Geology Reference
In-Depth Information
highs, often severely eroded, complicated by mud volcanism and diapir-
ism. On the other hand, most of the gas-condensate, gas-condensate-oil,
oil-gas-condensate and gas-oil fields are associated with buried structures.
Thus, the tectonic criteria constitute a controlling factor in the exploration
success.
The release of liquid hydrocarbons from the vapor-gas mixture is not
uniform and is most intense in the area of relatively high pressures. The
amount of liquid released per 0.1 MPa pressure drop continuously decreases
with decreasing pressure reaching zero at maximum condensation pres-
sure (Velikovskiy and Yushkin, 1959). Most of the liquids drop-out in the
first bottles (Figure 6.2-a). Similarly, the main reserves of “retrograde” oil
in multilayered fields must be associated with the high-pressure intervals,
i.e., the deepest productive horizons. This is supported by actual field data
(Gadzhi-Kasumov, 1969) indicating that the maximum oil reserves in PS/
RBS are associated with heaviest oils.
Numerous studies in the South-Caspian Basin show that with increas-
ing depth along the regional trends (from the flanks toward the basin's cen-
ter), there is increasing gas saturation in PS/RBS. The oil accumulations are
gradually replaced by oil-gas and, farther down, by oil-gas-condensate and
gas-condensate accumulations. In the same direction, the stratigraphic top
of upper productivity is lowered, and the number of hydrocarbon-satu-
rated intervals decreases.
For instance, in the oilfields of the Apsheron Peninsula's central anticli-
nal zone of Balakhany-Shah Deniz (Balakhany-Sabunchi and Surakhany
fields), oil saturates the entire section of the Middle Pliocene Productive
Series and the Apsheronian Stage (Upper Pliocene). Going down the
regional dip (the oil-gas and gas-oil structures Garachukhur, Zykh, Gum
Adasy, the gas-condensate-oil Bakhar field and gas-condensate Shah Deniz
field), the upper limit of productivity is lowered to the Lower Balakhany
Formation (lower part of Upper PS). A similar situation occurs in the
Gala-Zyrya area where the first structure is saturated with oil through the
Apsheronian Stage and the second structure contains gas, condensate and
some oil from the Nadkirmakinskiy sand (NKP) of the lower PS.
A similar situation is observed in the eastern part of the basin. The
entire RBS Section and partly the Apsheronian Section are oil-saturated
in the most uplifted and eroded Cheleken structure. The next structures,
600-800 m down the regional dip (Pri-Cheleken Dome, and Zhdanov and
Gubkin banks) contain mostly gas-oil accumulations with small amounts
of condensate in gas caps. Lower yet (by 1,400 m), the traps in the struc-
tures of LAM and Barinov banks are dominated by the gas-condensate.
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