Geology Reference
In-Depth Information
A similar situation was observed on a number of structures on the
Southwestern Apsheron (Lokbatan well 221, Puta well 539, Kushkhana
well 297, Kergez well 123, Kyzyltepe well 70, etc.). There, intense mani-
festations on reaching and penetration of the Upper Miocene often forced
weighing of the drilling mud to 2.25-2.40 g/cm
3
(Ali-Zadeh, Putkarzadeh
and Salayev, 1968; Rachinsky, Vezirova and Fridman, 1962).
Typical of AHPP's in the Oligocene-Miocene sediments is their ubiqui-
tous presence. They are developed regionally over the local highs as well as
in-between structures. Besides, significant strengthening in the intensity
of their manifestations is recorded in the direction of regional dip and of
increased thickness of individual stratigraphic components. For instance,
at Ajiveli and Umbaki prospects the Diatomaceous formation's sediments
at depths 0-1,000 m have almost normal pressure. On the adjacent deeper
subsided structures Utalgi, Touragay and some others drilling through the
Oligocene-Miocene required the application of weighted mud (2.20-2.40
g/cm
3
). Significant gas shows and rock-formation associated with AHPP
did not allow penetrating the Oligocene-Miocene except in a few wells
(Utalgi-7, Utalgi-8, etc.) (Durmishyan, Muradian and Rachinsky, 1972,
1973; Rachinsky, Vezirova and Friedman, 1972). Somewhat better drill-
ing conditions and only modest AHPP manifestations were observed in
local zones with significant deposition gap between Miocene and Pliocene.
In these cases, it was usually possible to drill through significant thick-
ness of the Miocene in quiet conditions. Apparently, it was caused by their
partial paleo-relaxation during the denudation period (north flank of the
Karadagh prospect).
In Oligocene-Miocene reservoirs on strongly eroded structures at shal-
low depths (as mentioned above they are represented by thin and small
low-permeability sandstone and siltstone lenses) formation pressure most
commonly is hydrostatic or only slightly above the hydrostatic (Kobystan:
Utalgi-III prospect, Maikopian horizon IV; Ajiveli prospect, Maikopian;
Ragim prospect, Chokrackian) (see Figure 3.1). Wells there tested water
usually at no greater than 3-10 CMD with wellhead pressure in shut-in
wells of no greater than 2.5 MPa (Rachinsky, Vezirova and Friedman,
1972). In the North-Apsheron zone of highs at Shimali Absheron, in the
South-Apsheron trough at Djanub-2 (Yuzhnaya-2) and in the north of the
Baku Archipelago at Duvanny-More prospect, the depth to the Oligocene-
Miocene is 2-5 km. Formation pressure in the reservoir beds there is close
to the geostatic. This conclusion is based on the intensity of the manifesta-
tions and density of mud used for the penetration (about 1.95-2.20 g/cm
3
).
Such formation pressure in closed-elastic reservoir lenses within thick clay
sequence and pore pressure approaching the geostatic pressure indicates
