Methods/Steps Used To Evaluate The Source Rocks

How does organic matter generate Hydrocarbon?

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.

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.

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 • 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 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 refers to rocks from which hydrocarbons have been generated or are capable of being generated. They form one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been deposited in a variety of environments including deep water marine, lacustrine and deltaic. Oil shale can be regarded as an organic-rich but immature source rock from which little or no oil has been generated and expelled. Subsurface source rock mapping methodologies make it possible to identify likely zones of petroleum occurrence in sedimentary basins as well as shale gas plays.

Rock rich in organic matter which, if heated sufficiently, will generate oil or gas. Typical source rocks, usually shales or limestones, contain about 1% organic matter and at least 0.5% total organic carbon (TOC), although a rich source rock might have as much as 10% organic matter. Rocks of marine origin tend to be oil-prone, whereas terrestrial source rocks (such as coal) tend to be gas-prone. Preservation of organic matter without degradation is critical to creating a good source rock, and necessary for a complete petroleum system. Under the right conditions, source rocks may also be reservoir rocks, as in the case of shale gas reservoirs.

There are three types of Source Rock:

·         Type 1 Source Rock- During deep burial when they are subjected to thermal stress they generate waxy crude oil. The algae which remains in deep lakes (in anoxic conditions) is the major source of type 1 Source Rock.

·         Type 2 Source Rock- During thermal crack they produce both oil and gas. In the marine environment, under the anoxic condition, the deposition and preservation of bacterial remains form these types of rocks.

·         Type 3 Source Rock- They are formed by terrestrial plant material.

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.

Other types of source rocks

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.

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

·         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

·         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.

·         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.

·         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.

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 )

Question 4. Explain the following

     i. Anoxic environment

     ii. Oxic environment

Anoxic environment

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. 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.

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.

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.

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