Generation and Migration
I. Introduction- Origin of oil
A theory of the origin of the oil must explain a series of facts:
(1) Most hydrocarbon deposits occur in sedimentary rocks, most of the
sedimentary rocks with oil are marine.
(2)Many hydrocarbon deposits occur in porous and permeable sandstones
and carbonate rocks, which are totally enclosed in other impermeable rocks.
(3) Other minor geologic occurances:
a) in basement rocks, not voluminous
b) igneous and metamorphic rocks, but commercial deposits are always
laterally continuous with sedimentary rocks
c) Taces of hydrocarbons occur in stony chondritic meteorites
(1) Differences between old and young oils:
a) Old oils contain more even-numbered chains than young oils (these
tend to have odd number chains).
b) Old oils contain more than 50% light hydrocarbons, which are rare
in young sediments.
(2) Similarities between crude oils and organically produced hydrocarbons
a) both have optical property of levorotation
b) Both contain specific complex organic molecules (porphyrins- containing
a 5-member ring with a N substituted for a carbon, and steroids- complex
heavy hydrocarbons, Typically C25)
c) There are biomarkers that can be identified in particular source
beds and crude oils.
(3) Similar fingerprinting can be done using gas chromatography.
II. Origin- Inorganic vs. Organic
A. Inorganic origin
(1) Extraterrestrial occurances of hydrocarbons used to support theory
that hydrocarbons may be inorganic;
(2) If hydrocarbons are abiotic, they should be contained in the mantle
and migrate to their positions in the crust. Test of the theory is to look
at occurences of hydrocarbons. Would hypothesize an association with igneous
rocks, deep crustal structures and faults:
(1) Gases, bitumen, and liquids in vesicles and inclusions. None
of these are commercial. This type of "igneous" hydrocarbons
most likely to have originated from mantle sources, but also may be from
alteration of intruded sediments prior to freezing of magma.
(2) Trapped hydrocarbons where igneous rocks intrude sediments.
These are relatively common although form only a very minor part of commercial
deposits. "Fingerprinting" of the oil and kerogen of the sediment
shows that the oil is derived from the sediment and not carried in with
the magma. The heat from the intrusion alter the organic matter, so not
of igneous origin.
(3) Hydrocarbons in weathered igneous basement. These deposits
usually underly unconformities that separate the igneous rocks from overlying
sediment. The porosity is provided by weathering, and thus the oil must
have migrated into the basement rocks after they were emplaced and weathered.
Unlikely that the oil migrated along with the deposits.
(3) Major test of the abiotic origin of oil.
Tommy Gold (Faculty at Cornell and Nobel Laureate(?)) is major proponent
of mantle source of hydrocarbons.
Suggested that the Siljan ring- meteorite crator in the Precambrian
basement of the cratonic shield of Sweden would be likely place to allow
hydrocarbons to escape from the mantle. There had been reports of oil in
the overlying Ordovician and Silurian sediments.
Drilled two wells, one drilled to 6957 m. The well was abandonded because
of lost circulation problems- drill mud flowed into the formation. Suggests
there is porosity and permeability at depth- ie possible oil reservoir.
Only 85 barrels of oil was produced from the well- later found that
the material was contamination from the drilling mud.
BUT- Only 1 in 10 wells discover oil, and probably fewer in places
where there is little subsurface information. Therefore, they may need
to drill more wells to find the mantle oil.
(4) Summary- there is almost certainly abiotic oil, extraterrestrial
and some associated with igneous rocks, but it is not commercial. This
leaves us with primarialy a biogenic origin.
III. Organic Origin/processes of preservation
Total carbon on earth surface- 2.65 x 1020 g, ~82% is in CO3 rocks,
~18% is in hydrocarbons.
The reduction of oxidized carbon is caused by photosynethsizing plants
and algea. They take energy from the sun and convert CO2 into glucose (C6H12O6).
The glucose molecules are the basis of constructing more complex organic
molecules by animals and plants. These complex molecules include:
(1) proteins- largely animal molecules containing C, H, O, N, S, P-
they are found as amino acids
(2) Lignin- Largely plant molecules, ound in the higher order plants.
Composed of aromatics.
(3) carbonhydrates- these have formula Cn(H2O)n. They are the sugars,
and polymers of sugars called cellulose, starch, and chitin.
(4) Lipids- organic molecules that are insoluble in water. They are
fats, oils, and waxes. Contain C, H, and O. Contain 5 carbon atoms- C5H8.
In most cases, these complex molecules are oxidized to CO2, largely
by bacteria. In exceptional cases, the complex organic molecules are preserved,
buried, and converted into hydrocarbons.
See Fig. 5.4 in Seeley
A. Preservation of Organic Matter in Ancient Sediments
The distribution and preservation of organic matter in ancient sediments
is difficult to predict, but this is what is critical, because it is the
fundamental reason why a particular sedimentary basin may or may not be
The oceans have contain diverse and voluminous organism since the precambrian
radiation. However, the composition of the organic matter may have changed-
(1) the amount of lignn may have increased relative to the other organic
(2) the amount of phytoplankton may have increased with two major blooms
(a) in the late Ordovician to early Silurian and (2) in the late Cretaceous.
Because marine organisms are the source of oil, it is important to understand
their distribution and how they may be preserved. The way to do this is
look at the modern distribution and mechanisms of preservation
B. Productivity and preservation of organic matter- modern environments
OM(sediment) = Production/destruction
There is a link between the production and destruction, however- when
production increases, the destruction decreases. This means the processes
the control the amount of organic matter in the sediment are not linearly
i.e. destruction = f(production)
It is possible (although not terribly easy) to determine the rate of
production in modern oceans. It is also possible (although even more difficult)
to determine the rate of destruction in modern oceans.
It is much more difficult to determine production and destruction rate
in ancient sediments, particularly for terrestrial environments.
There are large differences between marine and terrestrial environments-
Why would you care about these kind of questions in petroleum geology
class- because the oil is where the organic matter is.
(1) Marine Organic matter- preservation
"In the sea, as on the land, all organic matter is originally formed
by photosynthesis"- NOT TRUE. There are "chemosynthetic"
biota that derive their energy from chemical reations rather than photo
reactions. But these are relatively minor; they are unlikely to form commercial
Considering only photosynthesis, two important parameters for photosynthesis:
(1) amount of light- controlled by water depth, the latitude, and turbidity
of the water.
(2) chemical conditions in the water- proper fertilizers. Ocean waters
are "limited" in nitrogen and phosphorous. There is not enough
of these elements in most of the ocean's waters to support production of
organic matter. Thus most organic matter will be produced in areas where
there is nitrogen and phosphorous introduced to the water.
Because of the controls on production, the distribution of organic matter
in the world's oceans are not uniform.
(1) in the center of large oceans- oceanic gyres. These are "marine
(2) in the polar oceans.
(1) along the eastern margins of the Atlantic and Pacific Oceans
(2) at the boundary between the polar and equitorial oceans
(3) along the equator.
These distribution of productivity have to do with oceanic circulation.
The major oceanic circulation patterns allow upweling to occur where
water is brought to the surface, into the photic zone, from depth. Called
upwelling. It occurs only in certain places in the worlds oceans because
the oceans are typically stratified- the warmer water is on the top, and
prevents the deeper, colder, more dense water from coming to the surface.
This is partly the signficance of El Nino.
The deeper water contains more nutrients (N and P) because there has
been organic matter decay and diffusion from the sediments.
Upwelling is not really a deep phenomenon- the "deep water"
comes from 200 to 400 m water depth.
b) conditions favorable for preservation
Two important factors (1) sedimentation rate and (2) bottom water oxygen
content (oxidation of the organic matter)
(i) Sedimentation rate:
Look at Fig. 5.7
Different types of sediment show the same pattern of relationship between
sedimentation rate and amount of organic matter.
In general the amount of organic matter is positively correlated with
sed rate, to a particular rate, and then there is an inverse correlation
at greater sedimentation rates.
The explanation for the positive correlation is that at low sedimentation
rate, organic matter is retained on the seafloor in contact with oxygenated
water and oxidized. So that faster burial removes the sediment rapidly
from contact with the oxygen.
The negative correlation results from dilution effect- with more rapid
sedimentation there is more material other than organic matter so that
the organic matter is reduces the concentration.
Note that the accumulation of organic matter (essentially concentration/sed
rate) could still go up, it is just the concentration that decreases.
(ii) Bottom water oxygen content:
The amount of oxygen in bottom water controls how much organic matter
will be oxidized, but the amount of oxygen in many instances relates to
the amount of organic matter produced.
The amount of bottom water oxygen largely depends on the stratification
of the water body:
(1) Lakes- warm water stratification allows mixing in the upper
water layers, but stagnation in the bottom layers. As organic matter decays
in the lower layers, the oxygen is used, until none is left and much of
the remaining organic matter will be preserved.
Modern examples are in the African lakes. Ancient examples in Tertiary
lakes of Colorado, Utah, Wyoming (e.g. Green River oil shales and fossil
fish- no bioturbation).
(2) Barred basin- salinity stratification in regions where there
is high evaporation rates. Anoxic conditions similar to lakes can occur.
This type of situation occurs inthe Black sea- where very organic rich
sediments are being deposited. To a certain extent the same thing is occurring
in the Mediterranean, although not as strongly as the Black sea.
(3) Upwelling zones- In these regions there is sufficient organic
matter settling thorgh the water column that the water becomes depleted
in oxygen between ~200 and 1500 m. Thus along continental shelves and slopes,
where this low oxygen water contacts the sediment, there is enhanced preservation
Examples are California, Peru, Namibia &Gabon
(4) Anoxic ocean basins There are no modern examples of this,
but it is believed that the cretaceous ocean may have gone anoxic in a
manner similar to that of lakes. In these instances, the deep circulation,
driven by cold downwelling currents at the poles, or saline downwelling
currents at the equator stop. It is these currents that "ventilate"
the bottom water so that when they turn off, there is no way to oxygenated
water to the bottom of ocean.
Most of the evidence is from the Cretaceous Atlantic, which was just
opening at that time. This means that the anoxic basins may not be a global
(2) Organic productivity and preservation in Modern Continental Environments
There is little reason to talk about this preservation, because most
of continental organic matter is altered to coal, rather than liquid hydrocarbons.
Essentially, the production and preservation depends on availability
of water and temperatures. Only sediments that would be preserved are important,
and thus upland areas, which are eroded, would not be a place for preservation.
The remaining continental environment is swamps, these produce coal.
IV. Formation of Kerogen
What happens to all the organic matter (i.e. bug guts) as it gets buried?
Obviously it is subjected to higher pressure and temperatures. Three major
steps. Of course there are gradations between steps, like all things in
A. Shallow diagenesis of organic matter
With burial into the sediment, the Eh of the pore water continues to
decline. The rate at which is declines, and the starting value all relate
to the oxygen continent of the overlying water (e.g. well oxygenated or
At each level, different species of bacteria use different elements
or compounds as the oxidants to oxidyze the organic matter.
This redox reaction is how the bacteria gain energy to live, and the
specific compound they use determines the amount of energy they can get
from the reaction.
The chemical composition of organic matter is generally taken to contain
C, H, O, N, and P. The C/N/P ratio is very constant in marine organic matter
and is generally said to be 116/16/1. This is called the Redfield ratio
after its discoverer. The ratio is different and more variable in continental
(1) Reactions of organic matter:
Look at the reactions on p. 200 and 201
The series of major oxidants are:
(a) Oxygen, this provides 30 kcal/moles of energy
(b) Nitrate (NO3) and Nitrite (NO2). Most of the oxygenated nitrogen
species at the pH and Eh of seawater are as nitrate. This reaction provides
(c) Sulfate This reaction provides 5 kcal/mole of energy
(d) Carbon dioxide. This reaction provides the least amount of
energy. ~4 kcal/mole
(e) other oxidants: There are other minor species that can act
as oxidants. These include Fe and Mn oxides and Iodate. They are similar
in energy produced to the nitrate. In other words, Fe and Mn oxides are
reduced at about the same level as the nitrate. This is seen as a rapid
increase in the amount of dissolved Fe and Mn in the sediment (Reduced
Fe and Mn are very soluble, oxidized Fe and Mn are not).
(2) Inorganic reactions
There are several inorganic reactions that also occur. They are important
because they can control the physical properties of the sediment- e.g.
the porosity and permeability which are important for migration of hydrocarbons.
(a) Iron minerals- The formation of reactive sulfide and soluble
Fe(II) allows the precipitation of pyrite and siderite. Both of these minerals
are very common in organic rich (ie. black) shales.
(b) Other carbon minerals- Calcite and dolomite can form at shallowburial
depths, but with increased burial, there is a lack of Mg ions and so calcite
becomes the major carbonate mineral. The Ca for the calcite comes from
the dissolution of biogenic calcite in the sediments. The only source for
Mg is from seawater and if sedimentation is fast enough, then it is rapidly
B. Chemistry of Kerogen
After these shallow diagenetic reactions, the remaining solid organic
matter is now kerogen. It is distinguished as being insoluble in organic
solvents. There are three types of kerogen depending on the organic matter
that was the precursor. These three types generate three different types
(1) Type I- Algal
Higher in H/O ratio than the other kerogens- typically 1.2 to 1.7. The
H/C ratio is 1.65 (note, these are all weight ratios). The organic compounds
are typically lipids (fats).
(2) Type II- combination algal and zooplankton and phytoplankton
Has intermediate H/C and H/O ratio to those of Type I and Type II
(3) Type III- generally from woody (land) plants- Humic material
Rich in aromatics, but low in aliphatic compounds. It has a very low
H/C ratio and higher H/O ratio. Generally undergoes diagenesis to form
coal- the only liquid hydrocarbon it produces is methane.
Thus, when evaluating an province it is important to determine the amount,
as well as the type of kerogen present.
C. Maturation of kerogen
As the kerogen is subjected to deeper burial and increased P and T,
it begins to release HC. The rate and types of hydrocarbons released depend
on the rate of heating and the length of time available for heating.
Empirical evidence for the T for oil vs gas generation are:
k = Aexp(-E/RT)
Where: k is reaction rate constant (1/m.y.), A is frequency factor (1/m.y.),
E is the activation energy (kJ/mol), R is the gas constant (kJ/mol K),
and T is the temperature in kelvins.
a) The C-S bonds are weaker than C-C bonds so that the kerogen can decompose
at lower temperatures (ie. lower activation energy)
b) Recent paper (Lewan, 1998 Nature, v. 391, p. 164) proposes that the
S released during organic matter diagenesis catalyzes the reaction.
In general, reaction rates double for every 10°C increase in temperature.
But the Arrhenius equation only gives the rate that hydrocarbons
are generated. Thus, to calculate the total volume (= $$$$) you also need
to know the amount of time that the kerogen has been at the oil generating
(2) Techniques for determining the quantity of generated hydrocarbons
a) Level of Organic Maturatiy (LOM)
b) Time-temperature index (TTI).
x% = [1-exp(-STTI/100)]*100
(3) Techniques for determining temperature (Paleothermometers)
The major unkown variable in the TTI calculations is the temperature
that the organic matter has been subjected to. The problem is that the
geothermal gradient may have changed with time, so that the temperature
at the bottom of the hole is different from the temperature in the past.
There are a variaty of techniques to determine the temperature that
organic matter has reached.
a) Carbon ratio technique Not well calibrated. Compares residual
carbon after pyrolysis at 900°C and the total C in the sample. Ratio
of Cr/Ct. Idea is that "fresh" OM has more easily removed C than
"old and altered" OM. This means the Cr/Ct ratio increases with
b) Electron spin resonance The number of free electrons vary
with maturity and can be measured. There are problems associated with recycling
of organic matter and original variations in the organic matter- difficult
c) Pyrolysis The heating of source rocks and detection of the
hydrocarbons that are given off. Most common technique is through Rock-Eval
(1) S1 represents free hydrocarbons in the rock (ie. already generated)
(2) S3 represents the release of carboxyl groups as CO2
(3) S2 represents the hydrocarbons that are generated by break down
of the kerogen present inthe rocks.
Fig. 10-8, Hunt
d) Gas chromatography There is an evolution of the distribution
of n-alkanes. Calibrate the technique.
e) Clay mineral analysis Various clay minerals alter diagenetically
to new minerals at specific temperatures:
f) Fluid inclusions
g) Pollen color Spores and pollen are colorless when formed.
As they heat up they gradually become darker.
h) Vitrinite reflectance
Many observations indicate that HC found in reservoir beds (porous and
permeable) did nt originate there:
(1) HC form at depth through increased T and P. Must have moved away
(2) HC found in secondary porosity- HC must have flowed in after the
(3) HC typically found in the highest portion of laterally continuous
porous and permeable beds- implies upward and lateral migration
(4) Oil, gas and water are stratified according to their densities.
Implies they are free to move laterally and vertically.
Two types of migration:
(1) Primary migration- The initial movement of hydrocarbons from
the source rock into permeable carrier beds and reservoirs
(2) Secondary migration- subsequent movement of the hydrocarbons
through permeable beds. Driven by bouyancy of the fluids and the migration
occurs when the HC are fluid.
A. How does primary migration occur?
Major questions still as to how the HC migrate out primary migration
The problem is that most source rocks are fine grained, and thus generally
have low permeability. The porosity of the source rocks is generally low
by the time that they are buried into the oil generating window, which
implies two things:
(1) there is little additional compaction driven migration of water
from the pore space
(2) What little permeability present originally is now gone.
The major problem is a size problem:
Pore throats in shales at 2000 m depth are on the order 50 to 100Å,
but individual, large hydrocarbon molecules range from 10 to 100Å.
Thus droplets of oil are likely to be too large to pass through the pore
throats, particulalry when they are water wet. That is when there is water
adhered to the clay that also reduces the size of the throats.
An alternate explanation is that the source beds are not water wet,
but have continues oil phase with no water. This may be true in very rich
beds, but lean beds it is unlikely to occur.
(1) Release of interstitial water in clay minerals
Although most of the pore water in sediments is removed through
compaction by the time oil generation is reached during burial, there is
still the bound water (interlayer water) in clay minerals present in the
This bound water is released from the clays at temperatures that are
in the oil window. The depth depends on the geothermal gradient
Appears that there is a correspondence between the depth of the release
of water from clay mienrals and the maximum number of tops of oil reservoirs.
Is this proof that dehydrationof clays is responsible for primary oil migration?
What are problems?:
(1) Depth of drilling. Likely that the depth of the maximum will shift
downward as more deep wells are drilled and more deep reservoirs are discovered
(2) Correlation does not require causality
(3) Oil migrates in the subsurface, and migration is vertical. Thus,
it is likely that the generation of oil occurred at depths below the location
where it is found (i.e. the reservoir).
(4) The figure is from the Gulf coast which contains much smectite clays.
Other oil producing regions have little clay material.
(2) Interstitial hydrocarbons
Not only water, but hydrocarbons may also be included in the interlayer
sites of clay minerals. May provide a mechanism for primary migration-
the hydrocarbons will be released along with the water.
Overpressuring is when the pore fluid pressure are greater than hydrostatic
for some reason. Processes include:
(1) dewatering of clays, which provide excess water to the pore space
increasing the mass, without necessarily increasing the volume of the pore
(2) conversion of solid kerogen to liquid hydrocarbon. There is a volume
change with this process where the liquid is greater volume than the original
volume taken up by. May drive primary migration (e.g. see Bredehoeft article).
(3) Cementation of porosity reducing volume, without changing the volume
of fluids present in the pore space
(4) Rapid loading of sediment, preventing escape of fluids and increasing
the lithistatic load on the pore fluids.
B. Explusion Mechanisms
Several theories, none satisfactory:
Idea: Petroleum is expelled from source rocks before they have been
converted to water insoluble molecules. They are expelled when they are
ketones, adics and esters.
Problem: Concentrations of these compounds are low in in source rocks.
The compounds are easily adsorbed onto the surfaces of clay minerals. They
should not evolve into hydrocarbons in the reservoir bed once they migrate
(2) Expulsion at high temperatures
Idea: HC are more soluble at high temperatures than low temperatures,
e.g. see Figs. 5.26 and 5.27 for relationship. At T > 150°C, 50
to 200 ppm HC can be soluble in water. This is one to two orders of magnitude
greater than at T < 100°C. Also lower weight hydroacrabons (the
gases) are faily soluble even at lower T
Problem: These temperatures are greater than the oil generating window.
At these higher temperatures, porosity and permeability may be destroyed.
Also the hydrocarbons will thermally breakdown
Idea: Micelss are "colloidal organic acid soaps". One end
is hydrophobic, the other hydrophylic. They can link water molecules with
hydrocarbon molecules allowing migration. Hydrocarbon types have different
solubilities with different micelles, and it turns out the distribution
of these hydrocarbons matches the distribution of micelles.
Problem: For sufficient HC to have migrated requires a very high micelle/HC
ratio, but there are only trace quantities of micelles. Also, micelles
molecules are larger than pore throats in clays, so they would not fit
through the openings.
Idea: High CO2 concentrations will precipitate calcite, reducing pore
volumes and increasing pore pressures. CO2 also lowers the viscosity of
oil, allowing it to flow more freely. Also some HC may migrate out of the
source bed as gases, and then subsequenty condence.
Problems: precipitation of CO2 will also reduce permeability. Also most
CO2 is generated during diagenesis, and before catagenesis, and so may
be lost from the system. Primary migration of HC gases can't explain primary
migration of the heavier hydrocarbons.