Introduction
It is a flammable liquid which occurs naturally in rock formations beneath the Earth’s surface. It consists mainly of hydrocarbons—organic molecules made of hydrogen and carbon atoms along with traces of sulfur, nitrogen, oxygen, and metals. Petroleum fuels uses in transportation, power generation, and industrial processes.
The liquid component constituted the crude oil, which is a basic unit of petroleum.
Chemical composition:
It primarly consists of hydrocarbons with a small amount of different compounds:
1. Sulfur Compounds: Petroleum often contains sulfur compounds, which removed during the refining process to meet regulatory standards.
2. Nitrogen Compounds: It Found as amines or pyridines, also removed during refining.
3. Oxygen Compounds: Present in low concentrations as alcohols, ketones, or carboxylic acids.
4. Trace Elements: Includes metals such as nickel, vanadium and iron.
Theory of petroleum origin:
Two primary theories explain petroleum formation:
1. Organic Theory (Biogenic Theory- most accepted):
- Petroleum forms from ancient marine organisms buried under sediment.
- Over millions of years, heat and pressure transform organic matter into hydrocarbons (diagenesis → catagenesis).
- Supported by fossil evidence in oil-rich sedimentary rocks.
2. Inorganic Theory (Abiogenic Theory- less common):
- Proposes petroleum originates from non-organic mantle processes involving CO₂ and methane.
- Lacks widespread scientific acceptance due to limited evidence.
Classification of crude oil :
Crude oil is classified based on density and hydrocarbon content:
A. By Hydrocarbon Type ( Reffered as Van Krevelen Diagram)
i. Praffin or aliphatic hydrocarbons
ii. Naphthene or cycloparaffin type hydrocarbons
iii. Aromatic hydrocarbons
iv. Resins & asphaltenes
i. Paraffins (or aliphatic hydrocarbons): Paraffins are most common hydrocarbon group in crude oil, also known as alkanes. It's a straight or single chains (e.g., methane, propane). High energy content and low reactivity compare to others.
ii. Naphthenes (or cycloparaffin type hydrocarbons): Cyclic hydrocarbons with saturated bonds, often referred to as cycloalkanes. They have a ring structure composed of carbon and hydrogen atoms. Naphthenes are commonly found in crude oil, especially in petroleum fractions with higher boiling points. They have different ring sizes, such as cyclohexane, cyclopentane, and cycloheptane.
iii. Aromatics: Aromatics are hydrocarbons that contain a benzene ring or similar aromatic rings, which single or double of carbon alternatively. Highly stable compounds with strong aromatic properties. Common aromatic hydrocarbons found in crude oil include benzene, toluene, ethylbenzene, and xylene.
iv. Resins & Asphaltenes: Heavy, complex molecules(mixture of aromatic and polar hydrocanbon)often referred to as the non-volatile fraction of petroleu, affecting viscosity. Resins are the soluble components, whereas asphaltenes are the insoluble components.
B. By Density (API Gravity):
main categories of crude oil depending on density:
- Light Crude (API > 31.1°): Low density, which make it flow easy also easy to refining (e.g., Brent, WTI).
- Medium Crude (API 22.3°–31.1°): Middle range of density. Heavier hydrocarbon molecules compared to light crude oil.
- Heavy Crude (API < 22.3°): Higher density and lower API gravity. Contain more complex hydrocarbon molecules, which produce heavy products such as diesel fuel, fuel oil, and asphalt.
- Bitumen (API < 10°): Extremely low API gravity and is highly viscous. Near-solid, requires specialized extraction.
API Gravity Formula:
The API gravity detemine the relative density of petroleum liquids compared to water.
API gravity = (141.5 / Specific gravity) - 131.5
Also,
Specific Gravity = (141.5 / (API Gravity + 131.5))
Example: For API gravity of 35:
Specific Gravity = (141.5 / (35 + 131.5))
= 141.5 / 166.5
≈ 0.850
Formation of petroleum :
Petroleum, or crude oil, forms through a complicated process that takes millions of years. This process happens in several steps :
1. Organic Material Accumulation: Dead plankton or algae settle in marine and lake beds.
2. Sediment Deposition: Layers of mud or sand bury organic matter.
3. Burial and Compaction: The weight of the sediment layers causes increased pressure on the organic material below which buried deeper within the Earth's crust, causes compaction.
4. Heat and Pressure: It experiences more temperatures(1-3 degrees Celsius per 100 meters of burial depth) and pressures due to the heat flow from the Earth's interior and the geologic processes.
5. Diagenesis: During chemical and physical changes of these material(breaking down and rearrange) transform to the kerogen.
6. Catagenesis and Thermal Cracking: With continued burial kerogen convert into liquid and gaseous hydrocarbons ( larger organic molecules into smaller hydrocarbon molecules, makes pertroleum) through thermal cracking.
7. Migration: Upwards movement of these newly formed petroleum through porous rock until it trapped.
8. Petroleum Traps: When petroleum gathers in underground formations called traps, which are either structural or stratigraphic.
9. Reservoir Formation: Once petroleum is trapped, it forms a reservoir within the porous and permeable rock layers, typically composed of sandstone, limestone, or fractured shale. The reservoir work as a storage for petroleum.
10. Exploration and Extraction: Exploration uses seismic surveys, test drilling, and well trials to find and evaluate reservoirs. Once a site is viable, production wells are drilled and oil is recovered in stages—first by natural reservoir pressure (primary), then with water or gas injection (secondary), and finally with enhanced methods like steam or chemical flooding (tertiary).
Maturation of Petroleum :
Here are the main aspects of petroleum maturation:
1. Kerogen Conversion: With continued burial kerogen convert into liquid and gaseous hydrocarbons ( larger organic molecules into smaller hydrocarbon molecules, makes pertroleum) through thermal cracking.
2. Oil Window: As organic material matures under rising heat and pressure, it yields different hydrocarbons at specific conditions. The “oil window,” where oil forms around 60 °C to 150 °C
3. Gas Window: Beyond the oil window, higher temperatures cause the cracking of heavier hydrocarbons and produces the natural gases. Occurs at temperatures above 150 degrees Celsius.
4. Thermal Maturation Indicators: Geologists and petroleum experts use several indicators to assess the level of maturation and the potential presence of petroleum in a reservoir. These indicators include vitrinite reflectance, which measures the reflectivity of a type of organic matter under a microscope; pyrolysis analysis, which analyzes the released hydrocarbons from heated rock samples; and biomarkers, which are specific organic compounds that canprovide information about the source and maturity of petroleum.
5. Maturation and Petroleum Quality: Maturation level impacts petroleum quality: immature oils contain more waxes and less energy, while further maturation boosts liquid hydrocarbons, reduces impurities, and alters viscosity, density, and overall composition.
6. Over-Maturation: When temperatures and pressures excess, organic material can become over‑mature or thermally break down and forming graphite.
Kerogen :
Kerogen is the insoluble organic matter trapped in sedimentary rocks like oil shale and source beds. When buried and heated, it “cracks” into liquid and gaseous hydrocarbons. Which determine the quantity and quality of hydrocarbons.
the main types of kerogen:
1. Type I Kerogen: It considered to be the most oil-prone,which originates from organic material such as algae and other marine plankton. It contains a high proportion of lipids, proteins, and other hydrogen-rich organic compounds. It is typically associated with marine source rocks.
2. Type II Kerogen: It produces a mix of liquid hydrocarbons, including crude oil, and gaseous hydrocarbons, such as methane.Type II kerogen is derived from a mixture of marine and terrestrial organic matter.
3. Type III Kerogen: Type III kerogen tends to yield more gas than liquid hydrocarbons and It is derived from terrestrial organic matter such as woody plant material, coal and peat.
4. Type IV Kerogen: It is mainly associated with coal deposits and has limited significance as a petroleum source.It has no ability to generate either gas or oil. Originates from the highly carbonaceous material such as fossilized wood, lignite, and coal.
Migration of petroleum :
When hydrocarbon move from source to reservoir rock and become accessible for extraction is called petroleum migration.This movement occurs in two stages:
1. Primary Migration: First step in the movement of hydrocarbons from the source rock to nearest reservoir rocks. It happens due to the pressure from deep burial and the maturation of organic material. Depending on the geology, hydrocarbons may move upward into shallower layers or laterally through the same sedimentary layer.
2. Secondary Migration: Hydrocarbons can move father after primary migration through pores, faults and fractures, driven by pressure difference. This stage leads to the deposits of hydrocarbons in traps, which economically significant quantities.
Causes of migration :
1.Buoyancy and Density Differences: When hydrocarbon are less denser than the surrounding rocks and fluids such as water or brine, which creates buoyant force and allowing them to move upward through interconnected pore spaces, fractures, and faults. Depending on the pathways available, this movement can be vertical or lateral within the subsurface
2. Pressure Differences:
As heat transforms organic material into hydrocarbons, pressure also increases in the source rock. This heavier pressure pushes the hydrocarbons toward lower pressure areas, helping them move into nearby reservoir rocks where they can collect.
3. Permeability and Porosity : Migration mostly depends on the porosity and permeability of the rocks. Porosity means the rock has connected spaces where fluids can move, while permeability is the ability to move. for example- sandstones have both good porosity and permeability, offer ideal pathways for hydrocarbons to travel and gather in reservoirs
4. Structural Features: Structures like faults, fractures and folds influence petroleum movement beneath the surface. Faults and fractures can serve as open routes, letting hydrocarbons shift either upward or sideways. Which making it easier for fluids to flow.Some structures can trap or block migration, acting like seals that retain hydrocarbons and prevent them from leaking into other layers.
5. Capillary Forces: These forces can pull hydrocarbons through very small spaces like fine pores or narrow fractures—even against gravity. This is especially important in tight formations like shale, where permeability is very low but small fractures help the hydrocarbons move.
6. Fluid Properties: The movement of hydrocarbons is also influenced by their own physical properties. Less denser hydrocarbon can travel farther and more easily through rocks. In contrast, heavier hydrocarbons move more slowly due to their thickness.
Oil reservoir :
An oil reservoir is an subsurface rock formation where oil gathered and can be extracted. These reservoirs usually form in sedimentary rocks such as sandstone, limestone or dolomite, which have enough porosity and permeability.
1. Porosity: Percentage of voids within the rock formation. In oil reservoirs, it measure the amount of oil that the rock can store. Higher porosity means more oil can be held.
2. Permeability:Permeability is the capacity of a rock to transmit fluids, depending on interconnected pore spaces. Greater the permeability ,flow will be more easy, also making extraction easier
3. Reservoir Fluids: Oil reservoirs mostly consist of hydrocarbons ( crude oil and natural gas). The fluid composition varies, with different types of hydrocarbons and gases present depending on the geological setting.
4. Reservoir Pressure: The pressure inside an oil reservoir helps push the oil toward the surface. At first, the natural pressure may be enough to bring oil to the surface is called primary recovery. As pressure declines, methods like water or gas injection help maintain flow, allowing continued production through secondary or tertiary recovery.
5. Trap and Seal: For a reservoir to form, a trap must hold the oil in place and stop it from escaping. These traps may be structural or stratigraphic, caused by changes in rock type or porosity. Above the trap, a seal made of impermeable rock (such as shale or salt) prevents the oil and gas from moving further upward.
6. Reservoir Evaluation: Reservoir evaluation involves studying its size, porosity, permeability and fluid type using seismic data, well logs and core samples to measures its production capacity.
7. Recovery Methods: Oil recovery starts with natural pressure (primary). When it declines, water or gas is injected (secondary). Advanced methods follow if needed (tertiary).
Oil traps :
Types of oil traps:
1. Structural traps: Formed due to the deformation of rock layers(such as folding or faulting) which makes a closed structure that can hold oil and gas. Types of structural traps :
A. Traps formed due to folding:
i. Anticline: An anticline is an upward-arching fold in rock layers where the oldest rocks lie at the center. It can serve as an effective oil trap because hydrocarbons naturally rise and collect at the highest point of the fold. If sealed by an impermeable rock layer, the crest of the anticline can hold oil and gas, making it a prime target for petroleum exploration
ii. Syncline: A syncline is a downward fold in rock layers, forming a basin-like structure with the youngest rocks at the center. Though less common than anticlines, synclines can trap oil, especially if they are uneven or have complex internal structures. Oil may collect in the lower or tightly folded parts where conditions allow effective sealing and storage.
iii. Homocline: A homocline is a gentle, consistent tilt in rock layers without major folding. Oil can get trapped here when porous layers, like sandstone, dip beneath non-porous ones, like shale. If this setup blocks the upward movement of oil, it collects in the tilted porous layer, forming a trap.
B. Traps formed due to faulting :
Fault traps are created when rock layers shift due to faulting, causing a reservoir rock to be sealed against an impermeable layer.
1. Normal Fault Trap: Normal fault traps form when the hanging wall move down side relative to the footwall, creating a step-like offset. If permeable reservoir rocks in the hanging wall are sealed against impermeable rocks in the footwall, hydrocarbons can accumulate along the fault plane, forming a trap.
2. Reverse Fault Trap: Reverse fault traps form when the hanging wall is pushed upward over the footwall due to compressional forces. If the hanging wall has impermeable rocks and the footwall holds reservoir rocks, hydrocarbons can get trapped beneath the fault, creating a structural trap
3. Thrust Fault Trap: Thrust faults are low angled reverse faults where the hanging wall is pushed horizontally over the footwall. This movement folds and stacks rock layers, forming traps. Hydrocarbons can collect in these deformed reservoir rocks, sealed by overlying impermeable layers.
4. Transcurrent Fault Trap:Transcurrent or strike-slip faults involve horizontal movement of rock blocks along the fault plane. Traps form when this lateral displacement offsets reservoir and seal rocks, allowing hydrocarbons to accumulate where the reservoir rock is sealed against an impermeable layer.
2.Stratigraphic traps:
These traps are formed by variations in the rock layers or stratigraphy. Primary and secondary stratigraphic oil traps are two types of such traps.
2.1. Primary Stratigraphic Oil Trap:
It's forms during the deposition of sediments due to changes in rock type, porosity, or thickness. It doesn’t rely on structural deformation.
Main features:
- Formed during sedimentation.
- Caused by changes like facies variation, pinch-outs, or unconformities.
- Sealed by non-porous rocks like shale.
examples:
a. Sand Wedges
These are V-shaped sand bodies that penetrate into fine-grained sediments, formed by freeze-thaw in cold regions. Which helps hydrocarbons move and sometimes act as small reservoirs in otherwise tight rock layers.
b. Facies Changes
When a permeable rock like sandstone gradually changes to an impermeable onesuch as shale, oil can get trapped at the boundary. This transition creates a natural trap.
c. Sand in Clay or Shale
Sometimes sand beds form within thick clay or shale. These sand patches can store hydrocarbons if the surrounding mud acts as a seal.
d. Lenses
These are isolated pockets of reservoir rock, like sand, enclosed in non-porous layers. If sealed well, they can hold trapped oil or gas.
2.2. Secondary stratigraphic traps :
Formed after the deposition of sedimentary rock, which changes the pre existing rock properties, alterations in the depositional environment or tectonic activity and creating favorable conditions for the accumulation and trapping of hydrocarbons.
Examples:
Unconformities:
Angular unconformities occur mwhen younger rocks layers are deposited over eroded older rocks layers(tilted or folded), creating favorable conditions for hydrocarbon reservoirs.
3. Combination traps:
These traps form when folding, faulting or uplift is paired with variations in porosity, permeability, or lithology.
Example:
Salt domes:
Salt domes form when underground buoyant salt layers push upward through rock, creating perfect oil traps.
How Salt Domes Trap Oil and Gas
- Structural Traps
- Rising salt pushes rock layers upward and forming a dome-shaped structures
- Impermeable cap rocks (shaleor anhydrite) seal hydrocarbons below
- Reservoir rocks such as sandstone or limestone often surround the salt core
- Fault-Enhanced Traps
- Salt movement creates fractures and faults along dome edges
- These fractures can create permeability barriers
- Stratigraphic Traps
- Salt movement distorts surrounding sediments, causing:
- Sand channel formations
- Rock layer pinch-outs
- Unconformities
- These variations create perfect reservoirs.
- Salt as Hydrocarbon Reservoir
- Micro fractures provide porosity
- Surrounding rocks act as seals
Conclusion :
Formation
Created over millions of years from ancient marine life.Transforms under heat and pressure into hydrocarbons
Composition
Mainly hydrogen and carbon compounds. Contains sulfur, nitrogen, oxygen traces. The exact mix determines the type and quality of crude oil.
Exploration & Production
Finding petroleum involves geological surveys, seismic imaging, and drilling. Once located, wells are drilled to extract the oil from underground reservoirs.
Uses
Crude oil is refined into fuels (gasoline, diesel, jet fuel), lubricants, and raw materials for plastics, fertilizers and chemicals—supporting industries and daily life.
Environmental Impact
Oil extraction and use can cause pollution, habitat damage, and emissions. Burning petroleum products releases carbon dioxide, contributing to global warming.
Energy Transition
To reduce fossil fuel dependence, the world is turning to solar, wind, hydro and electric technologies—aiming for a cleaner and more sustainable energy future.
Economic Role
Petroleum powers economies, especially in oil-rich countries. It fuels transport, supports manufacturing and creates jobs across sectors.
Sustainability
The future of petroleum lies in balancing current energy needs with environmental goals through cleaner technologies, efficiency and greater reliance on renewables.
