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

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.

Migration
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.

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

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.

Trap
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.