I. Introduction- Origin of oil

A theory of the origin of the oil must explain a series of facts:

A. Geologic:

(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

B. Chemical

(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;

• The outer planets and their moons contain methane
  
• Carbonaceous chondrites contian traces of various hydrocarbons, including complex molecules
  
• Most people believe these hydrocarbons form from inorganic reactions (ie no extraterrestrial life) and thus hydrocarbons on earth may have formed in a similar manner.
  
  

(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:

• Methane is present in volcanoes (typically 1% of gases, up to 15%). In all cases, the volcanoes erupt through cover of sediments.
  
• Generally three types of occurences of hydrocarbons in volcanic rocks:
  

(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 productive.

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 species.


(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 related.

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- discussed separately

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 oil deposits.

a) Production:

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.

Low production:

(1) in the center of large oceans- oceanic gyres. These are "marine deserts"


(2) in the polar oceans.

High production:

(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 effect.

(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 geology:

(1) Diagenesis

• Shallow subsurface. Includes biogenic decay (bacteria) and abiotic reactions.
  
• Products include kerogen (the solid remaining) methane, carbon dioxide (bicarbonate at most pH), water, H2S
  
• The major change to the solid is that its oxygen content is reduced, but the hydrogen/carbon ratio isn't changed greatly.
  
  

(2) Catagenesis

• continued burial.
  
• fluid hydrocarbons is released from the solid matter. Initially liquids, and later at higher temperatures gas.
  
• The hydrogen/carbon ratio decreases, but the O/C ratio doesn't change much.
  
  

(3) Metagenesis.

• Continued burial, Pressure and temperatures approach metamorphic values.
  
• Largely methane expelled at these T and P.
  
• The H/C ratio continues to decline in the solid as the fluid H/C ratio increase. Remaining product is graphite.
  
  

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 stagnating).

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 organic matter.

(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

• This reaction generates CO2, nitrate, and phosphate from the organic matter.
  
• Oxygen is used up within the upper 2 to 3 mm of the sediment water interface, even when the bottom water is well oxygenated (seawater contains only about 300 µM of oxygen at saturation).
  
• when all the oxygen is used up, the next oxidized species to be used are:
  
  

(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 ~20 kcal/moles

• This reaction generates ammonia along with CO2 and phosphate from the organic matter and the reduced nitrate.
  
• when all nitrate and nitrite are used, the next oxidized species is
  
  

(c) Sulfate This reaction provides 5 kcal/mole of energy

• This reaction generates CO2, ammonia, phosphate from the organic matter as well as H2S from the sulfate. Because sulfate is the second most common ion in seawater (28 mM), this is probably the most common of all the redox reactions.
  
• Sulfate reduction can occur throughout the sediment column (down to the basaltic crust), or it can end within a few cm of the seafloor. The controls on sulfate reduction are the amount of organic matter present in the sediment and the sedimentation rate which controls diffusion.
  
• when (or if) all the sulfate is used up, then the next oxidizing specie is:
  
  

(d) Carbon dioxide. This reaction provides the least amount of energy. ~4 kcal/mole

• The products of this reaction are ammonia, phosphate, and methane.
  
• The CO2 is not energetically favored, but there can be large amounts of CO2 in the pore water. This means that large amounts of methane can be generated by this reaction- call microbial (or biogenic) methane.
  
  

(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 depleated.

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 of oil

(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:

• Oil generation: 60 to 120°C, called the "oil window"
  
• Gas generation: 120 to 225° C, called the "gas window"
  

(1) Kinetics:

Arrhenius equation-

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.

• The values for E and A can be determined for any source rock by heating it at various temperatures and measuring the yield of hydrocarbons.
  
• These values can range widely depending on the source rock. The value of E apears to be inversely correlated to the S content. Two possible reasons:
  

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 temperature.

(2) Techniques for determining the quantity of generated hydrocarbons

a) Level of Organic Maturatiy (LOM)

• This technique assumes doubling of reaction rate for each 10° C.
  
• Originally was made to be linear with a coal rank scale (e.g. bituminous through anthracite etc.). Found that it isn't logarithmic with the scale either, so not of much use.
  
  

b) Time-temperature index (TTI).

• Complex formulation to calculate a value of TTI that is based on values for E, A, and T and t for the amount of time that organic matter stays within a 10°C window. (p. 151 Hunt)
  
• There is also a second relationship to calculate the value of TTI for sediment that resides for a certain time at a certain temperature
  
• These equations are solved for each type of kerogen (different values of E and A) and then plotted graphically to simplify their calculations (see p. 151 in Hunt, Fig. 6-3).
  
• The next step is to sum up all values of TTI for each 10°C window or for periods of constant temperature. This provides a value of STTI, which is related to the % oil generated by:
  

• The importance of using this type of calculations is that it takes into account the kinetics of oil generation from kerogen. Because activation energy is so variable for each type of kerogen it is important to account for it.
  
  

(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 maturity.

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 to calibrate.

c) Pyrolysis The heating of source rocks and detection of the hydrocarbons that are given off. Most common technique is through Rock-Eval machine.

• Designed to heat organic matter, or sediment containing organic matter, and measureing the concentration of hydrocarbons and CO2 given off. They are measured with a Flame Ionization Detector and Thermal conductivity detector.
  
• Initially rock is heated to 300°C, then increase by 25°C/min. up to 550°C. There are three peaks (called P) or the areas under the peaks (called S) given off with this type of heating:
  

(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.



• S1 and S2 are converted to milligrams carbon by calibrating the instrument with standards.
  
• The total organic carbon (TOC) content of the sample is measured separately, but two indexes can be calcuated from the values for S1 and S2: the hydrogen index (HI) is the ratio fo S3/TOC, and the Oxygen index (OI) is the ratio of S3/TOC. These indexes can be plotted on a diagram that shows the type of organic matter present.
  

Fig. 10-8, Hunt

d) Gas chromatography There is an evolution of the distribution of n-alkanes. Calibrate the technique.

Fig. 5.18

e) Clay mineral analysis Various clay minerals alter diagenetically to new minerals at specific temperatures:

• smectite alters to illite at T of 80 to 120°C
  
• Kaolin and illite recrystallize to micas, or to chlorite in Fe/Mg rich environments
  
• The major problem with this is that the boundaries are not precise- there is a wide range in the distribution of the clays. Clays are completely independent of the organic matter.
  
  

f) Fluid inclusions

• These are inclusions contained in authigenic minerals. It is possible to estimate the temperature that the mineral formed by heating the inclusions until the vapor phase contained in the inclusion disappears (dissolves).
  
• These have long been used for hard rock studies beause the inclusions are larger and because they form at higher T, so easier to work on.
  
• Improved microscope optics allow work on small diagenetic minerals now. But there are many factors that go into choosing the correct inclusions to work on, as well as interpreting the data.
  
  

g) Pollen color Spores and pollen are colorless when formed. As they heat up they gradually become darker.

• This is a quick and easy technique for determining approximate temperatures, but relies on the interpretation of the person observing the pollen.
  
  
  

h) Vitrinite reflectance

• This technique is a modification of measure of the maturity of coal and is based on the how shiny the coal is- more mature is shinier.
  
• For oil measurement, the kerogen is separated from the sediment by dissolution with HF and HCl, mounted on a block with epoxy, polished and put in to a reflecting microscope, and the amount of reflectance is measured termed Ro.
  
• There is an empirical relationship between vitrinite reflectance and hydrocarbon generation: Ro of 0.6 to 1.5 is oil generation, Ro of 1.5 to 3 is gas generation, and over 3.0 is all graphite.
  
• This is probably the most widely used technique. Often measurements are made downhole, and anomalies can provide information about the geology (igneous intrusions, faults, unconformities, etc.).
  
  

V. Migration

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 after formation.


(2) HC found in secondary porosity- HC must have flowed in after the porosity formed.


(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 occurs.

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.

Fig. 5.20

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 sediment.

This bound water is released from the clays at temperatures that are in the oil window. The depth depends on the geothermal gradient

Fig.5.23

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?:

Fig. 5.24

(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.

(3) Overpressuring

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 space.

(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:

(1) Protopetroleum

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 there.

(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

(3) Micelles

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.

(4) Gases

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.