IFP SCHOOL
THE GRADUATE SCHOOL FOR ENERGY AND TRANSPORTATION PROFESSIONS
Where does petroleum
come from?
O
il (liquid hydrocarbon) and natural gas are fossil fuels, which come from the decomposition of organic
matter present in various concentrations in sediments. The progressive burial of sediments (subsidence
of several kilometers) means the organic matter is slowly transformed over millions of years.
THE
Micro-organisms
Lake
Plants
Continental
margin
Pollen
ORGANIC
Dead organisms are deposited on the
ground or sink to the bottom of the sea
or continental lakes. A large quantity of
organic matter is destroyed during its fall
between the surface and the bottom of
the ocean. Only 2% reaches the bottom of
the sea at shallow depths and only 0.02%
reaches the deep offshore sea bed.
Delta
Plankton, Algae
SEDIMENTS mainly consist of rock
fragments and minerals (clay particles,
fine sand grains …).
The organic matter which has been
preserved from destruction mixes with
mineral matter. The resulting dark
mud accumulates over time, forming
successive layers, several kilometers
thick in sedimentary basins (the sea
bed, lakes, deltas …).
MATTER
(plankton, bacteria, seaweeds, plants),
consists essentially of carbon, hydrogen,
nitrogen and oxygen. Depending on their
origin, various types of hydrocarbons
(«chemical types») can be defined.
SUBSIDENCE: The bottom of the basin
sinks under the influence of various geological
phenomena, which contribute to the burial
of sediments and to the preservation of the
organic matter.
As a result, pressure and temperature
increase within the accumulated sediments
(temperature increases about 3°C every 100
meters).
The petroleum system
SOURCE ROCK AND HYDROCARBON GENERATION
A fine-grained sedimentary rock containing at least 1 to 2% of organic matter can play the
role of a source rock.
Under the influence of temperature and pressure, the organic matter contained in the
source rock is gradually converted into kerogen, which is distributed throughout the rock
as small particles. The kerogen starts to mature at temperatures of 60 to100°C (at depths
below 2000m). The increasing temperature and pressure, as burial depth increases,
breaks and reorganizes molecules. The kerogen starts to generate hydrocarbons: oil first
(oil window) and gas at higher temperatures and pressures (gas window).
Hydrocarbon %
Biogenic origine gas
50
Temperature °C
- a source rock, which generates the hydrocarbons,
- a porous and permeable reservoir rock, which hosts
hydrocarbons after migration,
- a non-permeable cap rock, the seal, which will prevent the
hydrocarbons from leaking out of the trap.
60°C
Oil generation window
100
120°C
150
Gas generation window
Gas
Oil
200
(liquid hydrocarbon)
Natural
oil seep
Seal
(non-permeable)
Rock
EXPULSION, MIGRATION
AND TRAPS
The maturation process causes pressure
build-up in the source rock. When this
pressure exceeds the pressure of adjacent
layers, oil or gas is expelled to a porous and
permeable rock which allows fluids to flow
through it. This is called the reservoir rock
(usually 5 to 30% porosity). Hydrocarbons
are lighter than the water contained in the
rock. They migrate upward through the
porous and permeable rocks, or along faults.
Seal
(non-permeable)
Rock
Non-mature
(thermally)
Oil window
Reservoir
rock
Source
rock
Gas window
Reservoir rock sample
(microscope image, size 4 mm):
sandstones, carbonates …
Source rock: very fine
grained rock,
low permeability
Seal sample (10 cm):
claystones, anhydrite …
HYDROCARBON GENERATION: A SUMMARY
Organic matter
Living organisms
Kerogen
Sedimentation Preservation
Burial Maturation Chemical Molecular modification
Hydrocarbons
Expulsion
For more information:
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sources-d-energie/le-petrole
Migration
Trapping
Document printed on 100% FSC-certified paper, partially recycled in a printing plant with the Imprim'Vert printing
label. © CREAV Communication - IFP School, October 2014.
Hydrocarbons can migrate in the reservoir
over distances ranging from a few meters
to several hundred kilometers, until they
accumulate in a trap covered with a nonpermeable layer (the seal). In the trap,
hydrocarbons are separated according to
their density.