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The parameters that used in the exploration of such hydrocarbons are discussed below: 1. Determination of Ancient Shore Lines 2. Sedimentology 3. Post Depositional Alteration of Palynomorphs 4. Biostratigraphy and Palaeo- Environmental Analysis 5. Palynological Assemblages of Coal Seams.
1. Determination of Ancient Shore Lines:
The defining of ancient shore lines bears special significance in oil accumulation. It is generally believed that sediments parallel to sea shore are rich in oil. Most palynologists believe that those spores and pollen, which enter the fossil record, owe their distribution primarily to the atmosphere, especially to wind patterns.
The palynoflora from sediment of small ponds, soils, and swamp-generated peat, contain mostly autochthonous spores/pollen that dropped out of the air in the neighbour of the site of deposition. Most fossil pollen/spores in sediments have been transported sometimes very long distances, by streams and by ocean currents.
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For instance the spore/pollen flora of the Mississippi River delta contains spore/ pollen from the vegetation of North Dakota, Pennsylvania, Minnesota and all areas between those states and Louisiana, carried by the various tributaries of the Mississippi.
As the pollen rain is progressively less in the sea ward direction, sedimentary environments with pollen assemblages are limited to near shore marine and lacustrine waters. In modern marine sediments pollen and spores are frequently associated with diatoms, microforaminifera, dinoflagellates, hystrichospherids, and various oceanic plankton.
It is possible to determine distance and direction of ancient shorelines by the kinds and quantities of microfossils, as pollen and spores will decrease in density in a seaward direction, with a corresponding increase in marine forms.
In the figure 14.11, spores/pollen are shown as constituents of all environments except deep sea (more than 3000m), in decreasing abundance toward the open ocean. Spores/pollen provides only a link between terrestrial and true marine environments. The marginal and inner continental shelf environments contain only palynomorphs like spores/pollen, dinoflagellates cysts, and acritarchs. Fresh water environments lack foraminifera and nannoplankton.
Hoffmeister’s palynological group in Tulsa had discovered that spores/pollen are distributed in sediments according to principles that are basically sedimentological. Hoffmeister (1954) stated that the abundance of spores and pollen in sediment decreases sharply infact more or less logarithmically as one moves offshore. He also noted that there is a marked sorting among fossil spores/pollen, with smaller forms being relatively more abundant than larger forms, as one samples further offshore.
He stated that in a sample of shale the proximity of an ancient shoreline, which is the potentially important for oil exploration, is indicated where the ratio of large (70-120 pm) and small (20-50 pm) spores/pollen lies more or less between 0.25. Further the proximity of an ancient shoreline is indicated where the concentration of spores/pollen is about 7,500 per gram of sediment.
However, sediment size and bottom morphology are also important in a depositional process. Palynomorphs are much more abundant in sediment offshore from stream mouths than where no such drainage is nearby. It has also been shown that concentration tends to be lower on the continental shelf, and higher on the slope and rise.
2. Sedimentology:
Maceration residues prepared from sedimentary rocks contain a wide range of organic particles in addition to palynomorphs and these have been termed palynodebris. Several palynologists have made an effort to study such particles on a systematic basis, with the hope that such study could contribute to understanding of the total sedimentological picture.
Further the range of kinds of organic particles in sedimentary rocks also controls to some extent the production of hydrocarbons from the rock during thermal maturation.
These palynodebris have been classified into four principal types, viz.:
a) Amorphogen (structureless organic matter);
b) Phryogen (non-woody plant material, including palynomorphs);
c) Hylogen (from woody material);
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d) Melanogen (opaque organic matter).
Amorphogen and phryogen are more likely to produce liquid hydrocarbons in time than hylogen, and melanogen is least likely to be productive. Phryogen is usually dominant in marginal marine sediments, amorphogen is high in offshore marine sediments, and hylogen is most common in non-marine deposits.
Those particles, which originally were fragments of plants, are sometimes called phytoclasts. Masran and Pocock (1981) classified the phytoclasts based largely on botanical and coal petrological classification of particulate palynodebris (Table 14.3). Masran (1984) demonstrated the usefulness of the method in a study of North Atlantic Jurassic- Cretaceous sediments.
The organic sediment could be classified as marine or non-marine in origin and conclusion could be drawn about the post-depositional history of the sediment thus helping in exploration.
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Habib (1982) has classified the organic particles in palynological residues into five categories, Exinitic, Tracheal, Shredded xenomorphic, Micrinitic and Globular xenomorphic. Exinitic and tracheal residues typically occur in sediments that are rapidly deposited. They are comparatively rich in total organic matter. Micrinitic residues are typical of more slowly deposited sediments.
3. Post Depositional Alteration of Palynomorphs:
Post depositional oxidation can corrode or even destroy palynomorphs. Further post depositional heating causes chemical changes. All these affect the properties of the dispersed sporopollenin just like the coal series as it proceeds from peat to anthracite grades, with loss of H and O and concomitant enrichment of C and molecular condensation.
The process of coalification of the dispersed organic matter in sedimentary rock by thermal alteration is called catagenesis (50 -150° C). More precisely thermal up to 50° C as diagenesis (R0 up to 0.5), that in the 50-150° C range as catagenesis (R0: 0.5-2), that in the 150°C – 250°C range as metagenesis (R0: 2-4), and that above 250°C as metamorphism (R0 above 4).
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The principal observed change in spores/pollen exine along the carbonization- coalification route is the change of colour in transmitted light. Fresh exines of modern plants are pale yellowish to almost colourless. If exines are heated, e.g., by deep burial or proximity of the enclosing sediment to a lava flow, the colour intensifies from yellow to orange to brown, and ultimately black.
The length of exposure to the elevated temperature is also very important in carbonization. The exines of spores/pollen reach this point at about the Inkohlungssprung (a German word for ‘coalification leap’), the point in the natural carbonization process at which a quantum jump in molecular organization of coal occurs (Fig 14.12). It occurs in the low-volatile bituminous coal range at about 75% fixed carbon (- about 25% volatile matters).
As seen in the figure 14.12 palynomorphs consisting of compounds other than sporopollenin respond to geothermal maturation somewhat differently. Algal remains start with colourless but quickly undergo alteration in colour. Chitinozoans and scolecodonts, composed of pseudochitin and chitin respectively, start with dark yellow and darken at first slowly, then rapidly move to Inkohlungssprung.
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Spores/pollen exine start with yellow, alter to brown and black. Chitinous- walled fungal spores start either lighter or somewhat darker (pale brown) than spores/ pollen (usually pale yellow) but are bit more resistant to change than even sporopollenin. Acritarchs and dinoflagellates cysts are almost colourless and require more temperature exposure to darken. In other words when spore/pollen exines are already dark brown, acritarchs may be still relatively light in colour.
The first proposal of carbonization of dispersed organic materials in rocks to predict hydrocarbon potential of the rocks came from Gutjahr(1966). Baton (1980) suggested the seven-point scale for a measure of the carbonization (Table 14.4).
This colour of palynomorphs maybe compared with Pearson’s Colour Chart (Fig. 14.13) in which the colour is directly related to the numerical Thermal Alteration Index (TAI). Thus carefully recorded colour observation can be used for reconnaissance of the geothermal history of the rock.
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Fig. 14.13: Palynomorphs exine colouration with geothermal maturation. Thermal Alteration Index Scale (TAI) is based on Palynomorphs colour and on Vitrinite reflectance.
The course of carbonization or thermal maturation can also be observed by use of fluorescence microscopy of both exinate (spores/pollen in coal) and vitrinite (wood and bark tissue in coal). The particles in question are placed in an ultraviolet light beam, and the fluorescent light emitted is studied.
Teichmuller and Ottenjahn (1977) have shown that fluorescence microscopy very sensitively shows diagenetic changes of spore/pollen walls and other organic substances in the context of oil formation. They further demonstrated that fluorescence microscopy reveals changes in the macerals of coal, related to bitumen formation during coalification.
Van Gijzal (1981) reports that the principal stage of petroleum generation (Roil = 0.050 – 0.85) is characterized by double peaks in exinite fluorescence spectra, and the “oil death line” (Roil = 1.15 -1.35) is reached when fluorescence does not occur.
4. Biostratigraphy and Palaeo- Environmental Analysis:
Microfossils are abundant in the marine rocks, which are the most common forms of sedimentary rock in the crust of the Earth. Microfossils have many applications in petroleum geology and the two most common uses are biostratigraphy, and palaeoen- vironmental analysis.
The use of biostratigraphy as a tool in petroleum exploration is well established. Biostratigraphy is the differentiation of rock units based on the fossils contained in them. Various fossil groups can be found in different sedimentary environments, viz., terrestrial and marine. The frequently used fossil groups in biostratigraphy are listed in Table 14.5.
Most species of foraminifera are bottom- dwellers (benthic), but during the Mesozoic Era a group of planktonic foraminifera arose. These forms were free-floating in the oceans and as a result are widely dispersed than benthic species. Calcareous nannofossils are extremely small objects produced by planktonic unicellular algae.
They first appeared during the Mesozoic Era. Like the planktonic foraminifera the planktonic mode of life and the tremendous abundance of calcareous nannofossils make them very useful tools for biostratigraphy. The palynomorphs besides including fossil spore and pollen also include certain marine organisms such as dinoflagellates (red algae which make up the red tides in modern oceans).
The organic chemicals, which comprise the palynomorphs, get darker with increased heat. Because of this colour change they can be used to assess the temperature to which the rock sequence was heated during burial. This is useful in predicting whether oil or gas may have formed in the area under study, because it is the heat from burial in the Earth that makes oil and gas from original organic rich deposits.
Biostratigraphy plays a critical role in the building of geological models for hydrocarbon exploration and in the drilling operation that tests those models. The fundamental principle in stratigraphy is that the sedimentary rock in the Earth’s surface accumulated in layers, with the oldest at the bottom and the youngest on the top. (Fig 14.14).
The history of life on Earth has been one of creatures appearing, evolving, and becoming extinct (Fig 14.15). Putting these two concepts together, we observe that different layers of sedimentary rocks contain different fossils.
When drilling a well into the Earth’s crust in search of hydrocarbons, we encounter different fossils in a predictable sequence below the point in time where the organisms became extinct. In the figure 14.15, the extant species C is present in the uppermost layers. Species B is found only in the lower layers. The well does not penetrate any layers containing fossil A.
The point at which you last find a particular fossil is called LAD (Last Appearance Datum). In a simplified case, the LAD in one sequence of rock represents the same geological moment as the LAD in another sequence. These are the points of correlation between wells. Another well, drilled in this area should penetrate the same sequence, but most likely at different depths than the original well.
In addition to the LAD, another event is the First Appearance Datum (FAD). This may be difficult to recognize in a well, because rocks from higher in the well bore may slough off the wall and mix the rock from the bottom of the hole. However, in studies of rock units exposed at the surface of the Earth and in some cases from well bores, these FADs are extremely useful biostratigraphic events.
From figure 14.15 one can recognize that the range of the three fossils overlap for only a relatively short period of geologic time. As a consequence, if a sample of rock contains all three (A, B, & C), it must have been deposited during this interval of time (Concurrent Range Zone). This is yet another “event” which can be used to subdivide geologic time into biostratigraphic units.
In oil-company-oriented palynostartigraphy the operational task is to correlate sequences from one well with those from others. Correlations by palynofloras depend on real tops and bottoms based not on facies (migration) but on extinction of some forms and evolution of the other forms. In the time represented by some parts of the geologic column, plants were evolving rapidly, e.g., Middle and Upper Devonian thus spores are very effective for stratigraphy.
At other times dinoflagellates were more rapidly evolving than land plants, e.g., much of Jurassic, thus dinocysts are very effective for practical stratigraphy. Spore/pollen has big advantage for biostratigraphy over marine fossils, such as foraminifera, since they occur in all sedimentary environments. Dino- flagellate cysts mostly originate in marine environments. Thus palynostartigraphy can correlate levels in marine sediments with those of non-marine sediments.
5. Palynological Assemblages of Coal Seams:
Most bituminous coal contains abundant plant spores and majority of the spores are believed to be autochthonous in origin. The spores/pollen in coals occurs in a characteristic association, implying the occurrence of different plant communities associated with the deposition of the peat.
The Coal Measures of Britain (coal measure is used to describe a succession of sedimentary rocks comprising of claystones, shales, siltstones, sandstones, conglomerates, and limestones that are interstratified with beds of coal) indicate the existence of four such associations.
Each association is dominated by one or more characteristic species. The less common species are often restricted to a particular association and are therefore of considerable diagnostic value in interpreting the character of a composite assemblage. Some species appear to be generally distributed in coal.
Three of the spore associations are characterized by certain species of the genera Lycospora, Laevigatosporites and Densosporities respectively. The fourth association is characterized by the genera Crassispora, Fabasporites, and Puctatosporiteis. Spores of all these genera are among the most abundant in the Coal Measures.
The paleobotanical affinities of Lycospora strongly suggest that the parent vegetation was at least in part composed of arborescent lycopods. The assemblage characterized by Crassispora, (possible Sigillarian affinity) Fabasporities and Punctatosporites (probable Filicean affinity) is rich in species. The spores often corroded and associated with petrographic constituents suggest an allochthonous origin.
If it is accepted that within a basin of deposition peat formation takes place under several environmental conditions, it follows that there will be a corresponding number of vegetation and peat types. Thus in the coals it is reasonable to accept a relationship between the miospore floras and the petrographic types.
It can be seen that the Lycospore phase is mainly associated with coals rich in the microlithotypes, vitrites and clarite. The Densospore phase confined to durite and this durite has been referred as crassidurite, because of the thick- walled character of the spore exines (Fig 14.16).
Conclusion:
Energy is required nowadays in practically every aspect of our lives. It is needed to cook, to provide light and heat, to propel vehicles, to drive machinery in industry, etc. Coal and oil are the two major fuels in use today and their judicial harnessing is of extreme importance for long term benefit to mankind.
