The natural subsurface reservoir is a container of oil, gas, and water where they can move, and its shape is determined by the relationship between the reservoir rock and its surrounding poorly permeable rocks. The subsurface reservoir as only that part of the reservoir rocks where the oil and natural gas can form an accumulation. The natural reservoir remains as such regardless of the type of the contained fluids, or even if it is dry. A comparison can be made with the overpressure. The presence of fluids is necessary for the overpressure. No fluids of different densities, no surplus pressure. As for the reservoirs, they possess two important attributes: spatial limitation (which determines the volume and boundaries of the reservoir), and an internal structure that defines the type and nature of inter-reservoir migration. Indeed, these properties should be included in any definition of the reservoir as well as in the classifications being developed. Some attempts have been made to classify the oil and gas reservoirs on the basis of their relative size (local, zonal, basin-wide, regional, etc.) or their absolute size.
Three types of reservoir limitations can be identified: reservoir roof lateral, and reservoir base. The reservoir roof (top) may cover the reservoir
- In normal stratigraphic succession.
- With some depositional hiatus.
- May change its age along the strike.
It is improbable but not impossible that a reservoir may be capped by the surface of an impermeable over-thrust. It should be kept in mind the selectivity toward different fluids by caprock and a possibility of its transformation into a reservoir rock during epigenesis. Consequently, it is always important to indicate the exact nature of the fluid. The transformation of the caprock into a reservoir rock results in either disappearance of the reservoir or its conversion into a new reservoir (if there is another caprock above). Lateral limitations are caused by lithologic alterations (including cementation) and the permeability. Small accumulations may be laterally limited by faults. This is possible, but not typical, for the larger reservoirs, because the fault zones in their evolution can become migration paths for various fluids (oil, water, or gas). Fault zones are actually "communication windows" with the other reservoirs. The importance of the presence of the base (bottom) as a necessary reservoir component was not always clearly recognized, because it was believed that the accumulations were formed exclusively by the buoyancy (Archimedes forces). One must always keep in mind that the reservoir is an inseparable part of the hydrodynamic system. This system may be open or with a restricted communication to the surface (artesian), or of "elision" type (with an inverse pattern of hydrostatic pressure). It is not possible for such energy system to exist without a base (bottom). Reservoir rock is a rock capable of containing oil and gas and yielding them during production. The reservoir rock is characterized by: rock type; permeability type (intergranular, fracture, and/or combination of the two); the total, intercommunicating, and effective porosity; specific surface area; wettability of rock (oil-wet versus water-wet); fracture type (width, etc.); and fracture distribution. Reservoir is a natural subsurface container for oil, gas, and water. Its existence is predicated on the relationships between the reservoir rock and associated poorly permeable rocks. Reservoir is characterized by reservoir-rock type, relationship with impermeable rocks, reservoir capacity, its hydrodynamic conditions, reservoir energy, and structure. Trap is part of the subsurface reservoir where an oil or gas accumulation can form and be preserved. Its parameters include the reservoir type, reservoir-rock type, conditions of its formation, structure, and capacity. In a special case where the reservoir is lithologically limited from all directions, its parameters may coincide with those of the trap (the entire reservoir is represented by a single trap).
The following features are used in describing a reservoir:
- Type of the reservoir rock comprising the reservoir.
- Relationship between the reservoir and the surrounding impermeable rocks.
- Reservoir capacity.
- Depositional environment.
In terms of the relationship between the reservoir and its surrounding impermeable rocks, there are three major types of the reservoirs: bedded, massive, and lithologically limited in all directions. Bedded reservoir is a reservoir that is restricted at its top and base by low permeable rocks. The reservoir rock thickness in such a reservoir is more or less or at the edge of the reservoir development, which may result in a pinch-out of the reservoir rock. The reservoir rocks in bedded reservoirs are usually lithologically continuous, but may have a more complex nature. A bedded reservoir may have a single hydrodynamic system. Reservoir energy in bedded reservoirs is distributed in accordance with the hydrostatic or hydrodynamic environment of the artesian basins. However, reservoirs with that kind of energy distribution are typical only for the uppermost portion of the sediment cover. As a result of subsidence and sediment compaction and various secondary geochemical processes, reservoirs may be separated into diverse portions as a consequence of previously described phenomena. Leaving aside changes in the sediment composition, drastic changes occur in the major reservoir-rock properties (porosity and permeability). Even if prior to subsidence the reservoir rocks were reasonably uniform in terms of porosity and permeability, subsequent to subsidence non-uniformities appear between various portions of reservoir so that they may turn out to be totally separated from one another. An indication of such a change may be a change in a hydrodynamic drive from artesian to "elision" type and the appearance of abnormally high pressure. The beginning of the process involves (1) lateral fluid migration, and (2) gradual change in the reservoir energy. Potential energy of the accumulations relative to the total energy of the reservoir is small. As the sheet-type reservoir differentiates, lateral migration becomes increasingly more obstructed, with formation of numerous hydraulic fractures. Fluid migration from the reservoir to other favorable zones (if they are available) may become prevalent. An increase in the elastic potential energy is observed (Abnormally-High Formation Pressure, AHFP). Energy distribution becomes discrete. The difference in potential energy between the accumulations and the reservoir as a whole becomes smaller, and within some zones (blocks) they become identical. Thus, it is reasonable to recognize a separate type, i.e., differentiated sheet-type reservoir, which under certain circumstances becomes a bedded reservoir. Massive reservoir is a thick permeable sequence overlain at the top and restricted from the sides by low-permeable rocks. Its bottom may be at a depth that has not yet been penetrated by wells (Tengiz Field, Kazakhstan). Reservoir rocks comprising massive reservoirs may be homogeneous or heterogeneous. Homogeneous massive reservoir rocks may be carbonates and metamorphic or volcanic rocks. Their porosity and permeability is due to the presence of vugs and fractures. Porous and permeable zones in massive reservoir rocks are not stratigraphically related. Isolated high-porosity and high-permeability zones cutting through stratigraphic surfaces within a body of a massif are common. Buried reefs are often assigned to this reservoir type. Among the best examples are the Ishimbay group of fields in Bashkortostan, Russia, and Rainbow Oilfield in Alberta, Canada. Usually, the thickness (height) of massive reservoirs is greater than the width. The length of possible vertical fluid migration is similar or greater than the lateral migration within the beds. The flanks of the reservoir and its contacts with the contemporaneous sediments are steep (thus, the biostromes should be classified as the sheet-type (bedded) rather than the massive-type reservoirs). In as much as the bioherms are very similar to reef buildups, they should be considered as massive reservoirs. To form a trapping mechanism, beside the caprock, the massive reservoirs require isolating steep lateral limitations. As an example, numerous present-day coral reefs in the Indian Ocean do not form subsurface reservoirs not only because of the absence of caprock, but also due to the absence of lateral barriers (lateral isolation). Fresh water accumulating within such bodies floats on the surface of heavier seawater. The hydrodynamic system of the massive reservoirs is poorly studied. It is possible that they communicate at depth with the bedded (sheet-type) reservoirs and are, in effect, just a veriety of a sheet-type reservoir. Reservoirs lithologically limited from all directions include all types of reservoirs where the liquid or gaseous hydrocarbons present from the time of formation of the reservoir are surrounded from all directions with practically impermeable rocks. Fluid movement within such reservoir is limited by its size. There is some superficial similarity between the massive reservoirs and the differentiated sheet-type reservoirs. The similarity is in the limitation (lithologic isolation). The difference is in the timing of the emergence of the latter. The massive reservoir is a result of depositional processes, whereas the differentiated sheet-type reservoir is a result of stresses during the basin subsidence. The former type is originally small (certainly, any bed is a large lens, but this approach is not used here). The latter type is a separate portion of a previous, possibly large hydrodynamic system. Prevalent elastic energy is typical for both, but the latter type of reservoirs has a greater stress level. The capacity of any type of reservoir is defined by its size and reservoir-rock properties. Energy of a reservoir is associated with its capacity, and the energy is what is important for the extraction of oil and gas (and associated water). The identification of the above four reservoir types is tentative, because such well rooted concepts as reservoir rock and caprock (fluid barrier) are also tentative. Even in the same state, the same rock may be a fluid barrier to one fluid and a reservoir rock to the other, depending on the physiochemical properties of fluids and rocks (especially, wettability), and on the subsurface temperature and pressure. For instance, a prominent projection of a massive reservoir may be just a complication of a regional sheet-type reservoir and be a part of the same hydrodynamic system. This phenomenon is especially common in carbonate sequences. It is possible to imagine a conversion of a sheet-type differentiated (block) reservoir into a massive reservoir if the caprock loses its sealing properties over a fault or flexure. The hydrocarbon accumulation may then occur underneath another, shallower caprock.
