Generation, Migration and Trapping

Source rock and maturation

Hydrocarbons begin as organic matter deposited in an oxygen-poor environment where it could be preserved rather than decayed. Burial converts that organic matter into kerogen, and further burial subjects it to increasing temperature.

Generation happens within a temperature window. Too cool and the kerogen never converts; within the oil window, roughly 60 to 120 degrees Celsius, oil is generated; hotter still, in the gas window, the organic matter and any remaining oil are cracked to gas; hotter again and only residual carbon remains. Because temperature increases with depth, the maturity of a source rock is largely a function of how deeply it has been buried and for how long.

Migration

Hydrocarbons are less dense than the water filling the pore space, so they move upward and laterally through permeable rock until something stops them. Primary migration is expulsion from the source rock itself; secondary migration is movement through carrier beds towards a trap. Much of what is generated never reaches a trap at all — it disperses, reaches the surface, or is destroyed.

Reservoir rock

A reservoir must both store and yield fluid.

• Porosity — the fraction of the rock volume that is pore space. It determines how much fluid can be stored.
• Permeability — the ease with which fluid flows through the connected pore network. It determines how fast the rock will yield that fluid.

The two are related but not the same, and the distinction matters enormously. A chalk can have high porosity and almost no permeability; a fractured rock can have very low porosity and excellent permeability. High porosity with low permeability means a lot of oil that will not flow.

The common reservoir rocks are sandstones, whose properties depend on grain size, sorting and cementation, and carbonates, whose properties are dominated by later alteration — dissolution creating vugs and caves, or cementation destroying porosity — making them far more heterogeneous and harder to predict.

Traps and seals

The seal is an impermeable rock, usually shale, salt or tight carbonate, that stops upward migration. The trap is the geometry that holds the accumulation.

• Structural traps — anticlines, fault traps and salt domes, formed by deformation. Historically the majority of discoveries, and the easiest to see on seismic.
• Stratigraphic traps — pinch-outs, reefs, channel sands and unconformities, formed by variation in the rock itself. Harder to find and often overlooked in mature basins, which is where a good deal of remaining potential lies.
• Combination traps — both mechanisms together, which is common in reality.

Timing

The most easily forgotten element: the trap must have existed before the hydrocarbons migrated. A perfect structure formed after migration is empty, and this is a frequent explanation for a well that found everything expected except oil.