Hydrocarbon traps, migration and accumulation of petroleum

Hydrocarbon traps, it's definition working how they occur and their types

1. pResenters Ahmad nawaz 2k23-P&G-01 Muhammad wasi 2k23-p &g-03 yahya khan 2k23-p &g-10 Subject Petro physics Department Petroleum & Gas engineering presented to miss Momina khan  

2. Topics: Hydrocarbon Traps Migration and Accumulation of petroleum Properties of subsurface fluids

3. Hydrocarbon traps Introduction: A hydrocarbon trap is a natural geological structure in the Earth’s subsurface that prevents oil and gas from escaping and allows them to accumulate in one place. These traps form when porous rocks (that can hold oil or gas) are covered by non-porous, sealing rocks that stop the hydrocarbons from moving further upward.
Without such a trap, the oil and gas would continue to migrate toward the surface and be lost. So, hydrocarbon traps are essential for forming oil and gas reservoirs that can be discovered and produced. Essential Elements of a Trap Every effective trap includes: 1. Source Rock: where hydrocarbons are generated. 2. Reservoir Rock: porous and permeable rock to store hydrocarbons. 3. Seal Rock (Cap Rock): impermeable layer to prevent escape of hydrocarbons.

4. Why traps are important Without traps, hydrocarbons would migrate to the surface and dissipate.
Traps determine whether a petroleum system is commercially viable.
Exploration and seismic surveys often focus on identifying trap structures Classification of traps 1. Structural traps: Structural traps are a type of hydrocarbon trap formed by the deformation of rock layers due to tectonic forces such as folding or faulting. These deformations create shapes like anticlines (arches) or fault blocks, which allow oil and gas to accumulate beneath a sealing (impermeable) layer. Common types of structural traps: 1. Anticline Trap: Upward-arched rock layers trap hydrocarbons at the crest. 2. Fault Trap: Movement along a fault places a reservoir rock against a seal.

5. 2. Stratigraphic traps Stratigraphic traps are formed due to changes in rock layers (strata), not by folding or faulting. These changes can be in rock type, thickness, or porosity, which create natural barriers that trap oil and gas. Common types of stratigraphic traps: 1. Pinch-out Trap: The porous reservoir rock gradually thins out (pinches out) and disappears between impermeable rocks, trapping hydrocarbons. 2. Reef Trap: Reef rocks (like ancient coral reefs) are highly porous and surrounded by impermeable mud or shale, forming a trap. 3. Unconformity Trap: Older rock layers are eroded, and then new sediments are deposited above them, creating a seal and trapping oil and gas in the old porous rocks.

6. Migration and accumulation of petroleum Introduction: Migration of petroleum refers to the movement of hydrocarbons (oil and natural gas) from the source rock where they are generated to the reservoir rock where they accumulate. This process is crucial in petroleum geology, as it determines where oil and gas can be extracted. How Migration Begins 1. Formation of Petroleum: Organic matter (mainly from microscopic plants and animals) accumulates in fine-grained sedimentary rocks like shale. With increasing depth, pressure, and temperature over geological time, this organic matter is transformed into liquid and gaseous hydrocarbons. 2. Pressure Buildup: As more hydrocarbons are generated, pressure builds up within the source rock because it has low permeability (limited ability for fluids to pass through). This pressure eventually forces the hydrocarbons out of the source rock.

7. Movement Through Rocks Once hydrocarbons leave the source rock, they migrate through more permeable rocks like sandstones or fractured carbonates. This movement is driven primarily by: Buoyancy: Oil and gas are lighter than water, so they naturally move upward through water-filled pore spaces. Pressure Gradients: Fluids move from areas of high pressure to low pressure.
Capillary Forces: Interactions between fluids and the pores in rocks can affect how and where hydrocarbons move. Pathways of Migration Petroleum may travel: Vertically: Through cracks, faults, or fractures in the rock layers. Laterally: Along sloping permeable layers, often over long distances.
Migration continues until the hydrocarbons are either:
Trapped by an impermeable cap rock (forming an oil or gas reservoir), or
Lost to the surface through seepage (if no trap is present).

8. Final Accumulation The journey ends when hydrocarbons are halted by a structural or stratigraphic trap, typically sealed by a cap rock like shale or salt. This is where oil and gas accumulate in sufficient quantities to be extracted by drilling. Types of Petroleum Migration Petroleum migration occurs in two main stages: 1. Primary Migration Definition: The movement of hydrocarbons from the fine-grained source rock (usually shale) into the adjacent carrier bed (typically a porous rock). Process: As organic matter in the source rock is buried and subjected to heat and pressure, it transforms into oil and gas.
Because source rocks have low permeability, the generated hydrocarbons are expelled due to pressure buildup from compaction and gas expansion.

9. 2. Secondary Migration Definition: The movement of hydrocarbons from the carrier bed to the reservoir rock, where they accumulate. Process: Hydrocarbons move through porous and permeable rocks (sandstones, carbonates) under buoyant forces.
They travel upward due to their lower density compared to surrounding water.
Movement continues until hydrocarbons are trapped by an impermeable cap rock (like shale or salt), forming an oil or gas reservoir.

10. Accumulation of petroleum Accumulation of petroleum refers to the process by which oil and natural gas collect in a porous and permeable rock (reservoir rock), usually beneath an impermeable rock (cap rock) that traps the hydrocarbons and prevents them from escaping. Key Elements of Petroleum Accumulation: 1. Source Rock The origin of petroleum, where organic-rich sediments are buried and subjected to heat and pressure, forming hydrocarbons. 2. Migration Hydrocarbons move (migrate) from the source rock to the reservoir rock.
Primary migration: from source rock to a carrier bed.
Secondary migration: through permeable layers to the reservoir.

11. 3. Reservoir Rock A porous and permeable rock (e.g., sandstone or limestone) where hydrocarbons accumulate.
Must have enough porosity to store hydrocarbons and permeability to allow fluid flow. 4. Cap Rock (Seal Rock) An impermeable layer (like shale or salt) that traps hydrocarbons and prevents them from rising further. 5. Trap Structure A geological structure that holds the hydrocarbons in place. Common traps include: Anticline traps (arched rock layers) Fault traps (caused by rock displacement)

12. Properties of subsurface fluids Subsurface fluids are liquids and gases found beneath the Earth’s surface, primarily in the pores of rocks. These include water, oil, natural gas, and formation brines. Their properties are critical in fields like petroleum engineering, hydrogeology, and geothermal energy. Here are the key properties: Hydrostatic Pressure Gradient Definition: The rate of increase in fluid pressure with increasing depth due to the weight of the overlying fluid. Formula: P= p.g.h where: p = fluid density (kg/m³) g = gravitational acceleration (~9.81
m/s²) h = depth (m)

13. Lithostatic pressure gradient The lithostatic pressure gradient is the rate at which pressure increases with depth in the Earth’s crust due to the weight of overlying rock. Temperatur: Increases with depth due to the geothermal gradient.
Affects fluid viscosity, density, and phase behavior (e.g., oil becoming gas at higher temperatures). Geothermal gradient The geothermal gradient is the rate at which the Earth’s temperature increases with depth below the surface. It reflects how heat from the Earth’s interior moves toward the surface. Viscosity Definition: A fluid’s resistance to flow. Oil: More viscous than water or gas. Gas: Very low viscosity.
High-viscosity fluids flow more slowly in porous media.

14. Compressibility Describes how volume changes with pressure.
Gases are highly compressible; liquids are nearly incompressible.
Affects fluid flow and pressure response in reservoirs. Phase Behavior Subsurface fluids can exist in multiple phases (liquid, gas) depending on temperature and pressure.
Critical for predicting gas-oil ratios and production strategy.

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