INTRODUCTION:

Petroleum geochemistry is the branch of chemistry that deals with the distribution and composition of carbon compounds. Organic geochemistry is the study of the chemical composition of the earth, minerals, ores, rocks along with genesis, migration, accumulation and translation of petroleum.

Exploration companies have used petroleum geochemistry in hydrocarbon exploration. The most and major objective of exploration geochemistry, is to minimize 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 animals¹.

Organic matter is converted to hydrocarbons in the subsurface through various major three stages of transformations i.e. diagenesis, catagenesis and metagenesis. German scientist revealed a relationship between chlorophyll in living photosynthetic organisms and porphyrins in crudes of petroleum². This link provides a strong evidence of organic originof Petroleum. Marine phytoplanktons are the unique primary producer of the organic matter from the starting of the Precambrian till the Devonian. Since the Devonian higher terrestrial plants contributed the increasing quantity of ultimate production. At present cenario marine phytoplankton and higher terrestrial are approximately to produce about equal masses of organic carbon. On increasing 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 increase and this type of transformation phase is called 'catagenesis', during the catagenesis kerogen begins to decompose into smaller, more mobile molecules these are the precursor of petroleum. In the early stage of catagenesis, kerogens are still relatively large are called “bitumen”. In the late stages and final transformation stage, called ‘metagenesis’^{3, 4}. During metagenesis the primary products consist of small scale gas molecules. Further, kerogens formed from different organic matter, are chemically clear which has a significant effect on hydrocarbon generation⁵.

CHARACTERIZATION METHODS:

1. 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 these are in reservoir. Therefore for the geochemical studies, crude oil samples are collected at the wells 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 of hydrocarbons are commonly in the light fraction. For required minimizing the effects of sampling error the crude oil is distilled at 210°C. 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.

2. Analytical techniques in petroleum exploration:

Petroleum system comprises all those geological elements and processes that are necessary for an oil and gas deposit to occur in nature. Source rock, migration paths, reservoir rocks, seals, traps and the geological approach are the main elements of petroleum that design each of them^{6, 7}. Such systems involve a genetic relationship between the source rock and the petroleum accumulations, but proof of that relation force a geochemical correlation.

Application of biomarkers in petroleum:

Biomarkers are also called geochemical microfossils which are 30 nm in diameter and are greatly irregular in their stereochemistry. Geochemical fossils are formed from the living organisms that are found in sediments and oil^{8, 9}. Complex compounds of biomarkers may be derived from the terrestrial (mostly plants, marine pelagic (plankton etc.) and marine (mostly algae, bacteria and other microbes etc.). In the exploration of hydrocarbon biomarkers is the basic tool to utilize the stipulate of organic matter input, environmental discharge thermal maturity, oil-oil correlating and oil to source rocks correlating and age of the source rocks of petroleum ¹⁰.

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 steranescholestane. 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 plane¹¹.

Figure 1: Procedure for designing α (below the plane) and β (above the plane) hydrogens or other groups for a polycyclic biomarker¹¹

Commonly used biomarkers in petroleum exploration:

Normal Alkanes:

N- alkane is 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 a source which has 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 anteisoalkanes are associated with n-alkanes in plant waxes where they comprise a approximate number of carbon atoms (about 25-31) with an odd predominance

n-Alkanes

2-Methyl PentaCosane

Figure 2: 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) unit is the basic structural unit which is composed of carbon that is found in all biomarkers. The most common isoprenoids are pristane (C₁₉) and Phytane (C₂₀).

Figure 3: 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 carries upto 8 carbon atoms. Thus the series consists of C₂₇-C₃₅hopanes. These are believed to have originated from polyhydroxybacteriohopane.

Figure 4: Structures of Common Triterpanes

Figure 5: 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 inexhaustible in rocks, sediments and petroleum, because their precursors (Sterols) are so common in living organisms. Cholesterol has eight asymmetric centres and might be expected to show as many as 28 or 256 stereoisomers.

Stigmastane (C29) 24 n- Propylcholestane (C30)

Figure 6: Chemical Structure of various steroids

Porphyrins:

Porphyrins are characterized by 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, these 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 the characterization of the oils/condensates.

Identification of source and age related biomarker parameters/ratios:

n-alkane Distribution:

n-alkanes have their origin in plants and algal lipids. These are formed by the catagenesis of fatty acids and alcohols. The terrestrial plants produce normal alkanes with odd number of carbon atoms, especially 23, 25, 27, 29 and 31 atoms. On the other hand, marine algal lipids produce normal alkanes that have a maximum in their distribution at C₁₇ or C₂₂. Bimodal n-paraffin distributions, and those showed towards the range of nC₂₃ to nC₃₀, are usually associated with earthly higher plant waxes.

Figure 7: n-Alkane and Isoprenoid distribution in oils of different origin

Isoprenoid Distribution:

Pristane (Pr) and phytane (Ph) ratios are the commonly used isoprenoids distribution applied for correlation studies. Pristane and Phytane can be measured by GC analysis.

Isoprenoid/n-Alkane ratios:

Isoprenoid/n-alkane ratios, viz. Pr/nC₁₇ and Ph/nC₁₈provide valuable information on biodegradation and diagenetic conditions. These ratios are sometimes used in correlation studies too. Pr/nC₁₇ is a useful indicator for open marine (<0.5) and peat swamp (<1.0) environments. Both Pr/nC₁₇ and Ph/nC₁₈ decrease with maturity as with increasing maturity n-alkanes are produce rapid than isoprenoids.

Terpanes Fingerprinting:

Terpanes are derived from the bacterial (prokaryotic) membrane lipids, which are commonly found in petroleum. Tricyclic, tetracyclic, pentacyclic (hopane) terpanes mainly contributed to the terpanes fingerprints. The C₂₈and C₂₉ tricyclic are extensively used in correlation of oils and bitumens. The C₂₄-C₂₇ tetracyclic terpanes appear to be degraded hopanes and are more resistant to maturation and biodegradation. Hopanes are often pentacyclic triterpanes consists of 27 to 35 carbon atoms in naphthenic structure. These are derived from hopanoids in the precursor bacterial membrane.

Oleanane Index (OI):

Oleanane is measured by GC-MS analysis using m/z 191 mass-chromatogram. Oleananes are probably derived from betulins¹²and other pentacyclic triterpanes from angiosperms which were first prominent in the late Cretaceous (>100m.y.)¹³. Oleanane index usually use for comparing various petroleum sample and it is defined as:

Reservoir Geochemistry:

The main aim of reservoir geochemistry is to understand the distribution and origin of the petroleum, minerals and water in the reservoir interpretate for their possible spatial and compositional variation¹⁴. A better understanding of the fluids in the reservoir conducts 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 under the using of ratios of peaks compatible 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 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 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 these provide more detailed information about the thermal maturity, source rock depositional environment, non-biomarker analyses and biodegradation of oils. Different depositional environments are characterized by various accumulations of biomarker and organisms. Commonly accept classes of organisms include bacteria, algae, and higher plants. Biomarker parameters are also effective tools to determine the relative maturity of petroleum through the entire oil-generative window.

Applications of organic geochemistry:

The distribution, quantity and quality of organic matter (organic facies) are the point that help to decide the hydrocarbon probability of a petroleum source rock. Biomarker and non-biomarker geochemical parameters are best used together to provide the most reliable geological interpretations to help exploration, development, production and environmental problems. The strength of biomarker parameters is that these provide more detailed information about the thermal maturity, depositional environment, source rock, non-biomarker analyses and biodegradation of oils. Distributions of biomarkers can be used to correlate oils and extracts. For example, C₂₇-C₂₈-C₂₉Steranes or monoaromatics steroids distinguish oil source families with high precision. Different depositional environments are characterized by different accumulations of organisms and biomarkers. Often identify classes of organisms include bacteria, algae, and higher plants^{15, 16}.

CONCLUSION:

On the basis of above study 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.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

REFERENCES:

1. Tissot, B.P. and welte, D.H. Pertoleum formation and Occurrence, Springer- Verlag, New York. 1978; pp. 699.

2. Treibs, A. Chlorophyll and hemin derivatives in organic mineral substances. Angewandte Chemie. 1963; (49), pp 682-686.

3. Bhandari, A., Prasad, I.V.S.V., Kapoor, P.N., Varshney, Meenu, Madhavan, A.K.S., Pahari, S. and Singh, R.R. Depositional environment, distribution of source rocks and geochemistry of oil and gases, Krishna-Godavari Basin, Journal of Applied Geochem. 2008; 10 (1), pp 17-31.

4. Bhandari, A., Prasad, I.V.S.V., and Dwivedi, Prabhakar. Stratigraphic distribution of hydrocarbons in the Sedimentary Basins of India. Symposium in Applied Geochemistry in the evaluation and management of onshore and offshore Geo sources. Journal of Applied Geochemistry. 2007; 9 (1) pp 48-73.

5. Hunt, J.M. Petroleum Geochemistry and Geology. W.H. Freeman, San Francisco. 1979; pp 617.

6. Demaison, G.J and Huizinga, B.J. Genetic classification of petroleum systems using three factors: charge, migration and entrapment. In: The Petroleum system – From source to trap (L.B. Morgan and W.G. Dow, eds), American Association of Petroleum Geologists, Tulsa. 1995; pp. 73-89.

7. Hunt, J.M. Petroleum Geochemistry and Geology. W.H. Freeman and Company, New York. 1996.

8. Seifert, W.K. and Moldown, J.M. The effect of biodegradation on steranes and Terpanes in crude oil. Geochem. Cosmochim., Acta. 1979; (43), pp 111-126.

9. Seifert, W.K. and Moldown, J.M., 1980. The effect of thermal stress on source rock quality as Measured by hopane stereochemistry. Physics and chemistry of the earth. 1980; (12), pp 229-237.

10. Pandey, I.P., Joshi, H.C., Tyagi, Ashish Tiwari, Sadhana and Garg, Nitika. Study of the Parameters and Bio-Markers of Crude oils. Advances in Pure and Applied Chemistry, World Science Publisher, New York, United States. 2012. No. 3(1), pp 49-53.

11. Peters, K.E. and Moldowan, J.M. The biomarker guide interpreting Molecular fossils in petroleum and ancient sediments, Prantice Hall, Englewood Cliffs, NJ., U.S.A. 1993.

12. Grantham, P.J., Posthuma, J., Baak, A.Triterpanes in a number of Far-Eastern crude oils. In: Advances in Organic Geochemistry, J. Wiley and Sons, New York. 1983; pp 675-683.

13. Whitehead, E,V. Molecular evidence for the biogenessis of petroleum and natural gas.In;Proceedings of Symposium on Hydrogeochemistry and Biogeochemsitry, Vol. II(E.Ingerson,ed). The Clarke company, washington DC, 1973; pp 158-211.

14. Cubitt, J.M., England, W.A. The Geochemistry of Reservoirs, the Geological Society London, pp 1995; 321.

15. Pandey, I.P., Joshi, H.C. Physicochemical and Genetic Study of the Crude oils of Different Basins. International Journal of Science and Research (IJSR), 2015; 10 (4),pp.80-84.

16. Peters, K.E. and Fowler, M.G. Application of Petroleum Geochemistry to Exploration and reservoir management. Org. Geochem. 2002; (33), pp 5-36.

Received on 21.03.2017 Modified on 28.06.2017

Asian J. Research Chem. 2017; 10(4):431-435.

DOI:10.5958/0974-4150.2017.00071.2