Geology Reference
In-Depth Information
he
Jurassic and Valanginian
intervals at the base of all sedimentary
sequence in the basin were tested in wells at the prospects of the south-
eastern plunge of the Caucasus Major Meganticlinorium (Keshchay,
Begimdagh-Tegchay and Gozel-Tepeh prospects in the Gogran trough). A
test of the Middle Jurassic (interval 1,870-1,877
m) at Keshchay in well 1
produced a flow of the formation water at a rate of 70-80 CMD
6
with small
amount of gas. The wellhead pressure (excess formation pressure) was 15
MPa (Agabekov, 1963; Altovsly, 1958; Akhmedov, Salayev and Ismailov,
1961; Rachinsky, 1973). This determines the absolute formation pres-
sure value as 32.79-33.74 MPa with
К
AHFP
7
of around 1.75-1.80. On both
structureх in the region the wells tested water from these intervals usually
at no greater than 80 CMD. Only the wells intersecting large faults tested
up to 20-25 MCMD
8
(Keshchay, well 2, Valanginian).
The latter case is symptomatic as this flow was obtained from a depth of
only 242 m in the emergency blowout. The wellhead pressure at shutdown
of these wells was 14.1-22.3 MPa, which gives formation pressure value
on the order of 33-47 MPa with
К
AHFP
= 1.75-1.90 (Altovsky, 1958). At the
Gozel-Tepeh prospect formation pressure in well 2 at 5,800 m (Jurassic)
was 130 MPa with the excess over the hydrostatic being 72 MPa and
К
AHFP
= 2.24 (Figure 3.1). Well drilling was possible there with a heavy mud (mud
weight 2.64 g/cm
3
(Akhundov, 1957; Fertl, 1972, 1976). It is noteworthy that
formation pressures in all these wells cannot be generated at the expense
of elevations of these complexes' surface exposure in the directly adjacent
mountains, which excludes the probability of infiltration hydrodynamic
regime in them (Rachinsky, 1973).
The Jurassic-Valanginian stratigraphic range is composed in the South
Caspian Basin by maximum consolidated clayey slates, argillites, compact
clays with lens-shaped members of low-permeability fractured sandstones
sandstones, siltstones, gravelites, marls and limestones
9
. hese rocks do not
6
Cubic meters per day.
7
Abnormality factor.
8
Thousand cubic meters per day.
9
It is important to remember that the Mesozoic rocks in all mentioned regions/
areas in their geologic evolution experienced numerous tectonic regime of fre-
quently changing signs. These regimes included immersions and inversions of the
section, depositional lacunas and erosions. They all determined the incompatibil-
ity of mineralogical and reservoir properties to their modern depths. The range of
vertical motions may have reached 3-6 km (Apresov, 1947). Thus, the appearance
of the penetrated Mesozoic rocks reflects their previous metamorphism at much
greater depths, temperature and pressure.