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| Organic Matter |
Organic Matter
Organic matter is accumulated (mostly in a dispersed state) in predominantly clayey marine deposits. There are two major types of organic matter: humic and sapropelic. It was believed that the latter played a major role in oil generation, whereas the decomposition of humic organic matter resulted in the formation of coal and water-soluble (hence, easily dispersible) substances and gas. The decomposition of sapropelic matter gives rise to the liquid and gaseous compounds including hydrocarbons. The decomposition occurs as a result of heat flow and the energy of the sun accumulated by the organic matter. The hydrocarbons and some other substances formed from the decomposed organic matter are squeezed together with water out of the shales into the reservoir rocks. The hydrocarbons derived from the organic matter float in the water medium (gravitational theory) and move until trapped in the reservoir. Marine origin of oil source rocks appeared to be obvious, although it is unclear why the first oil-bearing sequences developed in different countries were continental or near-shore marine Paleogene and Neogene rocks. The studies of the present-day sedimentation indicated that all marine and almost all continental deposits contain organic matter. It was eventually recognized that only the presence of subaquatic sediments, either of marine or continental origin, was required. There were three ways for organic matter to "burn" in nature: combustion, smoldering, and rotting. The latter process was believed to be responsible for the organic matter formation in nature. This is an important issue for at least two reasons: rotting occurs without the supply of oxygen; from the outset, the process is believed to be isothermal.
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| Black shales are source rock, black colour is because of the baked organic matter. |
Source rock
Sediments may be classified as source rocks if they contain organic matter at least 2% or 43kg/m3. The absolute amount of organic matter buried in various genetic types of sediments depends on many factors, but mainly on the biological productivity of source organisms and by the facies environment during burial. Relative organic matter concentration also depends on the depositional speed. For example, due to a very slow rate of deposition, organic matter content in the Central Polar Basin reaches 1%. Generally, organic matter concentration in sediments widely ranges between trace amounts and 100% (in peat). The content of a dispersed organic matter in source rocks may be even lower, because part of it had been spent to generate oil and gas that have been expelled. Some other investigators believe that less than 1% of organic matter is converted into oil; thus, the expulsion loss could not significantly affect the residual carbon distribution in sediments. Effect of depositional rate on the qualitative and quantitative outcome of the organic matter transformation. First, the relative increase in the organic matter concentration is observed. Then, the rate reaches 50–200tons/km2/year and the depositional rate decreases due to the organic matter dilution by minerals. Other conditions being equal, the value and duration of AHFP also directly correlates with the depositional rate. The depositional rate in most known oil and gas basins is between 150 and 1000tons/km2/year. The major geologic factor in transforming organic matter into bitumen is the compaction of rocks under the overburden pressure. The oil and gas generation process is lengthy and continuous: Hydrocarbon compounds arise as a result of competition between two opposite trends: when subsidence prevails over uplift, during small as well as large oscillations of a given Earth's crust area. Oscillations of the Earth’s crust are the cause of relationships among the depositional processes, rock formation, and structural development. Sedimentation is responsible for the accumulation of organic matter in deposits. Lithification is responsible for the organic matter transformation, whereas the structural development (tectonic activity) is responsible for the formation of combustible fossil fuel accumulations, their metamorphism, and destruction. Oil and gas generation is, therefore, an unalienable part of the Earth’s crust evolution and involves dynamic processes. It is not just a simple mechanical displacement but consists mainly of complex transformations. These transformations consist of biologic, geochemical, physiochemical, and other changes. They manifest themselves jointly, but play different roles at different stages of oil and gas generation.
On the basis of laboratory studies, emerged a concept of clay-mineral transformation from montmorillonite to kaolinite during rock compaction. The limitations imposed on the role of clay minerals, as just catalysts in the transformations of organic matter. This concept, in turn, has been questioned, such a process could not proceed in nature due to insufficient supply of potassium in rocks. A number of people recorded the clay-mineral alterations during catagenesis. As an example, montmorillonite alterations during meso-catagenetic (MC) stage and cessation of this process upon reaching the MC4 sub-stage. In the process of alterations, the particles (sheets) change their orientation, which results in locking-up or opening of pores; hence, the filtration in clays acquires a random, unstable nature.
The following questions arise in studying the oil generation in source rocks (mostly shales):
- What is the nature of source rocks and where and how are they distributed in sedimentary basins?
- What is the physical state of hydrocarbons in source rocks?
- What forces (and at what stage) cause them to move to the reservoir rocks (primary migration/expulsion)?
- When and how do hydrocarbons move within the reservoir rocks to form oil and gas accumulations?
- What is the relation between (a) the oil and gas composition in the accumulations and (b) the environment of hydrocarbon generation and of formation of accumulations?
The term "micro-oil" has been introduced: The origin of oil begins with the living matter where the biochemical compounds are born that initiate formation of petroleum hydrocarbons or, to a smaller extent, where these hydrocarbons are born. Upon deposition at the bottom of a basin, and partially forming in the sediments due to the activity of organisms, these hydrocarbons and pre-hydrocarbons form a young micro-oil. Some people did not accept the term micro-oil because while building their bodies, cellular membranes and other cellular structural elements, plant and animal cells and, especially, some bacteria synthesize hydrocarbons. After death of the organisms and inclusion of their remains in the depositional cycle, the hydrocarbons contained in them may be decomposed by the microbial activity. The relative rate of hydrocarbon decomposition is lower than that of the other organic compounds. Thus, under favourable conditions some hydrocarbons may accumulate. The components of oil were not born all at once. It would be better to discuss not the source but the hydrocarbon generation stages that would correspond to the stages of lithogenesis. Various organic compounds formed from the moment of initial deposition, whereas mixtures of liquid hydrocarbons (crude oils) apparently formed during the formation of accumulations in reservoirs.
Various components of oil could have formed at different stages of organic matter transformation and at different lithogenetic stages. Thus, a complex chemical system called crude oil formed within reservoirs in the process of the formation of accumulation. That is why the term micro-oil was not accepted but view was hydrocarbons and non-hydrocarbon organic compounds dispersed in rocks are not macro- or micro-oil. Integrated geological and geochemical studies of the modern and ancient sediments provide ever growing body of data, which indicates that each stage of lithogenesis is accompanied by its own characteristic hydrocarbon generation. It should also be remembered that the parent organic matter is also changing simultaneously. A concept of a relation between the hydrocarbon generation and catagenetic stages was broadened into the doctrine of oil and gas generation cycles. As discussed later, the "stage-wise" and "cyclic" nature do not have the same meaning. In discussing the organic matter transformation into crude oil, it is necessary to consider the transformation factors. The major error implicit in most hydrocarbons-to-crude oil transformation concepts is the attribution of an exclusive role to a single factor. This will result in the detachment from the natural environment where all these factors are operative and intertwined. The transposition of laboratory experiments onto the natural environment, without a full account for its multifaceted character, always results in errors. Besides, an active energy influence on the source organic matter is attributed to each factor. At the same time, no serious consideration was given until recently to the energy facet of the issue, although it is only natural that organic matter in itself has the sufficiently high reserves of energy for the subsequent transformations. It was suggested that the living matter accumulate the sun energy, which is subsequently transferred to the organic matter. It appears that this is not accurate. The heat of the Earth itself should not be forgotten. All geologically "live" planets, including the Earth, release much more heat than they receive from the Sun. The processes of life cannot ignore that energy source. One confirmation of the above is the presence of life at great depth in the oceans, where the sunlight does not reach. No doubt, the energy stored in the living organisms or in organic matter is much greater than that in the oil or coal. The processes of transformation of matter with loss of energy are very common in the Earth’s crust.
