Role of Organic Geochemistry in Petroleum

A review on role of organic geochemistry in petroleum; characterization and applications of different basins

Harish Chandra Joshi

Abstract

Petroleum is a mixture dominantly of hydrocarbons with varying proportions of non-hydrocarbon constituents and traces of organometallic compounds. Generally Petroleum has an average composition of 85% carbon, 13% hydrogen, and 2% of sulphur, nitrogen and oxygen. The aim of study is to find out the physicochemical and genetic property of petroleum. In this study biomarkers, age specific biomarker and reservoir geochemistry can be used for the characterization, correlation and/ or reconstruction of the depositional environment as micro and macro fossils used by the geochemist.

Keywords: Biomarker, Genetic Characterisation, Kerogen, Geochemical Fossils.

Introduction

The name geochemistry was first used by the Swiss chemist, Christian Friedrich Schonbein in 1838. Petroleum geochemistry is the application of chemical principles to the study of the origin, migration, accumulation, and alteration of Petroleum (oil and gas) and the use of this knowledge in exploring and recovering Petroleum. Organic chemistry is the branch of chemistry that deals with the distribution and composition of carbon compounds. Geochemistry is the study of the chemical composition of the earth, minerals, ores, rocks and also is the study of the origin of petroleum. The major tasks of geochemistry can be summarized as follows:

• The study of the relative and absolute abundances of the elements and of the atomic species (isotopes) in the earth.
• The study of the distribution and migration of individual elements in the various parts of the earth (the hydrosphere, atmosphere and lithosphere etc.), and in mineral and rocks, with the object of discovering their distribution and migration.

Exploration companies have used petroleum geochemistry in hydrocarbon exploration. The most and major objective of exploration geochemistry, is to reduce the risk of drilling dry holes. Petroleum geochemistry is based on the organic origin of the oil and gas whereby organic matter obtained from dead plants and animals. Organic matter is converted to hydrocarbons in the subsurface through various major three stages of transformations diagenesis, catagenesis and metagenesis. German scientist Treibs (1936) reveal a relationship between chlorophyll-a in living photosynthetic organisms and porphyrins in Crudes of petroleum. This link provides a strong evidence of organic origin

of Petroleum. From the starting of the Precambrian till the Devonian, the unique primary producer of the organic matter were marine phytoplanktons. Since the Devonian an increasing amount of primary production has been contributed by higher terrestrial plants. At present cenario marine phytoplankton and higher terrestrial are estimated to produce about equal amounts of organic carbon. On increases the burial depth, porosity and permeability decrease, and temperature increases. Thus lead to the change a gradual halting of microbial activity and thus eventually called ‘organic diagenesis to a halt. As the temperature rises, thermal reactions become increasingly. This second transformation phase, called ‘catagenesis’, during the catagenesis kerogen begins to decompose into smaller, more mobile molecules. In the early stage of catagenesis, kerogens are still relatively large; these are precursors for petroleum and are called “bitumen”. In the late stages and final transformation stage, called ‘metagenesis’. During metagenesis the principal products consist of smaller gas molecules. Further, kerogens formed from different organic matter, or under different diagenetic conditions, are chemically clear which has a significant effect on hydrocarbon generation.

Characterization of crude oil by Analytical Methods

Firstly sampling of crude oils is required for their characterization. Oil should be collected as a single- phase sample under pressure conditions as they are in reservoir. Therefore for the geochemical studies, crude oil samples are collected at the well head under atmospheric pressure. Under these conditions light hydrocarbons of crude oils are lost completely or partly. Light hydrocarbon fraction gives the ideas only about the abundance and constituents of the light end of the oil. It is normally observed that the most abundant characteristics hydrocarbons are commonly in the light fraction. For required minimizing the effects of sampling error the crude oil is distilled at 210⁰C. The heavier fraction is considered the foremost part of the crude oil. It is used to describe the chemical composition of a crude oil and also to compare it with other crude oils.

Analytical Techniques in Petroleum Exploration

Petroleum system (Demaison, 1994; Hunt, 1996) comprise all those geological elements and processes that are necessary for an oil and gas deposit to occur in nature. These main elements are a petroleum source rock, migration paths, reservoir rocks, seals, traps and the geological approach that design each of them. Such systems involve a genetic relationship between the source rock and the petroleum accumulations, but proof of that relation force a geochemical correlation. organic geochemistry techniques available include surface geochemical prospecting, source rock geochemistry, crude oil geochemistry, natural gas geochemistry, biomarker geochemistry, isotope geochemistry etc.

Biomarkers in Petroleum

‘Biological marker’ or shortened to ‘Biomarkers’ (Seifert and Moldowan, 1981) are complex molecules derived from once living organisms they are found in sediments and oil and show little change in structure from their parent molecules (Peters & Moldowan, 1993 and Hunt, 1996). These compounds are also called as geochemical fossils (Eglinton and Cavin, 1967) because of their origin from living organisms. Such compounds may be derived from terrestrial (mostly plants, marine pelagic (mostly plankton) and marine benthonic (algae, bacteria and other microbes). Biomarkers are generally, microfossils less than 30 nm in diameter and are highly variable in their stereochemistry i.e. the spatial arrangement of atoms and groups in their molecules.

The common use of the biomarkers in petroleum exploration may be enumerated as follows:

1. Biomarkers are present in both and oil a source rocks so they provide vital information for the oil-oil and oil-source correlation.
2. Organic matter type (source of organic facies)
3. Depositional environment
4. Extent of thermal maturation
5. Degree of biodegradation
6. Information about the age of the source rock

α and β Geometry of Biomarkers

Steranes obtain from the diagenesis of natural products sterols. Diagenesis converts sterol via chemical dehydration and microbial reduction to a steranes cholestane. Cholestane molecule is drawn in three dimensions as follows. The hydrogen at the 3 position points up above the plane of the molecule and that at the 5 position points down below the plane (Peters and Moldowan 1993)

Commonly Used Biomarkers in Petroleum Exploration

Normal Alkanes: Normal alkanes are a homologues series of saturated hydrocarbons of general formula C_nH_2n+2. All linear n-alkanes from C₁ to C₄₀ and a few beyond C₄₀ derived from different sources have been identified in crude oils.

Iso- and Anteiso-alkanes: Isoalkanes are 2-methyl alkanes and quite a number of these have been observed in crude oils as have been the anteiso-alkanes, the 3-methlyalkanes. Iso and anteiso alkanes are associated with n-alkanes in plant waxes where they comprise a approximate number of carbon atoms (about 25-31) with an odd predominance

Figure 1. Showing common biomarkers like paraffins, Iso and ante-isoalkane

Acyclic Isoprenoid: These are special type of Iso-alkanes in which one methyl group is attached to every fourth carbon atom in straight. Isoprene (methyl butadiene) is the basic structural unit composed of carbon atoms that is found in all biomarkers. The most common isoprenoids are pristane (C₁₉) and Phytane (C₂₀).

Figure 2. Common Isoprenoid biomarkers in petroleum

Terpenoids: Terpenoids can be classified based on structural types into diterpenoids and triterpenoids Diterpenoids are categorized into bicyclic and tricyclic diterpenoids. Triterpenoids are grouped into tetra and pentacyclic. The most knowing are pentacyclic and among these are hopanes. Hopanes are pentacyclic triterpenoids comprised of four 6-membered and one 5-membered ring. There is a side chain which can contain upto 8 carbon atoms. Thus the series comprise of C₂₇-C₃₅ hopanes. They are believed to have originated from polyhydroxybacteriohopane.

Figure 3. Structures of Common Triterpanes

Figure 4. Structures of Common Tricyclic and Tetracyclic Terpanes

Steranes: Steroids can be classified as aliphatic and aromatic steroids (mono, di- and tri-aromatic depending on the number of aromatic rings). Steranes are a series of aliphatic steroids. The sterols in all eukaryotic organisms are precursors to the steranes in sediments and petroleum. Like the hopanes, steranes are abundant in sediments, rocks and petroleum, because their precursors (Sterols) are so common in living organisms. Cholesterol has eight asymmetric centers and might be expected to show as many as 28 or 256 stereoisomers.

Figure 5. Chemical Structure of various steroids

Porphyrins: Porphyrins are characterized by a tetrapyrrolic nucleus proved to be inherited from chlorophyll, the green photosynthetic pigment of plants and animals ,hemin, the red pigment of animal blood. These tetrapyrrolic organometallic compounds reported of the vanadium and nickel in petroleum. The major types of fossil porphyrin are deoxophylloerytrapyrrole (DPEP) and etioporphyrin (ETIO) porphyrin structure.

Age specific biomarkers

If biomarkers characterise a molecular record of life, they can be used for age determination. Certain age specific biomarkers like Oleanane present in oils derived from late Cretaceous or Younger. C11-C19 Paraffins, Odd carbon number prevalence in oil from many Ordovician sources. 24-n-propylcholestane, High in oils from Ordovician sources.Thus the biomarkers transport to the sources has proved to be of great help in geochemical characterization of the oils/condensates.

Reservoir Geochemistry

The main aim of reservoir geochemistry is to understand the distribution and origin of the petroleum, water and minerals in the reservoir and account for their possible spatial and compositional variation (Cubitt and England 1995). A better understanding of the fluids in the reservoir conduct to a better understanding in an area and prioritization of exploration thrusts. The principle factors responsible for difference in petroleum composition are the effect of organic facies variations, progressive source rock maturation, migration fractionation, gravity segregation, oil/water contact and non-uniform biodegradation of oil across the field. However these effects have been normalized by using ratios of peaks corresponding to compounds of similar molecular weight in the C₁₀⁺ region of the chromatogram.

The study of reservoir continuity is also the focus of the geochemical characterization to trace the nature and depositional conditions of the source organics, identification of the oil families and thermal maturity of the oils/condensates.

When a set of chromatographic peaks has been selected, a variety of techniques are available for grouping of this data. One way is to use a polar plot of selected ratios by a “star diagram” (polygon plot) by plotting each peak ratio on a different axis of polar plot. Each data point is plotted from the centre of the concentric circles outward. The points are then connected to create a star shaped pattern characteristic of each oil.

Applications of geochemical characterisation

Biomarker and non-biomarker geochemical parameters are best used together to supply the most authentic geological interpretations to help solve exploration, enlargement, production and environmental problems. Prior to biomarker work, oil and rock samples are properly screened using non biomarker analyses. The strength of biomarker parameters is that they provide more detailed information needed to answer questions about the source rock depositional environment, thermal maturity and the biodegradation of oils than non-biomarker analyses alone. Different depositional environments are characterized by different assemblages of organisms and biomarkers. Commonly accept classes of organisms include bacteria, algae, and higher plants. Biomarker parameters are also an effective means to determine the relative maturity of petroleum through the entire oil-generative window.

Conclusion

On the basis of above observation major conclusions which have been derived from the whole study are as follows:

The presence of complete range of normal alkanes upto nC₃₆ and in some cases upto nC_40. The presence of biomarker in oil indicates that oil may be terrestrial or marine. The terrestrial nature of the source is also strongly indicated by the steranes. Reservoir geochemistry of oils has been used to demonstrate the lateral/vertical continuity/compartmentalization.

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