Petroleum is the general term used for all the natural hydrocarbons found in rock. The petroleum is a complex mixture of hundreds of different hydrocarbons. The hydrocarbons fall into several natural series of which paraffin series is most familiar.
Origin of petroleum
It is now universally believed that petroleum and natural gas are of organic origin. They originate from slow decomposition of lower forms of marine organisms such as foraminifers, diatoms, algae, ostracods etc. the process of formation of petroleum may be summarized as follows.
- In coastal waters a large number of marine organisms thrive. Hence in offshore sedimentary basins huge amount of organic matter is deposited along with muddy sediments. Because in the bottom of stagnant water there is deficiency of oxygen the organic matter is protected from oxidation. Under such conditions anaerobic bacteria extract oxygen from the organic matter and transform it into fatty and waxy substances.
- During the millions of years of deep burial the organic matter is converted into oil and gas by the slow chemical reactions.
Migration of petroleum
The fine grained muddy sediments in which petroleum originates are called source rocks. The source rocks of petroleum are generally shales, silts and limestone. The petroleum migrates from the source rock into adjacent porous and permeable rock and accumulates there to form a pool. Such permeable rocks are called reservoir rock. The common reservoir rocks are sandstone, conglomerates, porous limestone, fractured shale and jointed igneous and metamorphic rocks. The causes for the migration of petroleum are:
- Compaction of the source rock
- Buoyancy effect
- Capillary effect
- Water flushing
In an oil pool the oil floats on the top water and above the oil there is usually a lens of natural gas.
Oil traps:-the oil traps migrates outward and upward from the source rock and passes into the porous reservoir rock. The migration of oil continues until it meets a suitable structure where its lateral as well as upward movement is checked. At such place the oil accumulates to from an oil pool. Such places are called oil traps.
- The porous reservoir rocks must have a favorable structure such as an anticlinal foil or dome to hold oil.
- There must be an impervious cap rock to check the upward migration of oil. The common cap rocks are shales, clay salt, gypsum and dense limestone.
- The structure deformation of rock must not be very severe. Intensely fractured rocks may render traps ineffective by causing leakage.
Types of oil traps
The oil traps are classified into two groups:
- Structure traps
- Stratigraphic traps
Structure traps:- the structure oil traps are formed as a result of folding, faulting and igneous intrusions.
- Anticlines and Domes:- the anticlines and dome are the most important because they form oil traps in practically all the large oil fields of the world. Here the oil and gas migrate up the limb and collect at the crest below a cap rock.
- Faults:- when a fault affects inclined strata a reservoir rock may be blocked off by an impervious shales thereby creating an oil trap.
- Salt domes:- where salt domes intrude into the sedimentary rocks good oil traps are formed. Here the oil accumulates near the upturned edges of the reservoir rock which are sealed by the salt.
- Igneous intrusions:- the volcanic necks and dykes may seal the upturned edges of reservoir rock to from oil traps.
Stratigraphic traps:- the stratigraphic oil traps are formed as result of lateral and vertical changes in the permeability of the reservoir rocks. These changes are caused by variation in the conditions during the deposition of rock. Some of the important stratigraphic oil traps are as follows.
- Unconformities in the rock sequence often give rise to oil traps.
- The shales which are the source rock of petroleum may contain lenses of sandstone. The oil may accumulate in these sandstone lenses.
Porous sandstone may wedge out thereby creating an oil trap