Petroleum System Elements

1) Petroleum Source Rock

a. Formation Process

fine-grained sedimentary rocks such as shale or limestone that are rich in organic material

form in environments where organic matter (mainly from microscopic plants and animals) accumulates faster than it decays, such as in deep lakes, marine basins, or oxygen-poor swamps.

Over millions of years, layers of sediment bury this organic-rich mud. The lack of oxygen slows decomposition, allowing organic matter to be preserved and incorporated into the sediment.

1) Petroleum Source Rock

b. Origin of Organic Material

• The organic matter comes primarily from plankton, algae, and other microorganisms that lived in ancient seas or lakes.

• When these organisms died, their remains settled to the bottom, mixing with clay and silt.

• The best source rocks are those with high total organic carbon (TOC), indicating abundant preserved organic matter.

2) Oil Generation

Temperature and Pressure Conditions:

• As sedimentary layers build up, the source rock is buried deeper, increasing temperature and pressure.

• Oil forms when the source rock is heated to about 60–120°C (the "oil window"), typically at depths of 2–4 kilometers.

• This process, called catagenesis, breaks down large organic molecules (kerogen) into smaller hydrocarbons (oil and gas).

• The term "kitchen" refers to the area in the subsurface where these conditions are met and oil is actively generated.

3) Migration Pathway and Petroleum Migration

a. migration process

• Once formed, oil and gas are less dense than the surrounding water-saturated rocks.

• Driven by buoyancy, hydrocarbons migrate upward and laterally through permeable pathways such as porous rocks, fractures, or faults.

• This movement continues until hydrocarbons encounter a barrier (seal/caprock) or become trapped in a reservoir.

3) Migration Pathway and Petroleum Migration

b. Migration pathways

• Typical pathways include interconnected pore spaces in sandstones or limestones, as well as faults and fractures that act as conduits.

• Migration is often a multi-stage process: first, hydrocarbons move out of the source rock (primary migration), then travel through carrier beds toward a trap (secondary migration).

4) Reservoir Rock

Formation and Composition

• Reservoir rocks are typically sandstones, though they can also be limestones or dolomites.

• Sandstones form from the accumulation and compaction of sand grains, often in river channels, beaches, or deltas.

• These rocks are valued for their high porosity (space to store oil) and permeability (ability to transmit fluids).

• The best reservoirs have well-connected pore spaces, allowing oil to accumulate and flow when drilled.

5) Traps

a. Fault Trap

• Created when a fault displaces rock layers, juxtaposing a permeable reservoir against an impermeable seal, blocking further migration.

5) Traps

b.Anticline

• Formed by the upward arching of rock layers (an anticline fold). Oil migrates to the crest of the fold, where it accumulates beneath the caprock.

5) Traps

c. Sand Pinchout

• Occurs when a reservoir sandstone thins out and terminates within impermeable rocks, trapping the oil at the pinchout point.

5) Traps

d. Role in oil accumulation

• Traps are essential for concentrating oil and gas in commercial quantities. Without a trap, hydrocarbons would continue migrating and escape to the surface.

6) Seal/Caprock

Functions and characteristics

• A seal or caprock is a layer of impermeable rock, such as shale, mudstone, salt, or dense limestone, that overlies the reservoir rock.

• Its low permeability prevents oil and gas from migrating further upward or laterally, effectively "capping" the accumulation.

• The effectiveness of a caprock depends on its thickness, continuity, and lack of fractures.