Petroleum System

Petroleum System

The Petroleum System consists of a mature source rock, migration pathway,reservoir rock, trap and seal. Appropriate relative timing of formation of these elements and the processes of generation, migration and accumulation are necessary for hydrocarbons to accumulate and be preserved.

The components and critical timing relationships of a petroleum system can be displayed in a chart that shows geologic time along the horizontal axis and the petroleum system elements along the vertical axis.

Exploration plays and prospects are typically developed in basins or regions in which a complete petroleum system has some likelihood of existing.

The petroleum system is a conceptual model that describes the processes that lead to the formation and accumulation of oil and gas. It consists of five essential elements:

  1. A source rock: This is a rock that contains organic matter that can be converted into oil and gas.
  2. A reservoir rock: This is a rock that can store oil and gas.
  3. A trap: This is a structure that prevents the oil and gas from escaping from the reservoir rock.
  4. A seal: This is a rock that prevents fluids from migrating from one reservoir rock to another.
  5. Overburden: This is the rock that lies above the reservoir rock and helps to protect it from erosion.

Source Rock Hydrocarbon Generation

The formation of hydrocarbon liquids from an organic rich source rock with kerogen and bitumen to accumulates as oil or gas.
Petroleum System
Petroleum System elements

Generation depends on three main factors:

  • the presence of organic matter rich enough to yield hydrocarbons,
  •  adequate temperature,
  • and sufficient time to bring the source rock to maturity.
  • Pressure and the presence of bacteria and catalysts also affect generation.
  • Generation is a critical phase in the development of a petroleum system.


The movement of hydrocarbons from their source into reservoir rocks.
  • The movement of newly generated hydrocarbons out of their source rock is primary migration, also called expulsion.
  • The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration.
  • Migration typically occurs from a structurally low area to a higher area in the subsurface because of the relative buoyancy of hydrocarbons in comparison to the surrounding rock.
  • Migration can be local or can occur along distances of hundreds of kilometres in large sedimentary basins, and is
  • critical to the formation of a viable petroleum system.


The phase in the development of a petroleum system during which hydrocarbons migrate into and remain trapped in a reservoir.


A subsurface body of rock having sufficient porosity and permeability to store and transmit fluids.
  • Sedimentary rocks are the most common reservoir rocks because they have more porosity than most igneous and metamorphic rocks and
  • they form under temperature conditions at which hydrocarbons can be preserved.
  • A reservoir is a critical component of a complete petroleum system.

Seal (cap rock)

An impermeable rock that acts as a barrier to further migration of hydrocarbon liquids.
Rocks that forms a barrier or cap above and around reservoir rock  forming a trap such that fluids cannot migrate beyond the reservoir. The permeability of a seal capable of retaining fluids through geologic time is   ~  10-6 to 10-8 darcies.  commonly
  • shale, mudstone
  • anhydrite 
  • salt, 
  • A seal is a critical component of a complete petroleum system.


A configuration of rocks suitable for containing hydrocarbons and sealed by a relatively impermeable formation through which hydrocarbons will not migrate.
Traps are described as
  • structural traps
    • Hydrocarbon traps that form in geologic structures such as folds and faults
  • stratigraphic traps
    • Hydrocarbon traps that result from changes in rock type or pinch-outs, unconformities, or other sedimentary features such as reefs or buildups
  • A trap is an essential component of a petroleum system.


The overburden is the rock that lies above the reservoir rock. It helps to protect the reservoir rock from erosion and from the intrusion of fluids from other formations.

The petroleum system is a complex system, but it is a valuable tool for understanding the distribution of oil and gas resources and for predicting where new discoveries might be made.

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