Petroleum System

Petroleum System

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. 

 Source Rock Hydrocarbon Generation
 

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

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.

Migration
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.

Accumulation

The phase in the development of a petroleum system during which hydrocarbons migrate into and remain trapped in a reservoir.

Reservoir

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 rock)

An impermeable rock that acts as a barrier to further migration of hydrocarbon liquids.

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.  commonly

• shale, mudstone
• anhydrite 
• salt, 
• A seal is a critical component of a complete petroleum system.

Trap

A configuration of rocks suitable for containing hydrocarbons and sealed by a relatively impermeable formation through which hydrocarbons will not migrate.

Traps are described as

• structural traps
  • Hydrocarbon traps that form in geologic structures such as folds and faults
    
    
    
    
    
• stratigraphic traps
  • Hydrocarbon traps that result from changes in rock type or pinch-outs, unconformities, or other sedimentary features such as reefs or buildups.
• A trap is an essential component of a petroleum system.
  
  
  

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Petroleum geology

Petroleum geology is a term used to refer to the specific set of geological disciplines that are applied to the search for hydrocarbons. It is principally concerned with the evaluation of seven key elements in sedimentary basins:

• Source
• Reservoir
• Seal
• Trap
• Timing
• Maturation
• Migration

 all these elements must be assessed via a limited 'window' into the subsurface world, provided by one (or possibly more) explorationwells. These wells present only a 1-dimensional segment through the Earth and the skill of inferring 3-dimensional characteristics from them is one of the most fundamental in petroleum geology. Recently, the availability of cheap and high quality 3D seismic data has greatly aided the accuracy of such interpretation. The following section discusses these elements in brief. For a more in-depth treatise, see the second half of this article below.

Evaluation of the source uses the methods of geochemistry to quantify the nature of organic-rich rocks which contain the precursors to hydrocarbons, such that the type and quality of expelled hydrocarbon can be assessed.

The reservoir is a porous and permeable lithological unit or set of units that holds the hydrocarbon reserves. Analysis of reservoirs at the simplest level requires an assessment of their porosity (to calculate the volume of in situ hydrocarbons) and their permeability (to calculate how easily hydrocarbons will flow out of them). Some of the key disciplines used in reservoir analysis are the fields of stratigraphy, sedimentology, and reservoir engineering.

The seal, or cap rock, is a unit with low permeability that impedes the escape of hydrocarbons from the reservoir rock. Common seals includee vaporites, chalks and shales. Analysis of seals involves assessment of their thickness and extent, such that their effectiveness can be quantified.

The trap is the stratigraphic or structural feature that ensures the juxtaposition of reservoir and seal such that hydrocarbons remain trapped in the subsurface, rather than escaping (due to their natural buoyancy) and being lost.

Analysis of maturation involves assessing the thermal history of the source rock in order to make predictions of the amount and timing of hydrocarbon generation and expulsion.

Finally, careful studies of migration reveal information on how hydrocarbons move from source to reservoir and help quantify the source (orkitchen) of hydrocarbons in a particular area.

Major subdisciplines in petroleum geology

Several major subdisciplines exist in petroleum geology specifically to study the seven key elements discussed above.

Analysis of source rocks

In terms of source rock analysis, several facts need to be established. Firstly, the question of whether there actually is any source rock in the area must be answered. Delineation and identification of potential source rocks depends on studies of the local stratigraphy, palaeogeographyand sedimentology to determine the likelihood of organic-rich sediments having been deposited in the past.

If the likelihood of there being a source rock is thought to be high, then next matter to address is the state of thermal maturity of the source, and the timing of maturation. Maturation of source rocks (see diagenesis and fossil fuels) depends strongly on temperature, such that the majority of oil generation occurs in the 60° to 120°C range. Gas generation starts at similar temperatures, but may continue up beyond this range, perhaps as high as 200°C. In order to determine the likelihood of oil/gas generation, therefore, the thermal history of the source rock must be calculated. This is performed with a combination of geochemical analysis of the source rock (to determine the type of kerogens present and their maturation characteristics) and basin modelling methods, such as backstripping, to model the thermal gradient in the sedimentary column.

Analysis of reservoir

The existence of a reservoir rock (typically, sandstones and fractured limestones) is determined through a combination of regional studies (i.e. analysis of other wells in the area), stratigraphy and sedimentology (to quantify the pattern and extent of sedimentation) and seismic interpretation. Once a possible hydrocarbon reservoir is identified, the key physical characteristics of a reservoir that are of interest to a hydrocarbon explorationist are its porosity and permeability. Traditionally, these were determined through the study of hand specimens, contiguous parts of the reservoir that outcrop at the surface and by the technique of formation evaluation using wireline tools passed down the well itself. Modern advances in seismic data acquisition and processing have meant that seismic attributes of subsurface rocks are readily available and can be used to infer physical/sedimentary properties of the rocks themselves.

for see more : http://www.knowledgerush.com/kr/encyclopedia/Petroleum_geology/

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Petroleum Geology.. History

In the essay, "A thirst for oil…Man's search for petroleum" by R. Williams, he states that oil has a big influence on why and what man does. At first oil was used for liniment, medicine, and cementing walls.

 All over the world, there are natural places you can recover oil and gas, such as the U.S. and Canada, but the Middle East was the biggest supplier for U.S. oil after World War II.

Petroleum first uses included:

• Sealants, lubricants, and medicinal purposes.

• A product of petroleum, kerosene, was used as fuel for light and heating.

• The major current use for petroleum came with the invention of the automobile.

• Petroleum is not just use for fuel; oil is also used in plastics, fertilizers, and cosmetics.

Early on searching for oil and gas was more difficult, found only near natural oil and gas seeps.

Later on, geochemistry helped in finding that deposits under the earth's surface. Technology is aided in the search for oil, but instead of just a guessing game it became an actual science with satellites, drilling, and saline injections, helping to uncover what is beneath the surface of the earth.

Petroleum is a limited resource that the world is obsessed with using until it is gone. People are willing to do whatever it takes to get all the oil they can, no matter what it means.

 R. Williams writes, "From a substance seeping from that earth's surface, ignored by man for centuries, to a commodity that has unprecedented implications in global relations, petroleum has truly become a man's black goal."

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