A petroleum reservoir or an oil and gas reservoir (or system), is a subsurface pool of hydrocarbons contained in porous rock formations. Petroleum ( L petroleum, from Greek πετρέλαιον, lit Natural gas is a Gaseous Fossil fuel consisting primarily of Methane but including significant quantities of Ethane, Propane, In Organic chemistry, a hydrocarbon is an Organic compound consisting entirely of Hydrogen and Carbon. Porosity is a measure of the void spaces in a material and is measured as a fraction between 0–1 or as a Percentage between 0–100% This is a List of rock formations meaning isolated scenic or spectacular surface rock outcrops The naturally occurring hydrocarbons are trapped by overlying rock formations with lower permeability. Permeability in the Earth sciences (commonly symbolized as κ, or k) is a measure of the ability of a material (typically a rock or unconsolidated
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Formation
The crude oil found in oil reservoirs forms in the Earth's crust from the remains of living things. Petroleum ( L petroleum, from Greek πετρέλαιον, lit In Geology, a crust is the outermost solid shell of a planet or moon 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. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Microscopic is a term used to describe objects smaller than those that can easily be seen by the naked Eye and which require a lens or Microscope to see Plants are living Organisms belonging to the kingdom Plantae. Natural gas is a Gaseous Fossil fuel consisting primarily of Methane but including significant quantities of Ethane, Propane,
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. Schlumberger Limited is the world's largest Oilfield services corporation operating in approximately 80 countries with about 84000 people of 140 nationalities 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. "[1]
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. Water is a common Chemical substance that is essential for the survival of all known forms of Life. "Riverine" redirects here For the use of that term in Maritime geography, see there A lake (from Latin lacus) is a Terrain feature (or Physical feature) a body of Liquid on the surface of a world that is localized to the Coral reefs are Aragonite structures produced by living organisms found in marine waters with little to no nutrients in the water An algal mat is a layer of usually filamentous Algae on marine or Fresh water soft bottoms The term sedimentary basin is used to refer to any geographical feature exhibiting Subsidence and consequent infilling by sedimentation 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 Ohio River is the largest Tributary by volume of the Mississippi River. The Middle East is a Subcontinent with no clear boundaries often used as a synonym to Near East, in opposition to Far East. [2] 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. An oil field is a region with an abundance of Oil wells extracting Petroleum (crude oil from below ground [3]
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. [4]
Traps
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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 geologist is a contributor to the Science of Geology, studying the physical structure and processes of the Earth and planets of the solar system A reservoir can be formed by one kind of trap or a combination of both.
Structural traps
Structural traps are formed by a deformation in the rock layer that contains the hydrocarbons. Structural geology is the study of the three dimensional distribution of rock bodies and their planar or folded surfaces and their internal fabrics Domes, anticlines, and folds are common structures. In Geology, a dome is a deformational feature consisting of symmetrically-dipping Anticlines their general outline on a geologic Map is circular or oval In Structural geology, an anticline is a fold that is convex up and has its oldest beds at its core See also Folding The term fold is used in Geology when one or a stack of originally flat and planar surfaces such as Sedimentary 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
Stratigraphic traps are formed when other beds seal a reservoir bed or when the permeability changes (facies change) within the reservoir bed itself. Stratigraphy, a branch of Geology, studies rock layers and layering ( stratification) Permeability in the Earth sciences (commonly symbolized as κ, or k) is a measure of the ability of a material (typically a rock or unconsolidated The term " Facies " can also refer to distinctive Facial expressions associated with conditions such as Williams syndrome.
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. A salt dome is a type of structural dome formed when a thick bed of Evaporite minerals (mainly salt or Halite) found at depth intrudes vertically into [5]
Estimating reserves
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). Oil in place is the total hydrocarbon content of an Oil reservoir and is often abbreviated STOOIP, which stands for Stock Tank Original Oil In Place, or 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.
Production
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. La Brea Tar Pits (or Rancho La Brea Tar Pits) are a famous cluster of Tar pits located in Hancock Park in the urban heart of Los Angeles, California ( is a US state on the West Coast of the United States, along the Pacific Ocean. Trinidad ( Spanish: " Trinity " is the largest and most populous of the two major islands and
Drive mechanisms
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. When heating a Liquid consisting of two or more components the bubble point is the point where first bubble of Vapor is formed 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. An aquifer is an underground layer of Water -bearing Permeable rock or unconsolidated materials ( Gravel, Sand, Silt, or Clay 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. The water injection method used in oil production is where Water is injected back into the reservoir usually to increase Pressure and thereby
Location
- Active areas of onshore US oil reservoirs
- California
- Colorado
- Kansas
- Mississippi
- Montana
- Nevada
- North Dakota
- New Mexico
- Oklahoma
- Texas
- Wyoming
- Utah
- Active areas of existing sub-sea (offshore) oil reservoirs
See also
Notes and References
- ^ The Making of Oil: Birth of a Reservoir. California ( is a US state on the West Coast of the United States, along the Pacific Ocean. The State of Colorado ( or chiefly by nonresidents) is a state located in the Rocky Mountain region of the United States of America. Kansas ( is a Midwestern state in the central region of the United States of America, an area often referred to as the American " Mississippi ( is a state located in the Deep South of the United States Montana ( is a state in the Western United States. One-third of the state in the western part contains numerous mountain ranges (approximately 77 named of the northern Nevada ( is a state located in the western region of the United States of America. North Dakota ( is a state located in the Midwestern and Western regions of the United States of America. New Mexico ( is a state located in the southwestern region of the United States of America. Oklahoma ( is a state located in the South Central region of the United States of America. Texas ( is a state geographically located in the South Central United States and is also known as the Lone Star State. The State of Wyoming ( is a sparsely populated state in the western region of the United States. The State of Utah (ˈjuːtɔː or) is a western state of the United States. The North Sea is a marginal, Epeiric sea of the Atlantic Ocean on the European Continental shelf. The Niger Delta, the delta of the Niger River in Nigeria, is a densely populated region sometimes called the Oil Rivers because it was once a West Africa or Western Africa is the Westernmost Region of the African Continent. South America is a Continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a The Gulf of Mexico ( Spanish: Golfo de México) is the ninth largest Body of water in the world An oil field is a region with an abundance of Oil wells extracting Petroleum (crude oil from below ground Drilling is the process of using a Drill bit in a Drill to produce cylindrical holes in solid materials such as wood or metal Schlumberger Excellence in Educational Development. Retrieved on January 30, 2006. Events 1648 - Eighty Years' War: The Treaty of Münster is signed ending the conflict between the Netherlands and Spain Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar.
- ^ What is a Reservoir?. Schlumberger Excellence in Educational Development. Retrieved on January 30, 2006. Events 1648 - Eighty Years' War: The Treaty of Münster is signed ending the conflict between the Netherlands and Spain Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar.
- ^ Rise and Fall of the North Sea. Schlumberger Excellence in Educational Development. Retrieved on January 30, 2006. Events 1648 - Eighty Years' War: The Treaty of Münster is signed ending the conflict between the Netherlands and Spain Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar.
- ^ What is a Reservoir? - What are some characteristics?. Schlumberger Excellence in Educational Development. Retrieved on January 30, 2006. Events 1648 - Eighty Years' War: The Treaty of Münster is signed ending the conflict between the Netherlands and Spain Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar.
- ^ Evolution of the Reservoir. Schlumberger Excellence in Educational Development. Retrieved on January 30, 2006. Events 1648 - Eighty Years' War: The Treaty of Münster is signed ending the conflict between the Netherlands and Spain Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar.
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