How Does Organic Matter Generate Hydrocarbon?

How does organic matter generate Hydrocarbon?

Petroleum source rock is defined as the fine-grained sediment with sufficient amount of organic matter, which can generate and release enough hydrocarbons to form a commercial accumulation of
oil or gas 

A source rock is a rock that is capable of generating or that has generated movable quantities of hydrocarbons. Hydrocarbon generation is the natural result of the maturation of buried organic matter.

Organic matter (organic carbon) in sediments underlying the oceans is derived from different sources including the following: Marine phytoplankton, Phytobentos in shallow water with sufficient light, Bacteria, and Allochtonous (i.e., land derived) material.

The organic carbon produced in the water column varies from ~0.1% to 5%, depending on various factors such as the following:

·         Oxygen depletion in bottom waters or in sediment as a result of high organic input,

·         Adsorption of certain compounds to mineral particles,

·         Preservation of organic compounds as shell constituents,

·         Changes in the rate of deposition of sediment organic matter,

·         High input of terrigenous organic compounds, which are more stable than organic matter, and

·         Dominant input of argillaceous sediments where oxygenation of pore water is restricted.

Organic matter undergoes changes in composition with increasing burial depth and temperature. The three steps in the transformation of organic matter to petroleum hydrocarbons are termed diagenesis, catagenesis, and metagenesis. The general scheme of evolution of the organic fraction and the hydrocarbons produced is depicted. Petroleum hydrocarbons exist as gaseous, liquid, and solid phases, depending on temperature, pressure, burial time, and composition of the system.

Petroleum is generated from productive source beds which have high organic carbon content. This high organic carbon content resulted from the burial of plants and animals. Since all plants are not of equal carbon content and so do animals, the C range of the petroleum change when the carbon source changes. an indication on the carbon source for C range present in petroleum. Indication of the origin of the organic matter in the source beds Maturation is the ability of the rock to generate hydrocarbons, and indicates its maturity. Therefore, the more the rock quality is, the more mature it is. Maturation occurs through millions of years in which diagenesis and catagenesis processes take place. About 10 to 20% of petroleum is formed during diagenesis. Most petroleum is formed during the catagenesis and metagenesis of the residual biogenic organic matter. Converting biomolecules into petroleum is called maturation. the maturation process

Sediments rich in organic matter accumulate in areas of high organic productivity near the surface and with stagnant water at the bottom. High organic productivity is dependent on light and nutrients. Preservation of the organic matter in sediments is favored by lack of oxygen (anoxic environments).

v  Diagenesis

Diagenesis is defined as the chemical reactions that occurred in the first few thousand years after burial at temperatures less than 50◦C. During diagenesis: oxygen, nitrogen and sulfur are removed from the organic matter leading to an increase in hydrogen content of the sedimentary organic matter. In addition, presence of iron during diagenesis increases the probability of sulfur removal as the iron reacts with sulfur over long burial time and this increases petroleum economic value.

This phase occurs in shallow subsurface environments at low temperatures and near normal pressures. It includes two processes, biogenic decay supported by bacteria, and abiogenic reactions. Diagenesis results in a decrease of oxygen and a correlative increase of the carbon content. It is also characterized by a decrease in the H/O and O/C ratios. Methane is the only hydrocarbon formed at this stage. Stage I (Diagenesis) 0-60oC

v  Catagenesis

The reactions that occur between 60 and 200◦C are considered to be catagenetic in nature. During catagenesis, the organic compounds are exposed to diverse thermal degradation reactions that include double bonds reduction by adding sulfur or hydrogen atoms, cracking reactions and condensation reactions.

This phase is marked by an increase in temperature and pressure, and occurs in deeper subsurface environments.

It results in a decrease of the hydrogen content due to generation of hydrocarbons. Petroleum is released from kerogen during this stage. Oil is released during the initial phase of the catagenesis, at temperatures between 60 and 100 ºC. With increasing temperature and pressure (approximately 120-225 ºC), wet gas and subsequently dry gas are released along with increasing amounts of methane

v  Metagenesis

‘Metagenesis takes place at temperatures over 200◦C and is considered to be a type of very low-grade metamorphism. explains the changes occur in organic matter during diagenesis, catagenesis and metamorphism.

This is the last stage of OM maturation. It is referred to as drying gas zone. Liable product is mainly methane formed as result of cracking of already generated HCs. Some structural rearrangement of Kerogene occurs leaving around residual hydrocarbons. Kerogene of low initial hydrogen content has very limited capacity to generate HCs because it cannot supply sufficient hydrogen to produce fully saturated hydrocarbons.

As a rock containing kerogen and is progressively buried in a subsiding basin, it is subjected to increasing temperature and pressure. A source rock is defined as mature when it is reached to generate hydrocarbons. A rock that does not reach to the level of generation of hydrocarbons is defined as an immature source, and that which passed the time of significant generation and expulsion, it is considered as over-mature source rock. Generally, various parameters have been used for estimating source rock maturation. These parameters include vitrinite reflectance (Ro) and rock-eval pyrolysis data such as Tmax and production index (PI). The study of thermal maturation of source rocks is one of the main steps in the source rocks evaluation in the study area. This is because it is possible from the maturation stage to determine the position of the sediments with the respect to the oil generation. It can also help in oil exploration from knowing the relation between hydrocarbon generation, migration, and accumulation with the tectonics, which lead to the development of the structural traps in the study area.

Mechanisms of hydrocarbons generations from source rock

Organic Matter When 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) • Lignin All Of These + Time + Temperature + Pressure = Kerogen

Environment of the Transformation 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. 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.

Diagenes is 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.

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. 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 Quality 0.0-0.5 poor 0.5-1.0 fair 1.0-2.0 good 2.0-4.0 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

 take place in an essentially 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 higher temperatures, methane is formed and HCs from C13 to C30. Consequently, the carbon content of kerogen increases with increasing temperatures. Simultaneously, fluid products high in hydrogen are formed and oxygen is eliminated.

Question 2. What is source rock?

Source rock is defined as the fine-grained sediment with sufficient amount of organic matter, which can generate and release enough hydrocarbons to form a commercial accumulation of oil or gas. Source rocks are commonly shales and lime mudstones, which contain significant amount of organic matter.

Source rock is defined as any rock that has the capability to generate and expel enough hydrocarbons to form an accumulation of oil or gas. Source rocks are classified according to oil generation into three classes

·         Immature source rocks that have not yet generated hydrocarbons.

·         Mature source rocks that are in generation phase.

·         Post mature source rocks are those which have already generated all crude oil type hydrocarbons.

Is a sedimentary rock that contain sufficient organic matter such that when it is buried and heated it will produce petroleum. Is any rock from which hydrocarbons originate (usually shale or limestone). It should contain more than 5% organic matter and has the potential to generate petroleum.

Distinguished the petroleum source rocks into the followings:

Potential source rocks are immature sedimentary rocks capable of generating and expelling hydrocarbons, if their level of maturity were higher. Rock which contains organic mattering sufficient quantity to generate and expel hydrocarbons if subjected to increased thermal maturation.

Possible source rocks are sedimentary rocks whose source potential has not yet been evaluated, but which may have generated and expelled hydrocarbons.

Effective source rocks are sedimentary rocks, which have already generated and expelled hydrocarbons. Rock which contains organic matter and is presently generating and/or expelling hydrocarbons to form commercial accumulations.

Relic effective source rock an effective source rock which has ceased generating and expelling hydrocarbons due to a thermal cooling event such as uplift or erosion before exhausting its organic matter supply.

Spent source rock an active source rock which has exhausted its ability to generate and expel hydrocarbons either through lack of sufficient organic matter or due to reaching an over matured state.

Characterizing source rocks, To be a source rock, a rock must have three features:

·         Quantity of organic matter

·         Quality capable of yielding moveable hydrocarbons

·         Thermal maturity

The first two components are products of the depositional setting. The third is a function of the structural and tectonic history of the province.

Question 3. Explain methods/steps used to evaluate the source rocks?

How to evaluate the source rocks?

·         Quantity of organic matter (TOC)

·         Quality of organic matter (Kerogen types )

·         Maturation level of organic matter (Ro, Tmax, TAI )

v  Quantity of organic matter (TOC)

The quantity of organic matter is commonly assessed by a measure of the total organic carbon (TOC) contained in a rock. Total organic carbon (TOC) is the amount of organic carbon present in a rock is a determining factor in a rock's ability to generate hydrocarbons.

The quantity of organic carbon is relative to the total organic carbon (TOC) content of rocks. Thus, TOC is the concentration of organic matter in the rocks and is expressed by the weight percent of organic carbon. A value of about 0.5% TOC by weight percent is considered a minimum or threshold for an effective source rock. For shale gas reservoirs, in general, values of about 2% are considered a minimum and may exceed 10–12%

v  Quality of organic matter (Kerogen types )

Quality is measured by determining the types of kerogen contained in the organic matter.

Kerogen is normally defined as that portion of the organic matter present in sedimentary rocks that is insoluble in ordinary organic solvents. The soluble portion, called bitumen.

Kerogen is a fine-grained, amorphous organic matter. It is not soluble to normal petroleum solvents, like carbon disulfide. Its chemical compositioin 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.

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. Is quite rare because it is derived principally from lacustrine algae.

Occurrences of Type I kerogens are limited to anoxic lakes and to a few unusual marine environments. Type I kerogens have high generative capacities for liquid hydrocarbons.

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. Arise from several very different sources, including marine algae, pollen and spores, leaf waxes, and fossil resin. They also include contributions from bacterial-cell lipids. The various Type II kerogens are grouped together, despite their very disparate origins, because they all have great capacities to generate liquid hydrocarbons. Most Type II kerogens are found in marine sediments deposited under reducing conditions.

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. Are composed of terrestrial organic material that is lacking in fatty or waxy components. Cellulose and lignin are major contributors. Type III kerogens have much lower hydrocarbon-generative capacities than do Type II kerogens and, unless they have small inclusions of Type II material, are normally considered to generate mainly gas.

Type IV kerogens contain mainly reworked organic debris and highly oxidized material of various origins. They are generally considered to have essentially no hydrocarbon-source potential.

v  Maturation level of organic matter (Ro, Tmax, TAI )

Thermal maturity is most often estimated by using vitrinite reflectance measurements and data from pyrolysis analyses.

Thermal maturity is the extent of heat-driven reactions that alter the composition of organic matter (e.g., conversion of sedimentary organic matter to petroleum or cracking of oil to gas.) Different geochemical scales, such as vitrinite reflectance, pyrolysis Tmax, and biomarker maturity ratios can be used to indicate the level of thermal maturity of organic matter. Many of the elements of basin modeling programs—maturation of source rocks, reservoir diagenesis, and porosity evolution—are affected by thermal and burial history. Thermal maturation data used to model these parameters are usually derived from fossils.

Methods or steps used to evaluate the source rocks

v  Pyrolysis analyses (Rock-Eval Pyrolysis)

Petroleum generation is a result of the burial diagenesis of organic rich sediments. Thermal stress without the presence of oxygen is responsible for hydrocarbon generation in a process called pyrolysis, as opposed to combustion. As TOC is measured in the lab by combustion, a rock can undergo laboratory pyrolysis to measure its total generation potential. This type of analysis is conducted after levels of TOC (total organic content) have been deemed adequate (appx. >1%). A rock with insufficient TOC will not generate hydrocarbons. In a pyrolysis analysis, a rock sample undergoes increasing temperature in an inert atmosphere where three peaks of released hydrocarbons can be measured. The first peak (S1) represents the volatilization of any previously generated hydrocarbons present in the rock, given that it has reached adequate thermal maturity. The type of organic material, or kerogen, determines whether generated hydrocarbons will be generally oil (sapropelic kerogen) or gas (humic kerogen), which has huge economic implications for companies interested in exploration and production.

v  Chromatography

A more in depth analysis can be conducted on the gasses released from pyrolysis through chromatography. Chromatograph “fingerprints” are generated based on the relative abundances of light to heavy carbon chains and their distributions in relation to oil-prone and gas-prone source rocks. Pyrolysis gas chromatography yields very specific geochemical data with far reaching applications. For this reason, the technique is most commonly seen in research settings

v  Seismic Evaluation

Using seismic data to determine a source rocks TOC content is possible and could potentially revolutionize how oil and gas companies choose to conduct source rock evaluations. Claystone source rocks have shown a predictable reduction in acoustic impedance with increasing levels of TOC. Increasing TOC also intensifies the vertical anisotropy seen in claystones. In seismic data, this produces a high amplitude negative reflection at the top of a potential source rock and a positive high amplitude reflection at the base, which can indicate levels of total organic carbon if calibrated correctly.

Ø  Geochemical Evaluation

Availability of viable source rocks constitutes a major factor governing the accumulation of hydrocarbon.  One of the essential steps in hydrocarbon exploration and exploitation is to understanding the source rock evolution. Organic geochemical characterization of source rocks entails assessing the hydrocarbon generation potential of sedimentary rocks by taking a look at the sediments capacity for hydrocarbon generation, type of organic matter, type of hydrocarbon that is expected to be generated and the sediments thermal maturity. It provides valuable information relating to concentrating exploration activities in particular places and reducing risks and costs.

Question 4. Explain the following

     i. Anoxic environment

     ii. Oxic environment

v  Oxic environment

An environment in which oxygen is involved or present. Poorer organic matter perseveration (0.2-4% TOC), Lower quality organic matter. Biological reworking is enhanced by: Presence of animal scavengers at interface, Bioturbation facilities diffusion of oxidants (02, SO4) in sediments. Lesser organic complexation with toxic metals, Oxic environment will lead to poor preservation of organic matter.

Under oxic conditions all sediments are mixed as the result of the actions of a variety of types of biota. Depending on the depth of biological activity, the sedimentary record will reflect this mixing process in the distribution of radionuclides.

Oxic (left) and anoxic (right) depositional environments generally result in poor and good preservation of deposited organic matter, respectively. The solid horizontal line separates oxic (above) from anoxic (below). In oxic settings, bottom dwelling metazoa bioturbate the sediments and oxidize most organic matter. In anoxic settings, especially where the oxic-anoxic boundary occurs in the water column, bottom- dwelling metazoa are absent and sediments are not bioturbated.

v  Anoxic environment

Anoxic environment is one that has no oxygen available. When we talk about anoxic environments, we are often referring to an aquatic environment with no dissolved oxygen or an underground environment (like soil or rock deep beneath the surface) without oxygen.

Anoxic conditions occur when the consumption of oxygen is greater than supply. Consumption of oxygen is controlled by organisms living and dying in the environment. When organic detritus accumulates on the sea bed it starts to degrade. During the degradation process oxygen is consumed. If the consumed oxygen is not replaced, anoxic conditions will be established.

The most common type of environment that may be anoxic is a body of water. Swamps or hypereutrophic water bodies are commonly devoid of dissolved oxygen. A hypereutrophic water body is one that is extremely rich in nutrients like phosphorus or nitrogen, creating an explosion of plant life. Because plants undergo photosynthesis to produce oxygen, we might first think that water bodies with a lot of plants would be oxygen-rich, but this isn't the case, especially if the primary plant matter is algae.

Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen and are a more severe condition of hypoxia. The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. This condition is generally found in areas that have restricted water exchange.

In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier as well as by a pronounced density stratification, in which, for instance, heavier hypersaline waters rest at the bottom of a basin. Anoxic conditions will occur if the rate of oxidation of organic matter by bacteria is greater than the supply of dissolved oxygen.

Supply of oxygen is controlled by water circulation patterns. At the surface the water takes up oxygen from the atmosphere. Through vertical circulation the oxygen is brought down to the sea bed. If there is permanent stratification in the water there will be no vertical circulation. Permanent stratification occurs as a result of density contrasts in the water. Density contrasts are caused by temperature or salinity differences.

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