Research - Background on the formation
of hydrocarbons
Information from Petroleum Geology for Geophysicists and Engineers
by Richard Selley
Five Major Types of Hydrocarbons
Kerogen- Kerogen is a fine-grained, amorphous organic matter.
It is not soluble to normal petroleum solvents, like carbon disulfide.
Its chemical composition is 75% C, 10% H, 15% other (sulfur, oxygen, nitrogen,
etc.). It is very important in the formation of hydrocarbons because it
is what generates oil and gas. Source rocks must contain significant amounts
of kerogen.
Asphalt- Asphalt is a solid at surface temperatures. It is soluble
to petroleum solvents. It is produced by the partial maturation of kerogen
or the degradation of crude oil.
Crude Oil- Crude oil is a liquid at surface temperatures. It is
soluble to normal petroleum solvents. It has four main groups of organic
compounds: paraffin, naphthalene, aromatics, and resins.
Natural Gas- Is a hydrocarbon gas. The major natural gases are methane,
ethane, propane, and butane.
Condensates-These are hydrocarbons transitional between gas and
crude oil.
Five Parameters for Hydrocarbon Accumulation
1. Source rocks are required to generate hydrocarbons. Generally,
has greater than .5% organic matter (kerogen) by weight.
2. Reservoir rocks are required to store hydrocarbons.
3. Seal or cap rocks are present to prevent the upward escape of
hydrocarbons from reservoir.
4. Traps occur where the source, reservoir and seal are arranged
in a way that the hydrocarbons can move from the source to the reservoir.
5. Source rock must have been heated sufficiently for oil (greater
than 60ºC) and gas (greater than 150ºC)
Three Phases of Alteration of Organic Matter
Diagenesis- Diagenesis occurs at the surface at normal temperatures.
The organic matter goes under bacterial decay, oxidation, dehydration and
decarboxylation. The resultant is kerogen. The porosity decreases 60% to
40%.
Catagenesis- Temperatures increase to 250ºC and kerogen generates
oil or gas. The porosity decreases to 10%.
Metagenesis- Occurs at 250ºC right when the kerogen is going
to change into oil or gas.
Catagenesis is the most important stage, and is different for the three
different types of kerogen.
Three Types of Kerogen
Type I (Algal)- It is very rich in hydrogen, low in oxygen and
contains lipids. It generates oil and is present in oil shales.
Type II (Liptinic)- It is made from algal detritus, phytoplankton
and zooplankton. It has aliphatic compounds and more hydrogen than carbon.
It can generate oil or gas.
Type III (Humic)- It has more carbon than hydrogen, and is rich
in aromatic compounds. It is produced form lignin in higher woody plants.
It generates gas.
Type I and Type II are usually found in marine environment and Type III
is found in continental environments. That is why there is the generalization
that marine produces oil and continental produces gas.
Migration of Hydrocarbons
Primary migration of hydrocarbons is the movement of oil and gas from
source rock to permeable carrier beds. Secondary migration is the movement
from permeable carrier beds to the reservoir beds. Secondary migration occurs
through porous rocks due to buoyancy and capillary and regional pressure
gradients.
Source rock:
Black shale (organic rich mud in marine settings)
Depth:5000 feet to 21000 feet
Temperatures: 120 F to 350 F
Above depth, lower temperatures: formation of gas (microbial)
Below depth, higher temperatures: Degeneration of oil and organic material
into methane
Reservoir rock:
- Sandstone
- Limestone
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