the conversion of organic matter to petroleum

Suez University

Faculty of Petroleum & Mining Engineering

The Conversion of Organic Matter to Petroleum

Student

Belal Farouk El-saied Ibrahim

Class / III

Section / Engineering Geology and Geophysics

The Reference / Pet. Geology

(F.K.North)

Presented to

Prof. Dr. / Shouhdi E. Shalaby

Organic MatterWhen an organism (plant or animal) dies, it is normally oxidized Under exceptional conditions: organic matter is buried and preserved in sediments The composition of the organic matter strongly influences whether the organic matter can produce coal, oil or gas.

Basic components of organic matter in sediments• PROTEINS• CARBOHYDRATES• LIPIDS (Fats)• LIGNINAll of these + Time + Temperature + Pressure = KEROGEN

Biomolecules in Living Organisms

Lipids, mostly fats, oils and waxes, have the greatest potential to be hydrocarbon sources. They are combinations of the fatty acids of the general formula CnH2nO2 with glycerol, C3H5(OH)3. An important example is glyceride C17H35COOCH3 formed from the stearic acid.Proteins are giant molecules that make up the solid constituents of animal tissues and plant cells. They are rich in carbon but contain substantial amounts of N, S and O.Carbohydrates are based on sugars Cn(H2O)n and their polymers (cellulose, starch, chitin). They are common in plant tissue.Lignin is a polymer consisting of numerous aromatic rings. It is a major constituent in land plants and converts to coal through desoxygenation.

We have examined the type of raw material needed and how it must accumulate in the natural environment. The next link in the process is to examine what happens to this organic matter (OM) when buried and subjected to increased temperature and pressure. One thing to remember is that not all of the organic carbon (OC) in sedimentary rocks is converted into petroleum hydrocarbons. A portion of the Total Organic Carbon (TOC) consists of Kerogen. We will look at the transformation of OM first to kerogen, then to petroleum hydrocarbons.

Environment of the Transformation

The only elements essential to the transformation of organic matter (OM) into petroleum

are hydrogen and carbon. Thus the nitrogen and oxygen contained in the OM must

somehow be removed while at the same time preserving the hydrogen-rich organic

residue. The formation of petroleum at this point must occur in an oxygen-deficient

environment, not be subjected to prolonged exposure to the atmosphere or to aerated

surface or subsurface waters containing acids or bases, come into contact with elemental

sulfur, vulcanicity, or other igneous activity, and have a short transportation time from the

time of death to that of burial. All of these conditions must be met in order to avoid

decomposition of the OM. All of this implies that as dead organic matter falls to the sea

floor (organic rain), the hydrocarbon constituents needed for creating the end product will

be preserved only if the water column through which they are falling is anoxic - lacking

living organisms, fall is rapid - the particle size must not entirely be microscopic, bottom

dwelling predators are lacking, and there is a rapid sedimentation rate - rapid deposition

buries the OM below the reach of mud-feeding scavengers.

Once the organic material is buried within the sea floor, transformation begins. It is a slow process that occurs to the OM. The general process can be illustrated by the following formulas:OM + Transformation = Kerogen + Bitumen (by product)Kerogen + Bitumen + more Transformation = Petroleum

There are three phases in the transformation of OM into hydrocarbons: Diagenesis, Catagenesis, and Metagenesis (Tissot, 1997). Diagenesis occurs in the shallow subsurface and begins during initial deposition and burial. It takes place at depths from shallow to perhaps as deep as 1,000 meters and at temperatures ranging from near normal to less than 60oC. Biogenic decay aided by bacteria (such asThiobacillus) and non-biogenic reactions are the principal processes at work producing primarily CH4(Methane), CO2 (Carbon Dioxide), H2O (Water), kerogen, a precursor to the creation of the petroleum, and bitumen.

Temperature plays an important role in the process. Ambient temperatures increase with depth of burial which decreases the role of bacteria in the biogenic reactions because they die out. However, much of the initial methane production begins to decline because it is the bacteria that produces the methane as a by-product during diagenesis. Simultaneous to the death of the bacteria however, the increased temperatures accelerate organic reactions.

k=A*e[-E/RT]

Arrhenius equation

R =Gas constant (0.008314 KJ/mol0K)T=absolute temperatureE=activation energyA=frequency factor

The dependence of chemical reaction rates upon temperature is commonly expressed by:

The Catagenesis (meaning thermodynamic, nonbiogenic process) phase becomes dominant in the deeper subsurface as burial (1,000 - 6,000 m), heating (60 - 175oC), and deposition continues. The transformation of kerogen into petroleum is brought about by a rate controlled, thermocatalytic process where the dominant agents are temperature and pressure

The temperatures are of non-biological origin; heat is derived from the burial process and the geothermal gradient that exists within the earth's crust. The catalysts are various surfactant materials in clays and sulfur. Above 200o C, the catagenesis process is destructive and all hydrocarbons are converted to methane and graphite. And at 300o C, hydrocarbon molecules become unstable. Thus thermal energy (temperature) is a critical factor, but it is not the only factor The time factor is also critical because it provides stable conditions over long periods of time that allows the kerogen sufficient cooking time - exposure time of kerogen to catagenesis. Thus the Catagenesis phase involves the maturation of the kerogen; petroleum is the first to be released from the kerogen followed by gas, CO2 and H2O.

The Third phase is referred to as Metagensis. It

occurs at very high temperatures and pressures

which border on low grade metamorphism. The

last hydrocarbons released from the kerogen is

generally only methane. The H:C ratio declines

until the residue remaining is comprised mostly

of C (carbon) in the form of graphite.

Preservation of Organic Matter

The biomolecules described before are reduced forms of carbon and hydrogen. Their preservation potential depends crucially on anoxic conditions, i.e. the absence of oxygen that could oxidize them.Stratified basins that prevent vertical circulation and thus the transport of oxygen to greater depths provide excellent conditions for this. An example is the Black Sea, which is salinity-stratified, but many lakes are also anoxic in their deeper waters because of thermal stratification or abundance of nutrients and lack of circulation.

Preservation of Organic Matter

Access to air (oxygen) rapidly - at geological scales - oxidizes organic matter and converts it into CO2 and H2O.The total carbon content in the Earth’s crust is 9·1019 kg (the

hydroand biosphere contain less than 10-5 of this). Over 80% of this is in carbonates. Organic carbon amounts to 1.2·1019 kg and is distributed approximately as follows:Dispersed in sedimentary rocks (~) 97.0 %Petroleum in non-reservoir rocks 2.0 %Coal and peat 0.13 %Petroleum in reservoirs 0.01 %This illustrates the low efficiency of the preservation process.

Total Organic Carbon (TOC)

If a rock contains significant amounts of organic carbon, it is a possible source rock for petroleum or gas. The TOC content is a measure of the source rock potential and is measured with total pyrolysis.The table below shows how TOC (in weight percent) relates to the source rock quality.TOC Quality0.0-0.5 poor0.5-1.0 fair1.0-2.0 good2.0-4.0 very good>4.0 excellent

TOC Types

TOC in sedimentary rocks can be divided into two types:

• Bitumen, the fraction that is soluble in organic solvents such

as chloroform

• Kerogen, (κεροσ = wax) the insoluble, nonextractable

residue that forms in the transformation from OM Kerogen is an

intermediate product formed during diagenesis and is the

principal source of hydrocarbon generation. It is a complex

mixture of high-weight organic molecules with the general

composition of (C12H12ON0.16)x

Conversion of OM to HC

The principal condition is that this conversion take place in anessentially oxygen-free environment from the very beginning of the process. Anaerobic bacteria may help extract sulfur to form H2S and N, in addition to the earlier formation of CO2 and H2O. This explains the low sulfate content of many formation waters.On burial, kerogen is first formed. This is then gradually cracked to form smaller HC, with formation of CO2 and H2O. At highertemperatures, methane is formed and HCs from C13 to C30.Consequently, the carbon content of kerogen increases withincreasing temperatures. Simultaneously, fluid products high inhydrogen are formed and oxygen is eliminated.

Dehydrogenization and Carbonization

The dehydrogenation and carbonization of organic source n be illustrated with the H:C ratio during theformation of coals:Source material H:C ratioWood 1.5Peat 1.3Lignite 1.0Bit. coal 0.8Anthracite 0.3-0.0Average, in weight %

Deoxygenization and Carbonization

The deoxygenation and carbonization of the source material is illustrated with the formation of petroleum:Source material O:C ratioOrganisms 0.35-0.6Pyrobitumen (kerogen) 0.1-0.2Petroleum (average) 0.004Average, in weight %

Source Rock Quality

The primary factor determining source rock quality is the level of TOC.Additionally, the quality of the source rock is better for higher H:C ratios before thermal maturation.As thermal maturation proceeds and HCs are formed, the kerogen will continuously deteriorate as a source for HC formation.

Lipid-rich kerogen

(phyto- and

zooplankton)

Sapropelic

kerogen (algae)

Humic kerogen

(land plants)

“Van Krevelen diagram” TAI, VR: Maturation indicators

Transformations with Depth

LOM = level of organic metamorphism; BTU = British

Thermal Unit; VM = volatile matter

Source: North, F.K. (1985) Petroleum Geology, Allen & Unwin


Temperature is the single most important factor in thermal maturation.

Rate of Maturation


Time is the second most important factor in thermal maturation

Rate of Maturation ctd.

Purposes of Maturation Indicators

• To recognize and evaluate potential source rocks for oil and

gas by measuring their contents in organic carbon and their

thermal maturities

• To correlate oil types with probable source beds through their

geochemical characteristics and the optical properties of

kerogen in the source beds

• To determine the time of hydrocarbon generation, migration

and accumulation

• To estimate the volumes of hydrocarbons generated and thus

to assess possible reserves and losses of hydrocarbons in the

system.

Lopatin’s TTI Index

V. Lopatin (1971) recognized thedependence of thermal maturationfrom temperature and time. He developed a method where in thetemperatures are weighted with theresidence time the rock spent at thistemperature. Periods of erosion anduplift are also taken into account. This

so-called time-temperature index TTIis still in use, although in variations.

The plot on the right shows a simpledepiction of it. Rock of age A enters theoil-generating window at time y, whilethe older rock B has been at that timealready in the gas-generating windowand will stay there until the present.


Other Maturation Indicators

Several approaches to quantify the degree of maturation have been

proposed aside from the TTI. Most of them are sensitive to

temperature and time.

• Vitrinite Reflectance (Ro) measures the reflectance of vitrinite (see

Kerogen maturation diagram) in oil, expressed as a percentage. It

correlates with fixed carbon and ranges between 0.5 and 1.3 for the

oil window. Laborious but widely used.

• Thermal Alteration Index (TAI) measures the color of finely

dispersed organic matter on a scale from 1 (pale yellow) to 5

(black). This index has a poor sensitivity within the oil window (TAI

around 2.5 to 3.0) and is not generally used.

• Level of Organic Maturation (LOM) is based on coal ranks and is

adjusted to give a linear scale.

Correlation of TTI, Ro, and TAI

The Oil and Gas Windows


The Oil and Gas Windows

A similar slide as before. It shows clearly at what temperatures oil generationpeaks.Gas generation diminishes above ~180°C

Oil Source Rock Criteria

The criteria for a sedimentary rock to be an effective oil source

can be quantitatively described. They are as follows:

• The TOC should be 0.4% or more

• Elemental C should be between 75% and 90% (in weight)

• The ratio of bitumen to TOC should exceed 0.05

• The kerogen type should be I or II (from lipids)

• Vitrinite reflectance should be between 0.6 and 1.3%

Source: Hunt, J.M. (1995) Petroleum Geochemistry and Geology, 2nd edition. W.H. Freeman & Co

This diagram shows thedevelopment of biomoleculesinto petroleum and, withfurther maturation, into gas(left branch at bottom) whichcauses the residues tobecome increasingly morecarbon-rich (right branch atbottom)

Summary: Origin and Maturation