, Muradian and Rachinsky, 1972, 1973).
In the east of the Apsheron Peninsula said interval corresponds with
the Lower portion of the Kala Fm. (Its basal conglomerates overlie the
Ponthian with angular and azimuthal unconformity.) In the southwest,
it corresponds with the lower portion of the Kirmaki formation, and on
the Central Apsheron, with Podkirmaki Formation. This basal interval is
mostly composed of uncompacted clay with small thin reservoir lenses
(except for the Podkirmaki formation). Its distinctive feature is intense
syngenetic AHPP, which are ubiquitous.
In a number of cases reservoirs in the entire section (including the Kala
Fm. closest to the PT base) pressure matching the hydrostatic is recorded.
But in the contact zone are observed intense manifestations of the abnor-
mal pressure sometimes reaching the lithostatic value. Examples are in
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Wherever in the South Caspian Basin clay Ponthian Stage sediments (Upper
Miocene) are deeper than 1.000 m their penetration is always difficult, with strong
jamming and blowout of drillstring, etc. Azerbaijan geologists and drillers call these
plastic sediments “swelling clays” and do not penetrate them unless urgently needed.
