Classification of Hydrocarbon accumulations based on types of traps

Although the classifications based on the form, type and genesis of oil and gas accumulations are called the ‘‘hydrocarbon accumulation classifications’’ they are in effect the trap classifications. These trap classifications may be grouped into three major types and four subtypes.

Traps formed by folding
Accumulations formed as a result of folding are usually associated with the bedded reservoirs. The complexity of structure (sometimes even isometric), size, and especially height are caused by the trap and reservoir position in the sedimentary basin. Over the central areas of tectonic plates, the traps are gentle and sometimes very large. Over the plate margins, transition zones and, especially, collision zones the folds are higher, steeper and with a clearly expressed trend. The accumulations may be classified using some other parameters (mentioned earlier), too. In particular, oil–water contours in such accumulations are closed and, in plan view, have oval or more intricate shapes, and form rings.

Traps formed within various buildups
Accumulations formed within various buildups are usually associated with the massive-type reservoirs. Most common are accumulations in biogenic buildups (reefs and bioherms). Sometimes, biostromes are mistakenly attributed to the same class. Included here are large accumulations with huge flow rates due to the presence of fractures and vugs in carbonates. Some investigators also include in this group the erosional projections of the metamorphic and volcanic rocks (fault-bounded or bounded by erosional surfaces), which may contain accumulations, e.g., White Tiger Field in Vietnam. 

Traps that are limited by the depositionally imposed facies changes
Lithologic and stratigraphic traps include facies pinch-outs, stratigraphic unconformities, and contact of the reservoir with the impermeable rock upsection. Such traps may be associated with the bedded reservoirs on the monoclines or on the flanks of anticlines. These traps may contain rather large accumulations. They may be associated with bedded reservoirs confined on every side. In such a case, they form large accumulations. Water saturation contours impinge on the trap (impermeable barrier). This type of accumulations is very common: about 50% of all known accumulations. Accumulations of Types I, II, and III are formed in accordance with the gravitational (anticlinal) theory. By far, not all known accumulations, however, belong in the described three types or combinations thereof. Also, not all of them formed in accordance with the gravitational theory.

Dominance of capillary forces over the gravity force
Oil or gas found in hydrophilic rocks occupy coarser-grained reservoir rocks, which are sealed by water-saturated fine-grained reservoir rocks. Examples of such accumulations associated with the relatively coarse-grained sandstone lenses example: The 100-ft sandstone in Appalachian oil and gas province, USA. Although the appearance of capillary forces is frequently observed, the formation and preservation of the accumulations cannot be attributed to these forces. It should be kept in mind that water and gas lenses exist within oil accumulations; water is sometimes encountered up-dip in pinched-out reservoirs exmaple: Productive Series of the Absheron Peninsula in Azerbaijan, and Maykopian sandstones in the Northwest Caucasus. 

Dominance of hydraulic forces 
The hydraulic forces can cause a tilt in the oil–water interface. A barrier (facies change, stratigraphic unconformity, and fault) often turns out to be a barrier due to the presence of pressure difference across it, rather than because of the appearance of an impermeable barrier in the way of fluid movement. The necessary condition for the preservation of hydraulically trapped accumulation next to a fault is a higher potential head of the water next to the fault zone than that of the productive formation (the surplus pressure is included). This condition may exist if, for instance, there is a communication along the fault between the accumulation and the reservoir with overpressure. In monoclines, the accumulations can be preserved if the potential head decreases down-dip in locations where the dip increases or the dip of the piezometric surface decreases. The latter is possible when the reservoir–rock properties change (capillary forces enter into play). The oil–water contact can close onto themselves (but crossing the structural contour lines on the top of the reservoir) or can about the trapping barrier. Neither of the described types, however, owes its existence to the hydraulic forces exclusively.
They can exist only under condition of the combined interaction of several different forces:

• Hydraulic and gravity. 
• Hydraulic and capillary. 
• Hydraulic, capillary and gravity forces. 

The effect of hydraulic forces is commensurate with that of gravity and capillary forces.

Oil and Gas trapped in Syncline

Gas accumulations in synclines
Gas accumulations in synclines or in monoclines devoid of structural highs. There is a gas accumulation in the Deep Basin monocline in Alberta, Canada. The latter accumulation resides in the Mesozoic sandstone, which is more than 3km high (the thickness of individual gas intervals is 10–150 m). The sandstone is water-saturated up-dip the gas accumulation, with an improvement in petrophysical properties. The gas reserves here are nearly 11.3 TCM. The gas accumulation of Milk River Field (Canada), with 250 BCM of reserves, is another similar example. The gas accumulation of San Juan Field (USA) resides in the Mesozoic sandstone in the synclinal part of the structure, with reserves of 700 BCM. The sandstone is water-saturated over the flanks. The porosity and permeability within the gas-saturated portion are 14% and 1 mD, respectively, whereas in the water-saturated portion, f ¼ 25% and k ¼ 100 mD.
To explain this phenomenon, the following two explanations may be suggested:

1. A rapid gas generation is currently occurring in the Mesozoic sandstones of the Milk River and San Juan fields at a temperature of 85–921 C. This gas is entering the reservoir at a higher rate that it is being removed from the reservoir. 
2. The reservoir rocks are hydrophobic. 

Capillary forces move the gas into the reservoir with finer pores and keep it there. In the case of the first explanation above, if the gas-saturated reservoirs are intercommunicated and form a single accumulation, the surplus pressure in its upper portion should be around 30 MPa. If f ¼ 25% and k ¼ 100 mD, the effect of capillary forces would be negligible and insufficient for retaining the accumulation. Also, if the porosity and permeability values are low along the axis of syncline, a high rate of gas input would be doubtful. The two explanations above assume that the gas is retained due to the change in rock properties. In the first explanation the fast gas input from clays is assumed (and from a larger area than the gas escape area). The second explanation subscribes to the action of capillary forces in hydrophobic rocks. It is quite possible, however, that the fluid properties change together with the changes in the reservoir–rock properties. A prolific gas generation occurred (and may still be occurring) in the Mesozoic sequence. The gas dissolves in water as soon as it is formed. Thus, the gas-saturated water enters the reservoir. In this case, the surplus pressure within the upper portion of the accumulation would be tens of times lower than that for the gas accumulations. Up-dip, the pore size increases (as indicated by the higher porosity and permeability), the capillary pressure and temperature decline, gas solubility in water drastically decreases (depending on changes in the reservoir conditions), and gas is released as a free phase forming a gas-in-water emulsion. A substantial portion of the released gas energy is spent for the formation of emulsion. The emulsion has huge surface area and, correspondingly, huge surface energy. Thus, the gas accumulation is insulated by the gas-in-water emulsion, which is only slightly movable and is elastic at reservoir conditions.
Three major reasons have been proposed in the above three explanations to elucidate the formation and existence of the accumulations:

1. The difference in the speeds of immigration and emigration, 
2. The capillary forces (and wettability), and 
3. Changes in the fluid properties due to changes in the formation temperature and pressure. 

The existing exploration, testing and production techniques are attuned almost exclusively to the accumulations where the gravity forces predominate. Discoveries of Type VI accumulations in such conditions is highly accidental. Their exploration, testing and production techniques must be established after the known accumulations of this type are studied in detail.