Geology Reference
In-Depth Information
was penetrated in a relatively small number of wells at some structures on
the southeastern plunge of the Caucasus Major meganticlinorium, in the
North-Apsheron zone of highs, western Apsheron-Balkhan Sill, the east-
ern plunge of the Balkhan zones of highs and Elbrus Trough.The complex
is almost totally composed of thick highly plastic clay sequence with rare
thin lens-shaped reservoir interbeds. This interval was rarely tested. The
reason was the lack in the penetrated Paleogene section of satisfactory res-
ervoirs and negative petrophysical data. The bulk of information about the
interval is based on observations while drilling, on field geophysical sur-
veys, well petrophysics and laboratory study of core samples and cuttings.
While drilling through the Paleogene clays several factors were impor-
tant for the pore pressure estimation in this interval. They include:
t
Swelling/blowout in boreholes.
t
Almost continuous sticking, “overpull”, pushing out of the
drill string by the rock spontaneous rise of drill tubing.
In some cases (Kobystan, Kelany prospect, well 1, Eocene), despite a
heavy mud (density 2.40-2.55 g/cm
3
), these manifestations have been so
strong that the pushed-up tubing twisted in spiral around the rig and the
rig had to be held in place using several tractors. About the same intensity
AHPP manifestations occurred while drilling wells for the Mesozoic at the
Fatmai and Kirmaku prospects (Central Apsheron) and at the Neftyanye
Kamni prospect (western Apsheronо-Balkhan Sill). That prevented wells
to be drilled to PTD (Durmishyan, Muradian and Rachinsky, 1973;
Rachinsky, Vezirova and Fridman, 1972).
Similar phenomena were widely recorded practically at all prospects in
the South Caspian Basin where these deposits have been penetrated. This
is an indication of a broad commonality in them of AHPP level close to the
geostatic (overburden) pressure. The following fact is demonstrative in this
connection. The roots of most mud volcanoes in the region (they extruded
to the surface huge amounts of plastic clay) are in the Paleogene deposits
(Atlas of Azerbaijan Mud Volcanoes, 1971).
he
Neogene
interval (
Oligocene and Miocene
section: the Maikopian
Formation, Chokrackian Horizon, Diatomaceous Formation) was pen-
etrated in a great number of wells at numerous prospects in Kobystan,
Apsheron Peninsula and in a number of offshore structures in the North-
Apsheron zone of highs, Apsheron and Baku Archipelagos, South-Apsheron
trough, Apsheron-Balkhan Sillа and Resht and Gogran (onshore) troughs.
At some prospects under appraisal located in the area of a significant depth
of this interval in the process of well drilling and initial production, intense
