Geology Reference
In-Depth Information
supported by this remarkable fact: In the Apsheron oil and gas area as a
whole, over 90% of the hydrocarbon reserves are in contact with alkaline
waters or with hard formation mixes with high foreign alkaline component
(so called “transitional” waters) (Rachinsky, 1970).
The noted circumstance is supported by the data in Table 10.1. It includes
information for the Apsheron Peninsula fields concerning the spatial dis-
tribution of oil reserves in the sections of local structures compared with
the extent of their saturation by alkaline waters.
Table 10.1 shows that major hydrocarbon amount of individual fields
is concentrated in the intervals with hydrocarbonate-sodium waters
2
and
associated with the 500-meter basal Productive Sequence interval, which
in most areas accounts for less than a half of the oil-saturated range. In
some cases, the reserves fraction in this interval is relatively lowered
(Balakhany-Sabunchi-Ramany, Surakhany, Bibieybat), i.e., the reserves
are more uniformly distributed in the section. In such cases, the transition
boundary between hard and alkaline waters in these fields usually occupies
the highest stratigraphic and physical position. Most of the section is taken
by alkaline waters, thereby reflecting, as in all other cases, the consistent
connection of the oil-saturation
with the ground water chemistry.
Of note is the trend toward the increase in thickness of the interval occu-
pied by the alkaline waters (percent of the vertical oil-saturation range)
and correspondingly of the oil reserves in it down the regional dip. This is
observed, in particular, in the anticlinal zones of Balakhany-Peschany Isl.
and Buzovny-Zyrya. In the fields Surakhany, Karachukhur and Kala, sec-
tion intervals saturated by hard formation mixes contain concentrated sig-
nificant hydrocarbon reserves. We interpret this association to be a result
of strong vertical cross-flows in these fields of mostly liquid oil caused by
the presence of a broadly developed fault system. The structures positioned
down the regional dip are more closed and less faulted. The scale of vertical
2
The statement of the association of 90% hydrocarbon reserves in the Productive
sequence with alkaline waters or formation water mixes with high enough alka-
line component does not contradict Table 7.1. Reviewing this Table, it needs to be
taken into account that the transition boundary from hard to alkaline waters is
not a genuine boundary with only one type-water of clearly-defined origin (syn-
depositional with the Productive sequence or alkaline invaded from below) on
either side of it. Strictly speaking, all waters in the Productive Sequence when it is
oil-saturated are mixes in varying proportions of syngenetic and foreign agents. In
this sense, the transition boundary is a zone above which the presence of alkaline
waters for purely chemical reasons does not result in the change in hydrochemical
type of formation mixes in these intervals of the section.
