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180 CLASSIFICATIONS OF OIL AND GAS ACCUMULATIONS

Fig. 10.1. Classification of hydrocarbon accumulations based on phase relationships. I – VII: Accumu-
lations in (a) mild and (b) harsh (very high temperatures and pressures) conditions. I – oil; 1a – oil and
solution gas; 2 – gas (dry); 2a – gas and condensate; 3 – gas – oil transitional zone; 4– oil accumulation
without distinct OWC; 4a – oil – water transitional zone; 5 – gas – water transitional zone; 6 – gas – oil –
water transitional. (After Eremenko and Chilingar, 1996, p.112.)
If subsidence is replaced by uplift, the reversal of these processes is practically
impossible. Most of the processes in the Earth’s crust are irreversible, e.g., impossible
for a well-compacted fractured reservoir rock to become a sediment if the basin
experiences an uplift. Different processes occur during the uplift, which result in the
destruction of accumulations. Groups of deposits shown in Fig. 10.1 are also pre-
sented in Table 10.3.
Pressure maintenance in the development (production) of accumulations in the
case of groups Ia–IVa is not to prevent the possible dropout of condensate. It may be
undertaken to accelerate the production schedule and to improve the fluid recovery.
Here, additional difficulties arise due to hydrophilic nature of rocks. Pressure main-
tenance in the groups Va and VIa does help prevent the condensate drop-out.
Pressure maintenance in the case of groups Ib–IIIb is necessary to improve the
condensate recovery. It is possible that some pressure decline in the accumulations
belonging to type IVb and, perhaps, (IIIb–VIIb) will not result in the loss of con-
densate. Inasmuch as the reservoir pressure here is usually much higher than critical,
the pressure may decline almost to a critical value. This can be determined by
laboratory analyses of the samples at reservoir conditions. In order to prevent the
breaking up of oil–gas–water mixtures (solutions) near the borehole zone (group IVb
or VIIb) not only the pressure but also the temperature must be retained.
The qualitative composition of the oil in accumulations may be described from a
prevailing hydrocarbon type (see Chapter 5). The gas condensates can be subdivided