Reservoir rocks are among the most important components of the petroleum system; without them, oil and gas production would be unrealistic and unprofitable. While source rocks produce oil, which is subsequently transported through migration processes, reservoir rocks ensure the storage and economic extraction of oil and gas. In the petroleum industry, it is often said that “the reservoir is the foundation,” as the quality, type, porosity, permeability, and physical properties of the reservoir directly determine the productivity and economic value of an oil field.
This article comprehensively analyzes the definition, properties, types, formation process, research methods and engineering problems of reservoir rocks and explains in detail the role of reservoir rocks in oil field exploration and development.
What are reservoir rocks?
Reservoir rocks are rocks that can store and transport oil and natural gas deposits . These properties depend primarily on their porosity and permeability .
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Porosity refers to the proportion of cavities in rocks that can contain petroleum, natural gas, and water.
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Permeability is the ability of rocks to transport liquids; that is, how well oil or natural gas can pass through the pores of the rock and enter a borehole.
If both rocks have the same porosity, but one has a higher permeability, it is significantly more valuable as a source of sedimentary material.
Which rocks are suitable as oil reservoirs?
In general, two main types of rock can be used as reservoir rocks:
1. Clastic sedimentary rocks
These rocks were formed by the deposition and consolidation of fragments of other rocks. The most important sedimentary rocks include:
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Sandstone
is a sedimentary rock with extremely high porosity and permeability and is known as a reservoir rock. Its granular structure forms a continuous network of pores, making it ideal for the storage and transport of oil and natural gas. -
Although an agglomerate
may exhibit good porosity, the adhesion of its grains and the porosity between them vary greatly.
2. Carbonate rocks
Carbonate rocks make up about 60–70% of the world’s oil reserves.
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limestone
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dolomite
Carbonate rocks are characterized by extreme structural complexity, resulting from metamorphic processes such as dissolution, which can lead to the formation of secondary pores. However, they also form highly productive sedimentary rocks.
Porous reservoir rock
1. Primary porosity
This refers to the porosity present in the formation of rocks, such as:
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Intergranular porosity in sandstone
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Pores between primary crystals in dolomite
2. Secondary porosity
It arises from processes that take place after precipitation:
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Dissolution of substances in carbonates
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Cracks and fissures
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Karst formations
In many carbonate deposits, secondary porosity is the most important factor for oil storage and production.
Permeability and its role in oil production
Permeability is usually measured in Darcy or Millidarcy.
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High permeability (>1000 mDracy) → Excellent performance
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Average permeability (10–1000 mDr.) ← Acceptable
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Low permeability (1–10 mDraki) ← Need for improved sampling methods
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Very low permeability (<1 mDracey) ← Unconventional resources such as shale oil
Although porosity and permeability are related, some rocks can have good porosity and very low permeability, such as many carbonate rocks.
The process of rock formation in sedimentary rocks
1. Precipitation
The main role of porosity in sedimentary rocks depends on the time of their formation:
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Precision of the parts
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How to create layers
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Sedimentary environments (deltas, coasts, rivers, shallow seas, coral reefs, etc.).
2nd location
In deeper soil layers, the weight of the sedimentary rocks reduces the porosity. This factor is particularly important for sandstone.
3. Subsequent rock formation
Changes that occur after a bowel movement:
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Cement fixation
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Solution
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dolomite
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break
During metamorphosis, porosity can either decrease or increase.
Basic properties of ideal storage rocks
1. High effective porosity
2. High permeability
3. Sufficient thickness for storage
4. Lateral rock continuity
5. Stable physical rock properties
6. No excess cement-filled voids
7. Ability to control cracking (if required)
The role of reservoir rocks in the formation of oil fields
The oil and gas system consists of five main elements:
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rough stone
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Sedimentary rock
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upper class
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Immigration mechanism
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Oil and gas separators (collectors)
In the reservoir layers, the oil settles and accumulates after its migration. Economically valuable oil fields could not exist without these layers.
Tank types are classified according to the type of flow.
1. Traditional tanks
Oil and natural gas are stored in environments with sufficient porosity and permeability and can be extracted using natural pressure.
2. Unconventional oil and gas reserves
For example:
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Oil shale
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Shale gas
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very hard sandstone
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Dense and cracked carbonates
These deposits have extremely low permeability and require the use of hydraulic fracturing.
Methods of exploring reservoir rocks
1. Examination of borehole measurement data.
The following are among the borehole measuring tools:
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range of values
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density
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carry
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neutron
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Magnetic resonance imaging
Physical parameters of petroleum rocks, such as saturation, porosity and permeability, can determine the properties of the petroleum .
2. Extraction of the pulp
Borehole drilling is the best method to directly study sedimentary rocks.
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Porosity test
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permeability
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thin slices
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SCAL and RCAL tests
3. Seismometer (Seismology)
It serves to identify traps, structures, and reservoir extensions.
4. Geology and numerical modeling
The 3D tank models include the following:
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Mechanical engineering
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Properties of storage tanks
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Pressure and temperature
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Fluid distribution
These models are necessary to predict the behavior of the reservoir during pumping.
Behavior of storage tanks during production
1. Tank pressure
The initial pressure and the drive mechanism of the oil tank are key factors in determining production quantities and productivity.
Initiating organization:
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Aerosol drift
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Air lift power plant
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Water flow
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Gravitational drift
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joint payment
2. Gas-oil ratio (GÖV)
Increased gastroesophageal reflux can be a sign of the following diseases :
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Near the fuel filler cap
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Low voltage
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open fracture
Yes.
3. Relationship between the properties of the reservoir rock and production.
Some rocks are compressed at relatively low pressure , leading to a reduction in porosity. This phenomenon can be observed in clayey and carbonate rocks.
Common problems in oil reservoir rocks
1. Asphalt and paraffin deposits
This prevents the flow of fluid into the pores.
2. Sand production
If this occurs in loose sandstone, it can lead to damage to equipment.
3. Sound source
In igneous rocks containing montmorillonite, this leads to reduced permeability.
4. Heterogeneity
The special properties of carbonate rocks also pose a serious challenge for mine development .
Increasing the productivity of sedimentary rocks
1. Basic methods
Utilize the natural pressure generated by the storage tank.
2. Useful methods
To maintain the pressure, water or gas must be injected.
3. Improved Oil Recovery (EOR)
Increase your yield with the following methods:
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Carbon dioxide injection
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Chemical injection
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thermal process
The choice of the appropriate method depends directly on the properties of the deposited rock.
The economic importance of sedimentary rocks
Oil fields with good reservoir rocks can:
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It has a sustainable production capacity for long-term security.
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Reduction of development costs
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Demand for complex technologies has declined.
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This ensures higher recycling rates for waste.
In contrast, the development of tight oil and gas fields, as well as unconventional oil and gas fields, is significantly more expensive.
In conclusion
Reservoir rocks are a fundamental and indispensable component of any petroleum system. From grain structure and rock type to subsequent formation processes such as sedimentation, fracturing, porosity, and permeability, all these factors influence their quality and properties. A comprehensive understanding of reservoir rocks is therefore crucial not only for exploration but also for field modeling, development, production, and management.