Geology Reference
In-Depth Information
In the above-described process, the reservoirs play a role of natural
separators wherein the condensed hydrocarbon liquids drop-out of the
vapor-gas mixture due to natural pressure gradients. The larger the volume
of mixture passing through the “separator”, the greater the volume of accu-
mulated liquid.
Various types of hydrocarbon deposits differing in the oil, gas and con-
densate contents, are presented in Figure 6.2. If an unlimited amount is
passed through the connected reservoirs (“separators”), the lower traps
may be filled up with the liquid phase to the spill-point. If the traps in a
field are in communication but the field as a whole is somehow isolated,
the vapor-gas mixture, once having filled the lower traps, stops moving
upward. This explains the existence of gas (gas-condensate) accumulations
with or without small oil rims (Figure 6.2-d).
Between these two extreme cases (purely oil or purely gas/gas-conden-
sate accumulation) there are two hydrocarbon accumulations with vary-
ing contents of oil, condensate and gas. Depending on specific geological
conditions, a single oil field can simultaneously contain traps with gas, gas-
condensate, gas-condensate-oil, oil-gas-condensate, gas-oil, oil-gas, and oil
accumulations (Figures 6.2-c, 6.2-b).
The latter option is uncommon in the Azerbaijan fields because faults,
which are the major avenues of migration, penetrate the entire Productive
Series.
At pressures below 9 to 13 MPa, vapor-gas mixtures are within the nor-
mal evaporation- condensation range of non-stabilized liquid. At this pres-
sure, the accumulations consist mostly of gas with a limited content of light
oil distillate (Figure 6.2-c). This is exemplified by small gas accumulations
of the Upper PS (Gala, Garachukhur, Surakhany fields).
Thus, gas (gas-condensate) accumulations represent the reservoir accu-
mulations of vapor-gas mixture, whereas the oil (and oil-gas) accumula-
tions are the “separator” accumulations. An oil accumulation forms under
conditions where a gas (gas-condensate) accumulation cannot be pre-
served because it is a result of re-formation of its precursor gas-condensate
accumulation from gas mixture, of which liquid hydrocarbons drop-out
and accumulate according to the laws of retrograde condensation. Thus,
each oil accumulation in the region went through the “gas-condensate”
cycle many times.
Inasmuch as the faults are major avenues of migration for the vapor-
gas mixture, there is correlation between the type of accumulation and
the degree of tectonic activity. Almost all of the oil and oil-gas accumu-
lations in South Caspian Basin are associated with significantly faulted
