Petroleum System Flashcards

What is a petroleum source rock and how does it form?

• A petroleum source rock is a fine-grained sedimentary rock (like shale or limestone) rich in organic material.

• It forms in environments where organic matter (mainly from plankton, algae, and microscopic marine life) accumulates faster than it decays, such as deep lakes, marine basins, or oxygen-poor swamps.

• Over millions of years, this organic-rich mud is buried by sediments, preserving the organic material due to low oxygen levels.

Where does the organic material in source rocks come from?

• The organic material comes from the remains of microscopic plants and animals (plankton, algae) that lived in ancient aquatic environments.

• After death, their remains settle to the bottom, mix with clay and silt, and are preserved in the sediment.

What is meant by the "oil kitchen" in petroleum geology?

The "oil kitchen" is a subsurface zone where the source rock is buried deeply enough to reach the right temperature (60–120°C) and pressure to generate oil.

How does the source rock generate oil?

• As sediment accumulates, the source rock is buried deeper, increasing temperature and pressure.

• At depths of 2–4 kilometers, organic matter is converted into hydrocarbons (oil and gas) through a process called catagenesis.

How does oil migrate from the source rock to the reservoir?

• Oil and gas, being less dense than water, migrate upward and laterally through permeable pathways such as porous rocks, fractures, or faults.

• This movement is driven by buoyancy and continues until hydrocarbons encounter a barrier (seal/caprock) or become trapped.

What are typical migration pathways for petroleum?

Interconnected pore spaces in sandstones or limestones

Faults and fractures acting as conduits

What is a reservoir rock and how does it form

A reservoir rock is a porous and permeable rock that stores hydrocarbons.

Sandstones are the most common reservoir rocks; they form from the accumulation and compaction of sand grains, often in river channels, beaches, or deltas.

What makes a good reservoir rock?

• High porosity (space to store oil)

• High permeability (ability to transmit fluids)

What is a trap in petroleum geology?

A trap is a geological structure that collects and holds hydrocarbons, preventing further migration.

What are the main types of traps? Describe them.

Fault Trap: Created when a fault displaces rock layers, placing a permeable reservoir next to an impermeable seal.

Anticline Trap: Formed by the upward arching of rock layers (anticline fold), with oil accumulating at the crest.

Sand Pinchout Trap: Occurs when a reservoir sandstone thins out and terminates within impermeable rocks, trapping oil at the pinchout.

What is a seal or caprock and what is its function?

A seal or caprock is an impermeable layer (such as shale, mudstone, salt, or dense limestone) that overlies the reservoir rock.

Its function is to prevent oil and gas from migrating further upward or laterally, effectively trapping the hydrocarbons in place.

What makes an effective caprock?

Low permeability

Sufficient thickness and continuity

Lack of fractures or faults that could allow hydrocarbons to escape