INTRODUCTION:
The word
petroleum is derived from Latin were Petra means rock or stone & Oleum
means oil it combinedly called as petroleum. It is Fossil Fuel. Petroleum is
generic name for hydrocarbon, including natural gas, crude oil and their
products. Petroleum is organic compound; it is a mixture of mainly carbon,
hydrogen with minor amounts of nitrogen, oxygen and sulphur and some impurities
(heavy metals, sand etc.). Types of Petroleum:
·
Crude
Oil
·
Natural
Gas
·
Semi-Solid
& Solid form
They were formed by the natural decomposition
of dead plants and animals which takes about several million years to form
petroleum under extremely high temperature and pressure condition beneath the
surface of the earth.
During decomposition process the organic matter
is get converted into kerogen, which is then converted into oil and gas
(Petroleum). This oil and gas were mostly occurs in the sedimentary rock
because those rocks have good porosity and permeability which is most important
for any rock to bear the naturally formed hydrocarbon in it.
Once the organic matter is converted into
petroleum it occurs in the source rock were the porosity and permeability will
be low so that they get migrated upwards to the more porous and permeable rock
layer which is mostly a sedimentary rock layer above this rock there will be an
impermeable rock or cap rock which prevents the movement of hydrocarbon to the
earth`s surface.
This rock layer which bears the oil and gas in
it is called as Reservoir which is basically storage medium where the oil and
gas get stored. During exploration these reservoir rock layer are identified by
the geologist by using the seismic waves and their approximate size and shape
can be determined then the field undergo further survey operation to obtain
additional and more accurate information about that particular reservoir rock
in order to drill and produce the oil and gas in an economical manner.
Once the well is drilled & completed the
well is ready to produce the hydrocarbon from the reservoir. The produced
hydrocarbon from the reservoir reaches the surface where it is separated into
gas & liquid phase and then the oil is obtained and then it is stored and
transported to the refinery for further separation components from that oil.
The various components of petroleum are
separated by using a fractional distillation column i.e. the separation is
based on the difference in their boiling points.
ORIGIN:
There are many theories were present regarding
the origin of petroleum. The theories must explain the following
characteristics associated with petroleum, the association of brine, presence
of nitrogen & sulphur compounds in it, presence of haemin & chlorophyll
in it, Its optically active nature. Three important theories are:
·
Mendeleev`s
carbide theory or inorganic theory
·
Engler`s
theory or organic theory
·
Modern
Theory
Mendeleev Theory: It states that
petroleum is formed by action of steam or water on metallic carbide on inner
portion of the earth`s crust in the presence of high temperature. But it failed
explain the presence haemin, chlorophyll, S, N2 & optical
activity.
Step 1:
CaC2 + 2H2O à C2H2 +
Ca (OH)2 . Step 2: 3C2H2
à C6H6.
Engler`s Theory: He proposed that
petroleum is formed by the decay & decomposition of dead animals under high
temperature and pressure. They explain the presence of brine, optically active,
S & N. It fails to explain the presence of chlorophyll in petroleum, resins
in oil.
Modern Theory: It is vegetable origin
of petroleum. It proposed that they formed by decomposition of both marine
animals and vegetable matter such as plants etc. to form the oil containing
chlorophyll and medium to high resin contents in it. It also explains the
presence of coal field near the oil deposits.
FORMATION:
The formation of
petroleum from organic matter defines the conversion of kerogen to oil and gas.
During burial of sediments increase in temperature results in progressive
change in the composition of kerogen.
There successive stages are diagenesis, catagenesis & metagenesis.
|
DIAGENESIS
Decrease in oxygen & increase in carbon
with depth.
Co2 & some heavy N, S, O compounds
are released.
Source rocks are considered as immature at
this stage.
|
|
CATAGENESIS
Decrease in hydrogen content and H/C ratio
takes place due to generation & release of hydrocarbon.
Main Oil generation and cracking produces
wet gas with increasing proportion of methane.
|
|
METAGENESIS
Begins at temperature exceeding 175ºC.
Rearrangement of aromatic sheet occurs.
Dry gas is generated at this stage.
|
MIGRATION:
It is a process of movement of oil and gas
between the porous and permeable rock layer near the source rock. There are
three types of migration they were:
·
Primary Migration: - It is process in which the hydrocarbon is
expelled from the source rock into the adjacent permeable carrier rock bed. It
occurs due to compaction of the source rock.
·
Secondary Migration: - The movement of hydrocarbon within the
carrier rock or reservoir beds from source area to trap. It occurs due to the
density difference and presence permeable formation.
·
Tertiary Migration: - The movement of hydrocarbon from one trap to
the another trap or seeps out.
ACCUMULATION:
Due to migration of
hydrocarbon from the source rock to the carrier bed then finally to the trap.
The trap is geological structure present beneath the surface of the earth they
mostly have impermeable layer as its top section which is important layer to prevent
the upward migration of hydrocarbon to the earth surface and helps the hydrocarbon
to get stored or remain inside the subsurface. These traps are the structures
were the hydrocarbons mostly get accumulated together after their migration
from source rock.
There two main type of mostly found they are:
· Structural
Traps
o
Anticlinal
Traps
o
Fault
· Stratigraphic
Traps
o
Pinchout
o
Truncated
o
Salt
Dome
REFERENCE:
Introduction to Petroleum Engineering Book by John
R Fanchi and Richard L. Christiansen.
Elements of Petroleum Geology Book by
Richard Selley.
by: Naveen Gokul
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