Petroleum and natural gas were formed from the remains of ancient marine organisms. Over millions of years, plankton and algae accumulated on the ocean floor and were buried under layers of sediment. The organic material was converted into oil and gas through heat and pressure over geologic time. These hydrocarbons migrated upward until they were trapped underground in reservoirs within porous rock formations by impermeable caps such as shale. Natural gas is primarily methane and formed similarly through the thermal maturation of buried organic matter. It is found in conventional reservoirs as well as unconventional sources like shale.

1. Origin of petroleum
and Natural gas

2. ORIGIN OF PETROLEUM
To be considered are these 3 stages:
•Formation of Petroleum
•Migration of Petroleum
•Accumulation of Petroleum

3. Formation of Petroleum
• There are 2 theories concerning the formation
of petroleum:
• The Inorganic theory
• The Organic theory

4. Inorganic Theory
• Berthelot (1860) & Dmitri Mendeleev (1902):
• Iron carbide in the earth’s mantle would react
with percolating water to form methane.
• FeC2+ 2H2O →CH4+ FeO2
• This theory is called Deep-Seated Terrestrial
Hypothesis.

5. • Sokoloff (1890):
• Hydrocarbons precipitated as rain from the
original nebular matter from which solar
system was formed.
• The hydrocarbons were the ejected from
earth’s interior onto surface rocks.
• This theory is called Extraterrestrial
Hypothesis

6. Problems with Inorganic Theories :
• No field evidence that inorganic processes
have occurred in nature.
• Commercial accumulations are restricted to
mainly sedimentary basins.
• Accumulations are absent from igneous and
metamorphic rocks.

7. Organic Theory
• Early marine life forms living on earth were
primarily plankton (rich in hydrogen & carbon)
• Over 95% of living matter in the ocean is
plankton.

8. • As these plankton died, their remains were
captured by the process of erosion and
sedimentation.

9. • Successive layers of organic-rich mud & silt
covered preceding layers of organic-rich
sediments & overtime created layers on the
sea floor rich in the fossil remains of previous
life.

10. • Thermal maturation processes (decay, heat,
pressure) slowly converted the organic matter
into oil & gas over periods of millions of
geologic years.
• Conversion of the organic material is called
Catagenesis.
• It usually occurs under anaerobic conditions.

11. • Clay & silt are carried together with the dead
organic remains & deposited under deltaic,
lacustrine & marine conditions to form Source
rocks.
• Black-coloured, organically-rich shales
deposited in a non-oxidizing, quiet marine
environment are considered the best source
rocks.

12. Shales

13. • Shale rock = 99% clay
mineral + 1% organic
material.
• Kerogen
 Insoluble
High molecular weight
Polymeric compound

14. • Thermal alteration of kerogen forms crude oil
by increasing the carbon contents.
• At shallow depths (< 3,000 ft), bacteria actions
on organic materials form Biogenic
Gas(natural gas).
• At great depths (high temperature &
overburden), Thermogenic Gas is formed.
• Later stages of thermogenesis will form wet
gas and condensate.

15. Supports for organic hypothesis
• Carbon & hydrogen are the primary
constituents of organic material, both plant &
animal.
• Nitrogen & Porphyrins are found in organic
matter & in many petroleum.
• Porphyrins are chlorophyll derivatives in
plants & blood derivatives in animals.

16. Migration of Petroleum
• Produced hydrocarbons migrates upward from
the deeper, hotter parts of the basin through
permeable strata into suitable structures.
• There are 2 stages of migration:
• Primary Migration –Kerogen transformation
causes micro-fracturing of the impermeable &
low porosity source rock, allowing hydrocarbons
to move into more permeable strata.

17. Secondary Migration
• The generated fluids move more freely along
bedding planes and faults into a suitable
reservoir structure.
• Migration can occur over several tens of
kilometers in lateral directions.

18. Accumulation of Petroleum
• Accumulation & storage occur when the
migrating fluids encounter an impermeable shale
or dense layer of rock.
• This is called a Trap.
• After accumulation, the fluids tend to stratify
according to their relative densities:
• Gas
• Oil
• Water

20. • If the migrating fluids do not encounter a trap,
they tend to flow to the surface or deposited
on the ocean floor.
• Examples are:
• Seepages
• Escaping natural gas
• Bituminous lakes

22. • Reservoirs are composed of either:
• Clastic formation –sandstone reservoirs made
from silicates (quartz, SiO2).
• Carbonate formation –carbonate reservoirs
made from detritus(coral or shell fragments).
• Reservoirs must be:
• Porous
• Permeable
• Trapped.

23. Anticlinal traps
• An anticline is a structural trap formed by the
folding of rock strata into an arch-like shape.
• The rock layers in an anticlinal trap were
originally laid down horizontally and then
earth movement caused it to fold into an arch-
like shape called an anticline.

25. fault trap
• This trap is formed by the movement of
permeable and impermeable layers of rock
along a fault plane.
• In some cases, there can be an impermeable
substance smeared along the fault surface
(such as clay) that also acts to prevent
migration. This is known as clay smear.

27. Stratigraphic traps
• Stratigraphic traps are formed as a result of
lateral and vertical variations in the
thickness, texture, porosity or lithology of
the reservoir rock.
• Examples of this type of trap are an
unconformity trap, a lens trap and a reef trap.

29. A salt dome trap
• A salt dome trap is an area where oil has been
trapped underground by salt pushing
upward.
• A salt dome trap is where a mass of salt from
deep in the Earth's crust is pushed upward
forming oil traps in a rock layer containing oil.
• Salt dome traps form where bodies of salt
flow upward and pierce overlying rock layers.

31. combination traps
• Traps with two or more trapping
elements are called combination traps

32. What is natural gas?
• Natural gas is a fossil energy source that formed
deep beneath the earth's surface. Natural gas
contains many different compounds.
• The largest component of natural gas is
methane, a compound with one carbon atom and
four hydrogen atoms (CH4).
• Natural gas also contains smaller amounts
of natural gas liquids (NGL, which are
also hydrocarbon gas liquids), and
nonhydrocarbon gases, such as carbon dioxide
and water vapor.
• We use natural gas as a fuel and to make
materials and chemicals.

33. How did natural gas form?
• Millions to hundreds of millions of years ago and
over long periods of time, the remains of plants
and animals (such as diatoms) built up in thick
layers on the earth’s surface and ocean floors,
sometimes mixed with sand, silt, and calcium
carbonate.
• Over time, these layers were buried under sand,
silt, and rock.
• Pressure and heat changed some of this carbon
and hydrogen-rich material into coal, some into
oil (petroleum), and some into natural gas.

35. Where is natural gas found?
• In some places, natural gas moved into large cracks and
spaces between layers of overlying rock. The natural gas
found in these types of formations is sometimes
called conventional natural gas.
• In other places, natural gas occurs in the tiny pores (spaces)
within some formations of shale, sandstone, and other
types of sedimentary rock.
• This natural gas is referred to as shale gas or tight gas, and
it is sometimes called unconventional natural gas.
• Natural gas also occurs with deposits of crude oil, and this
natural gas is called associated natural gas.
• Natural gas deposits are found on land, and some are
offshore and deep under the ocean floor.
• A type of natural gas found in coal deposits is called coalbed
methane.