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A petroleum reservoir or an oil and gas reservoir (or system), is a subsurface pool of hydrocarbons contained in porous rock formations. The naturally occurring hydrocarbons are trapped by overlying rock formations with lower permeability.
The crude oil found in oil reservoirs forms in the Earth's crust from the remains of living things. Crude oil is properly known as petroleum, and is a kind of fossil fuel. Scientific evidence indicates that millions of years of heat and pressure changed the remains of microscopic plant and animal remains into crude oil and natural gas.
Roy Nurmi, an interpretation adviser for Schlumberger described the process as follows: "Plankton and algae, proteins and the life that's floating in the sea, as it dies, falls to the bottom, and these organisms are going to be the source of our oil and gas. When they're buried with the accumulating sediment and reach an adequate temperature, something above 50 to 70 °C they start to cook. This transformation changes them into the liquid hydrocarbons that move and migrate, becoming an oil and gas reservoir."
In addition to the water environment mentioned, which is usually a sea but might also be a river, lake, coral reef or algal mat, the formation of an oil or gas reservoir also requires a sedimentary basin that passes through four steps: burial under miles of sand and mud, pressure cooking, hydrocarbon migration from the source to porous rock, and trapping by impermeable rock. Timing is also an important consideration; it is suggested that the Ohio River Valley could have had as much oil as the Middle East at one time, but that it escaped due to a lack of traps. The North Sea, on the other hand, endured millions of years of sea level changes that successfully resulted in the formation of more than 150 oilfields.
Although the process is generally the same, various environmental factors lead to the creation of a wide variety of reservoirs. Reservoirs exist anywhere from 1,000 to 30,000 ft (9,000 m) below the surface and are a variety of shapes, sizes and ages.
The traps required in the last step of the reservoir formation process have been classified by petroleum geologists into two types: structural and stratigraphic. A reservoir can be formed by one kind of trap or a combination of both.
Structural traps are formed by a deformation in the rock layer that contains the hydrocarbons. Domes, anticlines, and folds are common structures. Fault-related features also may be classified as structural traps if closure is present. Structural traps are the easiest to locate by surface and subsurface geological and geophysical studies. They are the most numerous among traps and have received a greater amount of attention in the search for oil than all other types of traps.
Stratigraphic traps are formed when other beds seal a reservoir bed or when the permeability changes ( facies change) within the reservoir bed itself.
An example of this kind of trap starts when salt is deposited by shallow seas. Later, a sinking seafloor deposits organic-rich shale over the salt, which is in turn covered with sandstone. As the Earth's pressure pushes the salt up, the shale is "cooked", producing oil that seeps up into the sandstone above. In some places, the salt breaks through the shale and sandstone layers into a salt dome that effectively traps the hydrocarbons beneath it.
After the discovery of a reservoir, a programme of appraisal will seek to build a better picture of the accumulation. In the simple text book example of a uniform reservoir, the first stage is to use seismic to determine the possible scope of the trap. Appraisal wells can be used to determine the location of oil-water contact and with it, the height of the oil bearing sands. Coupled with the seismic data, it is possible to estimate the volume of oil bearing reservoir.
The next step is to use information from appraisal wells to estimate the porosity of the rock. This is usually between 20-35% (the percentage of the total volume that contains fluids rather than solid rock). This can give a picture of the actual capacity. Through core samples, the characteristics of the reservoir fluids can be determined, particularly the shrinkage factor of the oil (how much the oil will shrink as a result of being brought from the high pressure, high temperature environment of the reservoir to stock tank conditions at the surface.
With this knowledge, it is then possible to estimate how many stock tank barrels of oil are located in the reservoir. This is called the Stock Tank Oil Initially In Place (STOIIP). As a result of studying things such as the permeability of the rock (how easily fluids can flow through the rock) and possible drive mechanisms, it is possible to then estimate the recovery factor (what proportion of the oil in place can be reasonably be expected to be produced). This is normally between 30-35%. This finally gives a value for the recoverable reserves.
The difficulty in practice is that reservoirs are not uniform masses. They will have a variable porosities and permeabilities throughout and may be compartmentalised, with fractures and faults breaking it up and complicating fluid flow. As such require a lot of effort and instinct to produce even an approximate picture of the reservoir properties for estimating reserves.
To obtain the contents of the oil reservoir, it is usually necessary to drill into the Earth's crust, although surface oil seeps exist in some parts of the world, such as the La Brea tar pits in California and Trinidad.
A good virgin reservoir will be under sufficient pressure to initially push hydrocarbons to surface. However, as the fluids are produced, in a static situation, the pressure will fall off and production will quickly falter with it. However, the picture is not static and often the reservoir will respond to depletion in a way that will help to maintain the pressure for a short time. Failing this, artificial drive methods may be necessary.
Solution gas drive
This mechanism (also known as depletion drive) depends on the associated gas of the oil. The virgin reservoir may be entirely liquid, but will be expected to have gaseous hydrocarbons in solution due to the pressure. As the reservoir depletes, the pressure falls below the bubble point and the gas comes out of solution to form a gas cap at the top. This gas cap pushes down on the liquid helping to maintain pressure.
Gas cap drive
In reservoirs already having a gas cap (the virgin pressure is already below bubble point), the gas cap expands with the depletion of the reservoir, pushing down on the liquid sections applying extra pressure.
Aquifer (water) drive
Below the hydrocarbons may be a ground water aquifer. Water, as with all liquids, is compressible to a small degree. As the hydrocarbons are depleted, the reduction in pressure in the reservoir causes the water to expand slightly. Although this expansion is minute, if the aquifer is large enough, this will translate into a large increase in volume, which will push up on the hydrocarbons, maintaining pressure.
Water and gas injection
If the natural drives are insufficient, as they very often are, then the pressure can be artificially maintained by injecting water into the aquifer or gas into the gas cap.
- Active areas of onshore US oil reservoirs
- North Dakota
- New Mexico
- Active areas of existing sub-sea (offshore) oil reservoirs