25 - What Is A Hydrocarbon System?...Basics Of Petroleum Geology
Hydrocarbons are extracted from reservoirs commonly made of sedimentary rocks. But a hydrocarbon reservoir is just one of the six elements that constitute a hydrocarbon system:
A source rock is a sedimentary rock that has a high concentration of organic material. It is a necessary element of the hydrocarbon system and without it there would have never been oil. The two main types of source rocks:
shale, consisted mainly of clay (most common)
limestone, consisted mainly of calcite (less common)
As other sedimentary rocks came off the land as a result of erosion, they accumulated on top of the source rock. Over millions of years, the sedimentation process buried the source rock (with its organic material) deep enough where temperature and pressure rose to a point it broke down the organic matter, which in turn released its hydrocarbon content. This process is called Thermal Maturation.
Thermal Maturation doesn't begin until temperature reaches 60 degrees Celsius (140 degrees Fahrenheit), which usually corresponds to a depth of around 2,000 meter.
Crude Oil and Natural Gas have different temperature ranges for them to be formed:
Oil : 60 -160 degree Celecius (140 - 320 degrees Fahrenheit). This is called the oil window. outside this range, oil can't be formed
Natural Gas: >160 degrees Celecius (>320 degrees Fahrenheit) which corresponds to a depth of around 5,000 meters. This temperature is high enough to further decompose the organic material into lighter hydrocarbon chains such as methane
The process of thermal maturation is a long process and takes millions of years. Accordingly, source rock should remain long enough in such a temperature range to form oil or gas.
Shale Oil and Shale Gas reservoirs require only two elements, Source rock and Thermal Maturation
Except for Shale Oil and Shale Gas, a Reservoir rock is needed to store all the hydrocarbons that broke out the organic material. There are two common types of reservoir rocks (beside the source rock in the case of shale oil and shale gas):
Sandstone reservoirs, the largest is Burgan field in Kuwait
Carbonate reservoirs, the largest is Ghawar field in Saudi Arabia
Porosity and Permeability are important factors for reservoir quality, good porosity defines reservoir capacity to store the hydrocarbon, while permeability defines the rocks ability to allow hydrocarbon to flow through the pores.
How does oil moves out the source rock and reaches the reservoir rock? a migration path is needed to achieve this. Migration occurs when hydrocarbons are expelled from the source rock and start to move to adjacent permeable rocks. They usually move upward due to density different from water (except for heavy oil).
Rock below surface are usually filled with brine (saline water)
As hydrocarbons migrate upward and reach the reservoir rock, they need to hit a rock that prevents further upward migration, in order to allow accumulation of hydrocarbon in the reservoir rock. This rock is called Cap rock and is:
impermeable (or low in permeability), such as shale
should overlie the reservoir rock
While cap rock is resistant to migration, it should have a shape that creates a trap to prevent lateral migration. There are two types of traps:
Structural caused by structural features such as anticlines, faults, and salt domes
Strati graphic caused by a change in rock properties making it less permeable
Hydrodynamic caused by a differences in water pressure due to a tilted hydrocarbon-water contact. an example is oil trapped in the Rubiales field in Colombia
A Hydrocarbon system usually requires all elements in order to become a prospect as per the Organic theory proponent.
However; as we thrive to discover more hydrocarbons, we developed technology that allows oil extraction from Shale Oil and Shale Gas, making it as a non-conventional reservoir, alongside with Oil Sands. Probably in the near future, we will see more non-conventional reservoirs in more frontier areas such as deep water.
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