Petroleum System

Petroleum system is a conceptual framework used in the field of geology and petroleum exploration to understand the processes and elements necessary for the formation, migration, and accumulation of hydrocarbons (oil and gas).

The Petroleum System consists of a mature source rock, migration pathway,reservoir rock, trap and seal. Appropriate relative timing of formation of these elements and the processes of generation, migration and accumulation are necessary for hydrocarbons to accumulate and be preserved.

The components and critical timing relationships of a petroleum system can be displayed in a chart that shows geologic time along the horizontal axis and the petroleum system elements along the vertical axis.

Exploration plays and prospects are typically developed in basins or regions in which a complete petroleum system has some likelihood of existing.

Petroleum System elements

The petroleum system is a conceptual model that describes the processes that lead to the formation and accumulation of oil and gas. It consists of five essential elements:

A source rock: This is a rock that contains organic matter that can be converted into oil and gas.
A reservoir rock: This is a rock that can store oil and gas.
A trap: This is a structure that prevents the oil and gas from escaping from the reservoir rock.
A seal: This is a rock that prevents fluids from migrating from one reservoir rock to another.
Overburden: This is the rock that lies above the reservoir rock and helps to protect it from erosion.

Source Rock

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

Source rocks are a specific type of sedimentary rock that plays a critical role in the formation of hydrocarbons, including oil and natural gas. These rocks are characterized by a high content of organic matter, primarily derived from the remains of marine organisms such as plankton and algae.

As source rocks become progressively buried, they are subjected to increasing pressure and temperature. This thermal maturation process plays a vital role in hydrocarbon generation. The breakdown of complex organic molecules within the source rock occurs through a series of geochemical reactions. The specific types and relative abundance of hydrocarbons generated depend on the initial organic matter composition (kerogen type) and the thermal maturity achieved.

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.

Maturation

Maturation refers to the process by which organic matter in source rocks undergoes thermal and chemical changes to produce hydrocarbons. The maturity level of the source rock influences the type and quality of hydrocarbons generated.

Migration Pathway

Migration pathway refers to the route or course that hydrocarbons (oil and natural gas) take as they move from their source rock to a potential reservoir rock. Understanding these pathways is critical for predicting where hydrocarbons might accumulate and informing exploration strategies.

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.

Migration Timing

The timing of hydrocarbon generation, migration, and accumulation is essential for the development of a petroleum system. Understanding the geological history and timing of these processes helps in predicting the presence of oil and gas reserves.

Accumulation

Accumulation refers to the process by which hydrocarbons (oil and natural gas) migrate from their source rock and become concentrated within a reservoir rock. These accumulations, if commercially viable, form oil and gas fields.

Reservoir Rocks

Reservoir rocks are sedimentary rocks that possess the necessary properties to store and transmit accumulations of fluid hydrocarbons (oil and natural gas) in economically viable quantities. Sedimentary rocks are the most common reservoir rocks because they have more porosity than most igneous and metamorphic rocks.These properties can be broadly categorized as porosity and permeability.

Porosity: Porosity refers to the pore space within a rock, expressed as a percentage of the total rock volume. It represents the void space available to store fluids. Ideal reservoir rocks have sufficient porosity to hold significant volumes of hydrocarbons.

Permeability: Permeability is a measure of a rock's ability to transmit fluids through its pore spaces. It is influenced by the size, shape, and connectivity of the pores within the rock. Highly permeable reservoir rocks allow for efficient flow of hydrocarbons, facilitating their extraction.

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 Rocks)

Seal rocks, also known as cap rocks, play a critical role. An impermeable rock that acts as a barrier to further migration of hydrocarbon liquids. These seemingly ordinary rocks act as silent guardians, ensuring the success of a potential oil or gas field. Seal rocks are fine-grained sedimentary rocks with exceptionally low permeability. This means they have minimal interconnected pore spaces, hindering the flow of fluids like water and hydrocarbons. This impermeability is the key characteristic that allows them to trap hydrocarbons within a reservoir rock.

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.

Types of Seal Rocks

While various rock types can exhibit sealing properties, some are particularly effective:

Shale: Shale, composed of clay minerals and tiny rock particles, is a classic seal rock due to its extremely low permeability.

Evaporites: Rock formations like anhydrite (calcium sulfate) or salt (halite) can be excellent seals due to their dense and tightly packed nature.

Trap Formation

A trap is a geological formation that acts like a cage. It allows hydrocarbons to enter a permeable reservoir rock but hinders their further upward movement. This trapping mechanism is essential for the concentration of oil and gas, leading to potentially recoverable resources.

Types of Traps

Traps can be broadly classified into two main categories based on the geological processes responsible for their origin:

Structural Traps: These traps arise due to tectonic deformation of sedimentary rock layers within the Earth's crust. This deformation can manifest as folds, faults, or a combination of both, creating geometries that favor hydrocarbon accumulation.

Fault Traps: Juxtaposition of impermeable formations against a reservoir rock due to fault movement creates a barrier to further hydrocarbon migration.
Fold Traps: Upward or downward folds in the strata can form a trap if an impermeable caprock overlies the reservoir rock at the apex or nadir of the fold.

Stratigraphic Traps: Unlike structural traps, these are formed by variations in the depositional environment or diagenetic processes within the sedimentary sequence. These variations can lead to changes in rock properties, influencing fluid flow and creating trapping geometries.

Pinch-out Traps: A reservoir rock that wedges out laterally against an impermeable seal can form a trap, similar to the thinning and termination of a permeable sandstone layer at the edge of a depositional basin.
Unconformity Traps: An unconformity, representing a surface of erosion or non-deposition, can juxtapose a permeable reservoir rock with an overlying impermeable layer, creating a trap.

Overburden Rocks

The overburden is the rock that lies above the reservoir rock. It helps to protect the reservoir rock from erosion and from the intrusion of fluids from other formations.

The petroleum system is a complex system, but it is a valuable tool for understanding the distribution of oil and gas resources and for predicting where new discoveries might be made.

Conclusion

The study of petroleum systems is fundamental to the success of oil and gas exploration, providing a systematic approach to understanding the geological processes involved in the formation and accumulation of hydrocarbons. Successful exploration and production efforts require a comprehensive understanding of the geological conditions and processes within a given basin or region.