CONVENTIONAL RESERVOIR:
The conventional reservoir is a porous rock formation that contains hydrocarbons that have migrated (move from one place to another) from the source rock.
OR
In other words, we can say that the fluid characteristics (oil or natural gas) of conventional reservoirs have got a permit to flow into the wellbores.
UN CONVENTIONAL RESERVOIR:
The unconventional reservoir has hydrocarbons that were formed within the source rock but never migrated. Because they are tight reservoirs such as tight-gas sands, shales, coal beds.
It requires special recovery Methods, Like Injection of Fluids (Gas & Water) & different EOR (Enhanced Oil Recovery) Methods.
SOURCE ROCK:
A source rock is a rock that has the capability of generating large quantities of hydrocarbons.
The source rock is might be shale which is a fine grain rock (indicate magmas that rapidly cooled at or near the Earth's surface) that comes mostly from clay.
Another example of the source rock is limestone which is composed of calcite CaCO3.
The first factor for drilling a well is to confirm whether a source rock is present.
To be a source rock, a rock must have three features:
• Quantity of organic matter. (Petroleum generation occurs over long periods of time—millions of years. In order for petroleum generation to occur, organic matter such as dead plants or animals must accumulate in large quantities. These organic matters can be deposited along with sediments and later buried as more sediments accumulate on top.)
• Quality capable of yielding moveable hydrocarbons.
• Thermal maturity. (The organic material in source rocks is transformed into oil at temperatures between 65°C and 150°C.)
•
RESERVOIR ROCK:
As we all know, Sandstone and Limestone are two Sedimentary Rocks which are used as a reservoir rock.
A good reservoir rock must be a good porous, permeable, and contains Hydrocarbon as well.
Main reservoir rocks are either sandstones or carbonates.
More than 60% of the world's oil reserves are found in sandstone reservoirs.
Sandstone is a sedimentary rock composed mainly of sand-sized rock grains cemented together.
CAPROCK:
caprock. A relatively impermeable rock, commonly shale, anhydrite, or salt, that forms a barrier or seal above and around reservoir rock so that fluids cannot migrate beyond the reservoir. ... The permeability of a cap rock capable of retaining fluids through geologic time is ~ 10-6-10-8 darcies.
Whereas a good reservoir rock is expected to have good porosity and permeability, a cap rock should have very low porosity and permeability. Without a cap rock, hydrocarbon fluids will continue migrating upwards from the source rock to the reservoir rock and upwards until they get to the surface.
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