Applying biomarkers as paleoenvironmental indicators to reveal the organic matter enrichment of shale during deep energy exploration: a review

Analysis of biomarkers in geological materials such as shales is very crucial because they can provide useful information on the depositional conditions and environments, organic matter input, thermal maturity as well as the geological age of shales in some cases. The paleoenvironment, and its impact on organic matter enrichment of the shales, plays a vital role in the exploration and development of the resource. Paleoenvironmental reconstruction can be conducted using elemental, isotopic, maceral, and biomarker proxies. However, the literature on the biomarkers for paleoenvironment reconstruction to reveal the organic matter enrichment of shales in many petroleum systems throughout the world is still insufficient. Hence, this paper seeks to critically review the application of biomarkers during paleoenvironmental reconstruction in shales. The uses of biomarkers as indicators of modern and ancient marine and brackish/saline lacustrine depositional environments are considered. This review shows that biomarkers could be used to establish the sedimentary depositional environments, redox conditions, and organic matter enrichments of shales that are critical to deep energy exploitation. Nevertheless, despite the fact that biomarkers are significant indicators of depositional conditions, secondary processes such as source facies, thermal maturity, migration, and reservoir alteration can greatly influence their uses as paleoenvironmental condition indicators in source rocks and oils. Hence, for a reliable paleoenvironmental evaluation, there is a need to combine isotopic, elemental and maceral proxies with biomarkers.


Introduction
Paleoenvironmental reconstruction has gained global attention of researchers because of its importance to conventional hydrocarbon resources and unconventional oil and gas exploration.Different depositional environments and conditions may have different assemblages of organisms, and thus contribute different biomarkers to the sediment.][3] The redox state of the environment in which shales were deposited can be inferred from the abundance and ratios of certain biomarkers in the shales. 32][3] A biomarker is a substance that maintains the structure of its biological precursor. 1,3,43][14][15][16] The components of sediment extracts and oils are a reection of both paleoenvironmental conditions and precursor compounds in the organisms that contributed organic matter (OM) at the time of sediment deposition, and thus can provide valuable information about the organic matter input and the prevailing depositional environment. 8,9The source facies can be distinguished by comparing structurally similar chemicals in sediments and crude oils with their likely biological precursors. 8,9,17,18The thermal history of the oil can be reconstructed by altering the biomarker structure. 3,17,18Some biomarkers are biochemical processes or environmental indicators.Triterpenoids and steroids were thoroughly investigated.During diagenesis, sterols are easily changed, whereas polycyclic terpenoids are more resistant 19 Any organic structures that emerge in sediments before the oil is expelled from the source rock can be used for correlation, not just biological structures, for example, diamondoids were formed during catagenesis. 20As a result, biomarkers from oil provide valuable information on depositional paleoenvironments, 21 maturity, 22 and organic input, as well as, in some instances, the age of the source rocks. 23,24][34][35] The essential structures of biomarkers are mainly preserved during sedimentation and diagenesis (Fig. 1). 3,6The term "diagenesis" refers to the biological, physical, and chemical changes that occur in organic matter in sediments before major heat (usually 50 °C)-induced alterations. 1,3,4,6Catagenesis is the thermal alteration of organic materials in rocks caused by burial and heating at temperatures between 50 and 150 °C under typical burial settings over millions of years. 1,3,4,6Biomarkers undergo structural changes during catagenesis that can be used to determine the degree of heating of their source rocks or the amount of oil expelled from these rocks. 3,6,36,37Furthermore, because biomarkers indicate a distinct group of contributing organisms, their distribution in an effective source rock serves as a ngerprint that can be utilized to link the rock to the expelled crude oil that may have traveled hundreds of kilometers. 3,6,36,37Before greenschist metamorphism, organic molecules are broken to gas at temperatures between 150 and 200 °C, a process known as metagenesis.Because of their volatility, biomarkers lose a signicant amount of concentration or are eliminated in these settings through several paths: (1) cracking of very mature particulate organic matter, (2) breakdown of residual oil in petroleum source rocks, and (3) secondary cracking of oil in reservoir rocks can all result in deep hydrocarbon gas accumulations. 5,7,38In the oil-oil, and oilsource rock correlation studies, biomarkers are commonly utilized. 36,37n-Alkane indices, in combination with sterane and aromatic ratios, can help distinguish between terrestrial and marine organic matter inputs; the ratio of hopanes to steranes can help distinguish prokaryotic versus eukaryotic input; and various saturated and aromatic hydrocarbon ratios can help suggest thermal maturity, lithology, and depositional environments. 19hale is dened as a ne-grained sedimentary rock that exhibits ssility and a more general classication of mudstone. 39Organic-rich shale refers to the shale with elevated total organic carbon contents (generally TOC >2.0% (ref.2][43][44][45][46] Organic-rich shales are widespread in salinized lacustrine basins throughout China, such as the Middle Permian Lucaogou Formation in Junggar and Santanghu basins in northwest China, 47,48 the Upper Jurassic Yanchang Formation in Ordos Basin in central China, 49 the Upper Cretaceous Qingshankou and Nenjiang formations in Songliao Basin in northeast China, 50 and the Eocene Shahejie Formation in Bohai Bay Basin in east China. 43,51These salinized lacustrine organic-rich shale (SLORS) have been proven as crucial source rocks for conventional oil, as well as sources and reservoirs for unconventional oil. 52The abundance of organic carbon and heteroatoms in shale layers, which are deposited in source rocks in the form of organic matter, leads to the generation of hydrocarbon through thermal maturation.The solid-state organic compound (known as kerogen) of these atoms breaks down and undergoes a signicant structural and compositional transformation during thermal maturation. 53,54o date, however, while there are many studies reported on the paleoenvironmental reconstruction in shales using elemental, maceral, and isotopic proxies, the literature on the biomarkers for paleoenvironment reconstruction to reveal the organic enrichment of shales in many petroleum systems throughout the world is still insufficient.Thus, the aim of this paper is to critically review the signicance of biomarkers during paleoenvironmental reconstruction in shales.

Lacustrine versus marine environments
6][57][58] Due to the smaller size and shallower depth, lakes generally receive a more turbulent inux.6][57][58] Lacustrine sediments Fig. 1 Generalized evolution of organic matter after deposition.Many biomarkers transform to other structures during late diagenesis and much of catagenesis before their destruction during late catagenesis and metagenesis (left). 3ypically contain tens of percent TOC, whereas deep ocean sediments contain only a few tenths.Lacustrine benthic creature diversity is lower, and their bioturbation depth is lower than that of marine fauna. 58In seawater, dissolved sulfate is a major ion, whereas it is usually low or absent in lakes. 59As a result, sulfate reduction is signicant in the microbial reworking of marine OM, but not in the reworking of lacustrine OM.Carroll and Bohacs (2001) suggested a three-fold lithofacies categorization for lacustrine petroleum source rocks that accounts for the most relevant characteristics. 58These lithofacies correspond to their algal-terrestrial, algal, and hypersaline algal organic facies, and comprise the uvial-lacustrine, uctuating pro-fundal, and evaporative lithofacies.All three lacustrine facies are found in the Eocene Green River Formation in Wyoming and the Upper Permian non-marine facies of the southern Junggar Basin in China. 3,6M has a varied pattern of increasing concentration further from the source of the organic matter at the margin of the basin toward deep water in deltaic or paralic environments dominated by terrigenous detritus. 59The Mississippi and Mahakam deltas are typically modern examples, whereas the Miocene Mahakam Delta and the Lower-Middle Jurassic in West Siberia are typically ancient examples (Fig. 2).Although most deltaic environments are very oxic, OM can also be preserved if it is buried quickly, successfully protecting it from metazoan attack (Fig. 3).

Oxic versus anoxic depositional condition
Biomarker compositions can be used to distinguish oils from various source rocks, but they can also be used to show regional differences in organic facies within the same source rock or between oils from the same source rock. 61Because biomarker patterns in oils are inherited from their source rocks, these uses are possible.Within the same source rock, lateral and vertical variations of organic facies emerge from differences in the type of organic matter and the characteristics of the depositional environment.][64][65] Understanding the paleo-oceanographic inuences on petroleum source rock deposition can help with regional mapping of organic facies and predicting promising regions for future development. 62,64,65Under certain atmospheric circumstances, aerobic microorganisms rapidly oxidize organic materials from dead plants or animals.Aqueous sedimentation of organic matter can occur under a variety of redox circumstances, with the availability of molecular oxygen being the most important factor.The terms in Table 1 can be used to characterize the various redox conditions and their corresponding microbial biofacies (determined by metabolism).

Oxic condition
Aerobic bacteria and other organisms decompose organic materials settling from the photic zone in an oxic deposition (Fig. 4).There are 6-8 ml of oxygen per liter in normal seawater.Biological oxygen demand (BOD) is created by respiratory processes.If enough organic matter remains aer all available oxygen has been used up, anaerobic organisms continue to oxidize it with other oxidants like nitrate or sulfate.In the water column or the bottom sediments, the boundary between aerobic and anaerobic metabolism (oxic versus anoxic environments) can occur.Metazoa, such as multicellular burrowing organisms like clams and worms, typically bioturbated benthic sediments that contain interstitial oxygen.Massive textures without lamination characterize oxic deposits preserved in the geologic record.Because of the combined effects of BOD and restricted replenishment by oxygenated water, lakes and oceanic basins can become oxygen-Fig.2 Map of total organic carbon (TOC) distribution for Lower-Middle Jurassic shales, including the Middle Jurassic Tyumen Formation in West Siberia. 60][68][69] depleted (stagnant).Basin shape, water temperature, and salinity gradients are all elements that inuence water recharge.For example, a thermocline is a layer of water in which the temperature decreases with depth more than the underlying and above water (Fig. 5).When the air temperature is high and the shallow water gets more heat than it loses through radiation and convection, thermoclines can form in lakes and the ocean.When the surface water heats, a small negative temperature gradient occurs.Although wind mixing of the surface waters lowers the surface temperature, the overall impact is downward heat transmission and the establishment of an isothermal layer warmer than the underlying water.As a result, a strong thermocline forms between the isothermal surface layer and the cooler water beneath it.Warm surface waters in the oceans usually extend to a depth of 150-300 m.The thickness of the underlying thermocline ranges from 300 to 900 m.Below the thermocline, the temperature drops more slowly and reaches 1-4 °C towards the bottom of the ocean.
Because of the humid atmosphere and lack of major seasonal temperature variations, equatorial lakes, such as Lake Tanganyika in the East African ri-lake system, are particularly Fig. 4 The O 2 -H 2 S-CH 4 level in oxic (left) and anoxic (right) depositional environments generally result in poor and good preservation of deposited organic matter, respectively. 71g. 5 Schematic showing the formation of a thermocline in watermass.If air temperature rises above water temperature for water having constant temperature versus depth (1), then the surface water begins to warm (2).Mixing of surface waters by wind lowers surface temperature, resulting in the formation of an isothermal layer having a higher temperature than the thermocline and deeper water (3, left).Further warming (4) and mixing of the surface water result in net downward transport of heat and a deeper, and commonly thicker, thermocline (5). 3 prone to severe thermoclines. 71Due to the thermocline, oxygen lost from deep water due to BOD is not rapidly replaced by mixing with shallower water, and anoxia develops.Anoxic conditions exist below 150 meters in Lake Tanganyika, which is 1500 meters deep.The Eocene Green River Formation in Colorado and Utah have laminated, organic-rich marlstones that were deposited in a vast anoxic lake.Non-marine Lower Cretaceous source rocks from China (Songliao Basin), Brazil (Lagoa Feia Formation), and West Africa (Bucomazi Formation) are also instances.
A halocline or density-stratied water column can form when fresh water is pumped into a silled marine basin with limited evaporation, especially when deep saline water is separated from open ocean water by the sill.The Black Sea is primarily a sluggish marine basin.Excess fresh water from riverine input pours into the Mediterranean Sea through a 27 m deep sill at the Bosphorus.The upshot of this positive water balance is lowsalinity surface water relative to the Black Sea's deeper, more saline water and a permanent halocline at depth.The halocline is a chemocline that distinguishes between oxic and anoxic conditions.At 80-100 m depth, the current chemocline is found within the photic zone, where hydrogen sulde rst develops and oxygen vanishes.Isorenieratene and related compounds found in Black Sea sediments suggest that photosynthetic green sulfur bacteria (Chlorobiaceae) have been active in the Black Sea for at least 6000 years and that anaerobic water penetration of the photic zone is not a recent phenomenon. 69The Upper Jurassic of West Siberia (Bazhenov Formation) and the North Sea (Kimmeridge hot shales), as well as the latest Albian in North America, are examples of anoxic marine source rocks deposited under similar conditions (Mowry Shale).

Anoxic conditions
OM and ne-grained sediments get concentrated in concentric or bull's-eye patterns in deep quiet water near source-rock depocenters in anoxic marine silled basins and anoxic lakes where sediment thickness is the greatest. 72The Black Sea and the Caspian Sea are modern instances of this concentric distribution of organic carbon, while the Upper Jurassic in West Siberia (Fig. 6), the lower Jurassic of the northern North Sea, and the lower Toarcian and Hettangian/Sinemurian of the Paris Basin are ancient examples.Anoxic conditions range from less than ∼1 wt% total organic carbon to more than 20 wt% total organic carbon (TOC).Turbidity currents and related gravity ows may muddle the aforementioned concentric distributions by transporting organic matter or sediments into deep water via pathways that aren't predicted by basic depositional models.
The implications of oxic vs. anoxic deposition on the quantity rather than the quality of preserved OM were examined by Pedersen and Calvert. 72Anoxic conditions appear to enhance the preservation of hydrogen-rich, oil-prone organic materials.Peters and Simoneit (1982), 73 for example, found identical TOC content in alternating layered (anoxic) and homogeneous (oxic) diatomaceous oozes in the Gulf of California, similar to. 74Higher Rock-Eval pyrolysis hydrogen indices and lower oxygen indices imply that the laminated zones in these sediments contain more hydrogen-rich organic matter than the homogeneous zones.If anoxia is unimportant in the preservation of OM, it is difficult to explain the widespread correlation between oil-prone, organic-rich petroleum source rocks and faunal or sedimentologic traits indicating anoxia, according to. 75The majority of source rocks are laminated and lack signs of active infauna.Biomarkers and supporting parameters in petroleum source rock extracts indicate anoxic conditions (e.g., high vanadium/ nickel porphyrin, low pristane/phytane, and high C 35 homohopane indices). 3ecause both metazoa (multicellular aerobic organisms) and aerobic bacteria demand higher amounts of oxygen, aerobic breakdown of organic matter is signicantly limited in anoxic or suboxic water (less than ∼0.2 ml oxygen/l water) (Fig. 4, right).Bioturbation of benthic sediments does not occur below ∼0.1 ml of oxygen/l water due to the absence of metazoa, leaving only anaerobic bacteria and probably some benthic foraminifera to rework the organic matter.Interstitial oxygen, nitrate, Mn 4+ oxides, Fe 3+ oxides, and sulfate are the oxidants employed by benthic organisms in general. 76,77Because sulfate is abundant in seawater (0.028 M), sulfate reduction is usually the major mode of respiration aer the advent of anaerobic conditions in marine settings. 78The absence of bioturbation allows the formation of tiny laminations that record depositional cycles, as seen in effective petroleum source rocks.Glacial varves in ords, for example, reect yearly depositional cycles that oen comprise a thin layer of dark-colored organic-rich clay grading upward from a layer of light-colored sand or silt.Laminated sediments at British Columbia's Saanich Inlet, a silted, ord-like area, contain up to 9% organic matter. 79Free hydrogen sulde (H 2 S) generated Fig. 6 Map of total organic carbon (TOC) distribution for marine shales of the Upper Jurassic Bazhenov Formation in West Siberia, deposited in a large anoxic silled basin. 85y sulfate-reducing bacteria is accumulated in euxinic sediments under marine, anoxic conditions. 80uring sediment deposition, traces of animal activity provide information about water chemistry. 81For example, few ichnofossils (trace fossils) other than very shallow Helminthoides burrows consisting of short, monospecic horizontal mining and grazing traces can be found in Lower to Middle Triassic mudrocks from the Barents Sea. 75Due to the lack of evidence for burrowing, these mudrocks were deposited in dysoxic to anoxic conditions, resulting in better OM preservation, high TOC, and high algal/amorphous OM.In marine sediments, Savrda (1995)  summarized connections between oxicity and burrow type or density. 81However, systematic correlations between redox conditions and bioturbation in lacustrine sediments are poorly established. 82Anaerobic degradation of organic materials is less effective thermodynamically than aerobic degradation. 83This nding backs up the widely held assumption that anoxia is to blame for the increased preservation of hydrogen-and lipid-rich organic materials in petroleum source rocks. 55Even when organic productivity is high, and oxic conditions present, organic matter is generally lost by sedimentation and diagenesis.Most polar locations in current oceans, for example, have high primary productivity, but low organic carbon in the oxic bottom sediments.Organic productivity, rather than anoxia, is the fundamental restriction on the buildup of organic-rich marine sediments, according to. 60,73They cited sources that suggested that the rates of the breakdown of organic matter under oxic and anoxic circumstances are equal and cannot be used to imply increased retention of OM under anoxic conditions, based mostly on laboratory incubation tests.They found no increase in carbon content in alternating anoxic and oxic sediments in the central Gulf of California. 74In comparison to analogous, oxygenated environments, data from anoxic sediments in the Black Sea reveal that organic carbon accumulation rates are not especially high. 84The distribution of current organic-rich sediments around the planet does not appear to be correlated with high productivity in the underlying water column. 55Because of the signicant circulation of cold, oxygen-rich waters that successfully satisfy all BOD imposed by settling the organic matter, surface waters near Antarctica, for example, display great productivity, but the underlying sediments are organic-lean.Modern organic-rich sediments are found in areas with high productivity and anoxia at the bottom of the water column. 3

Methods of biomarker analyses
The analytical methods applied in the analyses of biomarkers and characterizations are reported below.

Extraction
The shale samples are usually crushed with agate mortar and powdered to less than 100 mesh size before extraction.About 50 g powdered samples (although this generally depends on the extractable organic matter (EOM) contents in shale) are Soxhlet extracted with an azeotropic mixture of dichloromethane : methanol (93 : 7, v/v) for 72 h.To eliminate elemental sulfur from the extracts, activated copper powder is widely used.The excess solvent is then distilled out with a rotary evaporator to a 3 ml aliquot volume.Aer transferring the aliquot into a clean, weighed vial with a micropipette, the remaining solvent is removed under nitrogen gas ow at a temperature below 50 °C. 86Alternatively, microwave-assisted extraction (MAE) can be applied using hexane/acetone (1 : 1).Other solvents that can be used for the MAE include tetrachloroethylene, methylene chloride/acetone (1 : l), toluene/methanol (l0 : l), methylene chloride, and toluene/methanol (1 : 10).Soxtec extraction can also be employed using hexane/acetone (l : l), and that for ultrasonic extraction, methylene chloride/acetone (9 : 1) is used. 87

Column chromatography
To fractionate extracts, column chromatography (some automatic instruments are also used to separate the EOM into SARA) with silica gel/alumina as the stationary phase is commonly utilized.A standard glass column measures 50 cm in length and has a 0.5 cm internal diameter.DCM and light petroleum spirits are used to rinse the column twice (petroleum ether).The column is then rinsed with n-hexane and plugged with a cotton wool to serve as a resting pad for the stationary phase, which is silica gel (SiO 2 ).The stationary phase (SiO 2 ) is then added.Two (2 g) of alumina (Al 2 O 3 ) is used to stabilize the surface.The saturated, aromatic hydrocarbon and polar fractions are eluted using 70 ml of n-hexane, 70 ml of dichloromethane/n-hexane (2 : 1, v/v), and 70 ml of dichloromethane/methanol (1 : 1, v/v).Each fraction is recovered by carefully evaporating solvents on a rotary evaporator, followed by the removal of the remaining solvent under the inuence of a nitrogen gas stream. 87Gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS), and gas chromatography coupled to tandem mass spectrometry (GC/MS/MS) can now be used to analyze the recovered saturated hydrocarbon, aromatic hydrocarbon, and polar fractions. 88

Gas chromatography (GC)
The aliphatic fractions are commonly analyzed by capillary gas chromatography for the characterization of the n-alkanes, pristane, and phytane (GC).Typically, a Hewlett Packard 5890 Series II is used, which is equipped with a Gerstel on-column injector, an electronic pressure control (EPC), a fused silica capillary column (HP Ultra I) of 50 m length, 0.2 mm inner diameter, and 0.33 m lm thickness, as well as a standard ame ionization detector (FID).At a ow rate of 1 ml min −1 , hydrogen is widely utilized as a carrier gas (pressure controlled).The oven temperature is commonly programmed from 90 °C (hold time 5 min) to 310 °C at a rate of 4 °C min −1 (for some unusual biomarkers, some special temperature programs are also used).A Multichrom 2-online data system (Fisons) is commonly used to store and process retention times and peak areas.

Gas chromatography-mass spectrometry (GC-MS)
Gas chromatography-mass spectrometry (GC-MS) (Fig. 7) analyses are exceedingly sensitive, allowing for the analysis of very small amounts of material.0][91][92] The injector temperature must be higher than the boiling point of the primary substance in the mixture to be examined.The standard capillary columns employed in these analyses have a column length of 15 to 30 m.The ow rate of the mobile phase (gas) is modest, around 1 ml min −1 or less.The current GC-MS technique has a signicant restriction in that it can only analyze a limited number of volatile, thermally stable compounds.The individual biomarkers are commonly identied based on comparison of elution sequences, relative retention times, and mass spectra with the NIST Chemistry WebBook (https:// webbook.nist.gov/chemistry/)[98] 4.5 Gas chromatography-mass spectrometry-mass spectrometry (GC-MS-MS) Gas chromatography tandem mass spectrometry (GC/MS/MS) is a more powerful analytical technique.The most common application of GC/MS/MS is for trace quantitative analysis in complex matrices, such as geological samples. 99,100The system's ability to choose against the matrix (lower chemical noise) is an important performance component to consider.The signal-tonoise ratio (S/N) can be used to demonstrate this. 99,100Furthermore, an S/N ratio ensures that instruments are not contaminated during installation, while low-level precision and instrument detection limits (IDL) provide the full image. 99,100The physical and chemical properties of analytes of interest, as well as their interaction with the analytical column's stationary phase, are used to separate samples in a gaseous form for GC-MS/MS analysis.The analytes enter the tandem mass spectrometer (MS/MS) aer departing the analytical column, which is made up of two scanning mass analyzers separated by a collision cell. 99,100n the collision cell, fragments picked in the rst analyzer react with inert gas, resulting in additional fragmentation.These daughter product ions are then resolved and analyzed in the third quadrupole.Liquids, gases, and solids can all be analyzed using GC-MS/MS.The sample is directly introduced into the GC for liquids.Gaseous components are transferred directly into the GC using gastight syringes.Solvent extraction, outgassing, or pyrolysis analysis are all options for solids analysis.Following that, the analytes of interest are measured by comparing them to external or internal standards.GC-MS/MS is highly suited for the detection of unknown volatile components using mass fragmentation patterns and mass transitions associated with the unknown analyte, in addition to quantication.

Comprehensive two-dimensional gas chromatography time-of-ight mass spectrometry (GC × GC-TOFMS)
][103] Furthermore, considering the importance of biomarkers in geochemical and environmental studies, a precise and complete investigation of their composition in oils and source rocks is required.Capillary gas chromatography (GC), which is frequently hyphenated with mass spectrometry (MS), [90][91][92][93] and tandem MS (MS-MS) are the most popular techniques used to study biomarkers. 99,100Although the abilities of these techniques are well established, there are certain limitations.The fundamental constraints of both GC-MS and GC-MS-MS are the extremely complex mixture of crude oils and shales, as well as the poor resolution of GC.Oil mixtures are not properly clari-ed, despite the better separation capabilities of multiple reaction monitoring (MRM) or MS-MS techniques.Comprehensive two-dimensional gas chromatography (GC × GC) is one of the most powerful analytical methods for separating complex mixtures with great resolution.The GC × GC is great for separating complex matrices and identifying isomers and other molecules with similar chemical structures.In bidimensional space, different well-ordered chemical groups can be recognized, providing extra information on the molecular composition.For retrieving biomarker structural information encoded in complex petroleum samples, detection using a time of ight mass spectrometry (TOFMS) device is required aer the GC × GC separations.Because mass spectra are available for chemical identication and structure elucidation, the TOFMS can be regarded as a third analytical dimension.The non-scanning capabilities of TOF technology, as well as its rapid acquisition rate, are two of its most important advantages.On the one hand, the non-scanning capability is the most signicant advantage over scanning MS instruments (quadrupole, ion trap), which produce non-skewed peaks.The entire chromatogram is obtained with non-distorted mass spectra, allowing for rapid spectral deconvolution and a reliable comparison with commercially available MS libraries.Rapid acquisition, on the other hand, is required for accurately reconstructing the narrow peaks generated during GC × GC separation modulation.To enable chromatographic resolution, provide sufficient MS information per peak, and efficiently deconvolve the compounds that co-elute, a high acquisition frequency is required during the time unit.The GC × GC-TOFMS (Fig. 8) is ideal for analyzing complex samples, especially for identifying unusual compounds or potential biomarkers that are rarely detected in routine 1D GC or GC-MS analyses, due to its better chromatographic resolution, well ordered 2D structures, mass spectral information, and deconvolution.3 The same group compared the compositions of different oils a year later using multiway principal components analysis (MPCA) on data from GC × GC studies. 94 (2013) used GC × GC-TOFMS to examine information on oil maturity and biodegradation. 109,110Many saturated and aromatic biomarkers in extra heavy gas oil were successfully. 100,111Recently, Kiepper et al. (2014) used GC × GC-TOFMS to characterize the biomarkers in the crude oils from the Cumuruxatiba and Espírito Santo basins, Brazil. 112

Biomarkers for paleoenvironmental reconstructions
4][115][116][117][118][119][120] Diverse depositional environments may have different assemblages of organisms, resulting in different biomarkers being contributed to the sediment.For example, biomarker compositions differ signicantly in terrigenous, marine, deltaic, and hypersaline environments. 3Details of biomarkers for paleoenvironments based on n-alkanes, isoprenoids, terpanes, and sterane, as well as some aromatic biomarkers, will be discussed in the following sections.5.1 Saturated compounds 5.1.1Normal and branched alkanes.Coal, sediments, and petroleum all contain n-alkanes (m/z 85). 121They provide information about the biological sources of OM. 6,121 Due to their biosynthesis from fatty acids by enzymatic decarboxylation, organism usually have odd numbers of C-atoms. 121axes of higher plants are responsible for n-alkanes in the C 27 to C 31 range, which show a distinct odd over even predominance (OEP). 122Short-chain n-alkanes (C 15 to C 19 ) with an OEP are thought to originate from algae (marine environment), 2 implying that aquatic OM has a large role in deposition. 3,114,115Ratios can be used to express the inuence of various contributors on the distribution of n-alkanes.For example, the ATR HC (aquatic/terrigenous ratio of hydrocarbons, Wilkes et al. (1999)) is a parameter used to determine the amounts of aquatic to terrigenous derived n-alkanes, or short-chain to long-chain n-alkanes: 123 Long-chain n-alkanes, or organic matter derived from terrigenous OM, contribute signicantly to values of 0.5.The ratio depends on the maturity of the samples and is only distinctive for immature organic matter.Samples of low maturity normally show the biogenic distribution of nalkanes.With increasing organic matter maturity, the predominance of odd-numbered n-alkanes over evennumbered n-alkanes becomes less pronounced.Due to thermal cracking, an increase of short-chain n-alkanes can be observed, leading to equal proportions of odd and evennumbered n-alkanes. 121In estimating the thermal maturity of fossil fuels, the ratio of odd/even carbon-numbered nalkanes has been used. 2,124These ratios are expressed as the carbon preference index, CPI 125 or enhanced odd-to-even predominance, OEP. 124The CPI and OEP values less or greater than 1.0 indicate low thermal maturity while values around 1.0 suggest, but do not prove, that an oil or rock extract is thermally mature.The CPI or OEP values below 1.0 are unusual and typify low maturity oils or bitumen from carbonate or hypersaline environments. 3,126These ratios are affected by organic matter type and therefore are mostly applied with caution.
5.1.2Acyclic isoprenoids.These compounds have been found in all types of geologic samples. 127They are known constituents of plants, animal tissues, and bacterial cell walls.The presence of acyclic isoprenoids in the biosphere is due to the phytol side chain of chlorophyll-a.They are composed of three types of linkages: head-to-tail (the most common, including compounds such as pristane (Pr), phytane (Ph), and homologs up to C 45 ); tail-to-tail (squalane, perhydro-carotane, lycopane, and others); and head-to-head (C 32 -C 40 typical of thermophilic and other archaebacteria).The phytol side

Review
RSC Advances chain of chlorophyll-a gives rise to Pr (2, 6, 10, 14-tetramethylpentadecane) (I) and Ph (2,6,10,14-tetramethylhexadecane) (II) (Fig. 9).The most abundant isoprenoids in the aliphatic hydrocarbon fraction are pristane and phytane. 6,128hytane is the product of the dehydration and reduction of phytol, whereas pristane is derived from the oxidation and decarboxylation of phytol (Fig. 8).0][131] The pristane/phytane (Pr/Ph) ratio is a widely used geochemical parameter that has been utilized as a depositional environment indicator, but, it has low specicity due to thermal maturity interferences and preliminary assessment of OM inputs. 3It's also a common indicator of the redox conditions of the depositional environments.According to Ten Haven, 132 high Pr/Ph (>3.0) indicates terrigenous input under oxic conditions, while low Pr/Ph (0.8) indicates anoxic, hypersaline, or carbonate environments.Low Pr/Ph levels (<2) suggest reducing conditions in aquatic depositional settings such as marine, fresh, and brackish water, whereas high values (up to 10) indicate oxidizing conditions in peat swamp depositional environments. 133Pr/Ph values are also inuenced by maturity, and/or different precursors of pristane (tocopherols or chromas).Another important method for determining the source of organic matter is the Pr/n-C 17 ratio.It is also widely used as an indicator of the redox potential of the depositional environment.
Squalane (III) has been proposed as a halophilic archaea indicator and hence a molecular diagnostic of hypersaline environment. 134,135Crocetane (IV) has been identied as a marker for methane-oxidizing archaea because it is depleted in 13 C to a value as low as −100&. 136Thiel et al. (2001) discovered that the n-C 23 alkane, which is depleted in 13 C ( 13 C values less than −70), is an indicator of anaerobic methane oxidation. 137Methanogenic bacteria have been shown to contain isoprenoids such as penta-and tetramethylicosane. 138Archaea produce isoprenoids that are linked head-to-head. 139M derived from terrestrial sources is assigned to values above 0.6, whereas organic material generated from marine sources is attributed to values below 0.5. 6The pristane/normal alkane (Pr/n-C 17 ) and phytane/normal alkane (Ph/n-C 18 ) ratios have also been employed to determine redox conditions during sediment deposition (Fig. 10). 3,18,128,141The Pr/n-C 17 and Ph/n-C 18 ratios are also inuenced by maturity and biodegradation. 141otryococcane (V) is a branched hydrocarbon derived from botryococcene, an unsaturated hydrocarbon that has been linked to an organism that will only develop in a specic type of environment. 141Botryococcus braunii, a fresh or brackish water alga, was shown to have botryococcane concentrations of 70 to 90 percent in its senescent phase.Moldowan and Seifert (1980)  utilized the peculiar occurrence of this compound in Botryococcus braunii as evidence that certain oil deposits in Sumatra, 142 Indonesia, were formed primarily from the prehistoric source material in a fresh or brackish lagoonal-type environment. 134B. braunii can contribute both unsaturated hydrocarbons, which Fig. 10 Plot of Pr/n-C 17 against Ph/n-C 18 ratios of oils from Niger Delta. 18,141re potential precursors of botryococcanes, and long-chain nalkanes to freshwater (lacustrine) sediments since it exists as two physiologically distinct clonal races. 142For the rst time in an Australian crude oil, biomarkers found in coastal bitumens from the western Otway Basin conrmed the presence of substantial amounts of botryococcane. 143The waxy quality of the three main bitumen types found in the Otway Basin, as well as their botryococcane content, has been attributed to botryococcus blooms deposited in deep lakes under anoxic or microoxic circumstances. 144-Carotane (VI), a saturated hydrocarbon generated from a pigment with a C 40 carotenoid structure, was initially discovered in the Eocene Green River Shale of Colorado and has since been discovered in a variety of sedimentary rocks and crude oils.Although most carotenoids do not survive early diagenetic processes, b-carotane is well preserved in sediments and oils in many environments.[145][146][147][148][149][150] b-Carotane is well known as a marker for saline and reducing lacustrine environments as well as extremely restricted marine environments.11,151,152 Carotenoids are the biological precursors of b-carotane and are produced by algae, cyanobacteria, and higher plants.11,48,151,153 b-Carotane is abundant in the eocene oils of the Bohai Bay Basin.154 The Eocene Shahejie (Es) formation, particularly the third (Es 3 ) and fourth (Es 4 ) members, is the principal source rock for both conventional and "shale oil-producing" formations in the Dongying depression, as several studies have shown.[155][156][157] The levels of -carotane in the oils vary greatly between the Es

Review
RSC Advances linked to a higher contribution of marine algae. 1624][165] However, the fact that it may be found in sediments and oils of various ages suggests that there must be other sources. 3In marine-sourced oils, the C 23 TT is frequently prominent, whereas a higher amount of C 19 TT and C 24 tetracyclic terpane (TeT) usually indicates the presence of terrigenous organic. 13,162,166The ratio of C 26 TT to C 25 TT can be utilized to distinguish between marine and lacustrine source rocks. 3,167The relative distribution of tricyclic terpanes in combination with C 24 TeT has been widely used as a molecular parameter to distinguish terrestrial versus marine organic matter input, correlate crude oils and source rock extracts, predict source rock characteristics, and decipher source rock lithology. 3,13,167However, Samuel et al. (2010) recommended against such use because tricyclic terpanes and C 24 TeT distribution patterns in most oils are extremely similar globally. 168etracyclic terpanes (VIII) are found in a series ranging from C 24 to C 27 .9][170] Most marine oils formed from mudstones to carbonate source rocks contained abundant C 24 TeT. 171C 24 -C 27 tetracyclic terpanes are oen referred to as de-E-hopanes, or 17,21-secohopanes and are suggested to be more resistant against biodegradation and maturity effects than hopanes. 162nusual tri-and tetracyclic terpanes (C 21 tricyclic terpanes, C 25 tricyclic terpanes, C 27 tetracyclic terpanes, C 24 -des-Aoleanane, C 24 -des-A-lupane, and C 24 -des-A-ursane) have recently been reported in crude oils and source rock extracts from the Pearl River Mouth Basin, the Beibuwan Basin, and the Liaohe Basin, China, 169,172 and source rock extracts from Niger Delta Basin, Nigeria. 17According to Xiao et al. (2018), these uncommon compounds contain chemical structures that are comparable to oleanane, ursane, and lupane, and are thought to be derived from alcohols or ketone precursors found in higher plants. 169The higher abundance of these tri-and tetracyclic terpanes is likely due to the higher plant material input to the OM content of source rocks. 169Furthermore, the redox conditions and water depth in the depositional environment have a substantial impact on the distribution patterns of the compounds, and they may be easily generated under oxidizing conditions. 169 ).This means that Z1/(Z1 + C 24 TT), Y1/(Y1 + C 24 TT), and Pr/Ph are all constrained by the same depositional conditions, implying that these newly discovered tetracyclic terpanes were formed in both oxidative and reducing environments. 169,170The Z1/(Z1 + C 24 TT), Y1/(Y1 + C 24 TT), and (C 19 + C 20 ) TT/C 23 TT ratios are related to the Pr/Ph and (C 19 + C 20 )TT/C 23 TT ratios, as shown in Fig. 11a-d. 169,170As a result, these compounds are derived from source rocks containing a mixture of higher plant and marine sources. 169ternary diagram based on the relative abundance of C 19 -C 23 TTs (C 19+20 TT, C 21 TT, and C 23 TT) (Fig. 12) was successfully used to differentiate the depositional environments of source rocks and crude oils, and the study revealed that four distinct sedimentary environmental zones could be distinguished: marine/saline lacustrine, freshwater lacustrine, uvial/ deltaic, and swamp. 170,173,174Based on C 19 -C 23 TT, Ogbesejana et al. (2020) found that rock samples from the Niger Delta Basin received a mixed input of marine and terrigenous organic matter and were deposited under oxic to sub-oxic conditions in lacustrine-uvial/deltaic sedimentary conditions (Fig. 11). 170.1.4Hopanes.Hopane (IX) is the name given to a class of aliphatic biomarkers that have a pentacyclic terpenoic structure with a ve-membered E-ring.Sediments, hydrocarbons, and coals all contain hopanes in the C 27 to C 35 range. 175iomarkers for bacteria and cyanobacteria are considered as hopanoids. 176Hopanoids are commonly isotopically light (d 13 C values ranging from −50& to −60&). 177,178They are derived from natural precursors such as C 35 -bacteriohopanetetrol (Appendix 1) and similar C 30 -or C 35 -bacteriohopanoids found in the cell membranes of a wide variety of bacteria. 179,180These compounds are the counterparts of sterols, which are found in eukaryotes. 179,180Hopanes and their biological precursors have been investigated extensively for decades, yet their diagenesis and catagenesis transition processes remain unknown. 181,182hey are frequently used for both maturity and paleoenvironmental assessments. 6  C 30 hopane, in combination with other parameters such as C 30 n-propylcholestane and C 26 /C 25 TT, and the canonical variable from stable carbon isotope analyses, is the best way to differentiate marine from lacustrine crude oils. 3n the lacustrine oil samples from the Esprito Santo and Cumuruxatiba basins in Brazil, 114 3-methylhopanes and onoceranes were predominant, whereas the relative abundance of 2-methylhopane with an extended side chain was high, and onocerane levels were low only in the marine oil samples from the same basins. 114A new index based on methylhopanes was proposed.For oils with a lacustrine origin, the percentage of C 31 3b-methylhopane (3MH31) relative to C 30 hopane (H30), 100 × (3MH31/H30), was >1 while for oils with a marine origin, it was 1. 114 5.1.5Gammacerane.Green River Shale extracts were the rst to contain gammacerane (X). 184The origin of gammacerane, which is widely applied as a salinity indicator, is unknown.The sole known potential biological precursor of gammacerane is tetrahymanoI, a pentacyclic triterpenoid found in protozoa and fungi. 185Dehydration of tetrahymanol to create gammacer-2-ene, followed by hydrogenation, is most likely the path of digenetic conversion of tetrahymanol to gammacerane.Sulfurization and subsequent cleavage of tetrahymanol can also produce gammacerane. 186Gammacerane is found in a wide range of samples from varied habitats, and its value as a biomarker for salinity is reliant on its relative abundance rather than actual presence. 16In marine and nonmarine source-rock depositional systems, gammacerane indicates a stratied water column caused by hypersalinity at depth. 186In addition to b-carotane and related carotenoids, gammacerane is a prominent biomarker in various lacustrine oils and bitumens, particularly the Green River marl and oils from China. 11,14,135,146,187,188Certain marine crude oils derived from carbonate-evaporite source rocks are similarly high in gammacerane. 11,16,187,189,190Gammacerane can be used to distinguish between different petroleum families.For example, Poole and Claypool (1984) used gammacerane to distinguish oils and bitumens from different source rocks in the Great Basin. 191.1.6Steranes.Steranes (XI) are derived from sterols, which are widely distributed in plants and microorganisms, and are typically found in mature sediments and crude oils via diagenesis. 1273][194] Steranes are key indicators of source facies, depositional environments, and thermal maturity found in crude oils and sedimentary organic matters. 195Sterols are naturally occurring precursors to steranes and diasteranes. 139Sterols undergo changes that result in the formation of steranes and diasteranes during diagenesis. 196Sterenes and diasterenes are the most likely direct precursors of steranes and diasterenes, respectively. 139The hydrogenation produces the saturated isomers.The abundances of C 27 -, C 28 -, and C 29 -steranes and diasteranes provide some insight into the OM origin and depositional conditions (Fig. 13). 7C 29 -steranes are thought to be derived mostly from higher plants, whereas C 27 and C 28 -steranes are thought to originate from phytoplankton. 192However, signicant amounts of C 29 steranes have been found in oils and source rocks that are assumed to be mostly marine in origin. 15,197imilarly, Grantham (1986) published an important study that showed that the C 29 steranes in crude oils are not necessarily derived from terrestrial sources. 198

Aromatic compounds
Aromatic hydrocarbons are important constituents of petroleum and extracts of both recent and ancient sediments, [199][200][201][202] and they have the potential to provide important information on sedimentary environments, source input, migration, thermal maturity, and oil-source rock correlations. 203PAHs are not produced by living organisms and are essentially nonexistent in natural organic matter. 204During diagenesis and catagenesis, the bulk of PAHs in petroleum are the result of complex chemical changes of naphthenic and/or olenic biological predecessors. 200Only in favorable conditions, where a characteristic component of the naphthenic structure has been preserved unchanged, can the biological origin of specic PAHs be determined. 205PAHs distributions could be useful in a variety of applications in petroleum geochemistry.Abundance of certain PAHs in sediments and crude oils, such as 1,2,5-trimethylnaphthalene (1,2,5-TMN), 1,2,5,6 tetramethylnaphthalene (1,2,5,6-TeMN), 9-methylphenanthrene (9-MP), 1,7-dimethylphenanthrene (1,7-DMP), originate from diterpenoid and triterpenoid natural products. 200.2.1 Naphthalene.Fossil fuels contain a lot of naphthalene (XII) and its alkylated derivatives. 206,207The main precursors for methylated naphthalenes are terpenoids produced from terrestrial plants. 208Because of the impacts of source, thermal stress, and biodegradation, their distributions throughout the geosphere are highly varied. 208The ratios computed from methylated naphthalenes are thought to reect an increase in the abundance of the stable isomers relative to the less stable isomers, which is dictated by 1,2-methyl shi and methyl transfer in the naphthalene carbon skeleton. 207g. 13 Ternary plot of C 27 , C 28, and C 29 sterane distributions in source rocks from Niger Delta. 18,192.2.2 Phenanthrene.In aromatic fractions, phenanthrene (XIII) is a signicant component.The relative concentration of phenanthrene in freshwater source rocks was higher than in marine source rocks. 209The overall naphthalene/phenanthrene ratio ( P N / P P ) of terrestrial freshwater oils was 0.52, that of terrestrial saline oils was 0.64, that of terrestrial hypersaline oils was 0.60, that of marine shales oils was 1.41, and that of marine carbonates oils was up to 1.80.This could indicate that the precursors of phenanthrenes in the crude oils and source rocks investigated come from terrestrial organic matter rather than marine organic matter, 209 which would be consistent with prior ndings. 210,211However, Jinggui et al. (2005) showed that the composition and abundance of phenanthrene and alkylated analogs in the Tabei and Tazhong uplis' marine source rocks, as well as the Triassic and Jurassic freshwater source rocks (mudstones and coals) from the Kuche depression, were all very similar. 203The ndings revealed that, regardless of source, marine and terrestrial organic matter from various sedimentary environments have relatively signicant amounts of phenanthrene derivatives, and that the two types of organic matter are not distinguishable.The dibenzothiophene/phenanthrene (DBT/P) ratio alone is an effective predictor of source rock lithology, with ratios >1 for carbonates and ratios <1 for shales. 212Hughes et al. (1995) and Sivan et al. (2008) have successfully applied the cross plots of DBT/P vs. PrPh ratios to classify source rock paleodepositional settings (Fig. 14). 208,212The classication method is based on the assumption that these ratios indicate diverse Eh-pH regimes arising from substantial microbiological and chemical processes that occur during sediment deposition and early diagenesis.The DBT/P ratio evaluates the availability of reduced sulfur for incorporation into organic matter, whereas the Pr/Ph ratio evaluates the depositional environment's redox conditions. 212.2.3 Triaromatic steroids.Multiple factors determine the relative abundance of triaromatic steroids (TAS) (XIV), which can be utilized as markers for diverse source inputs and depositional settings.4][215][216] TAS can result from the aromatization of monoaromatic steroids and the loss of a methyl group (-CH 3 ).naphthofurans (BNFs) (XVI to XVIII) are the major oxygen heterocyclic aromatic compounds found in oils, coals, sediment extracts, and tar deposits.The source of DBFs in oils and sedimentary organic matter is still a point of contention.0][221] DBF, DBT, and biphenyl abundances in Permian rocks (East Greenland) were most likely produced from phenolic compounds in the lignin of woody plants. 222he bulk of natural compounds linked to dibenzofuran are lichen or higher fungus metabolites.The carbon-carbon oxidative coupling of orsellinic acid and its homologs appears to be the source of lichen dibenzofurans. 223As a result, Radke  et al. (2000) proposed that DBFs in crude oils and sediment extracts may be used as lichen biomarkers. 202However, more research is needed to conrm the lichen-DBF precursor- product link. 224Biphenyl with oxygen can produce dibenzofuran, according to simulation experiments and geological evidence. 205Similarly, methyl-substituted biphenyls can combine with methylated DBFs to synthesize methylated DBFs.In the laboratory, biphenyl derivatives are oen employed as reactants to synthesize dibenzofurans. 223To completely comprehend the origin and evolution of DBFs in the geosphere, further geological evidence and laboratory studies are required.Dibenzofuran and its alkylated homologs (abbreviated as DBFs) as well as benzo[b]naphthofurans have been used as key molecular markers in organic geochemistry.DBF occurrence and distribution are primarily inuenced by source rock type and/or depositional environment. 170,202,209,225Radke et al. (2000), Asif (2010), and Kruge (2000)  indicated that the relative abundance of alkyldibenzothiophene (ADBT) compared to alkyldibenzofuran (ADBF) may be used to identify depositional settings (Fig. 16). 202,205,229Variations in the relative abundances of uorenes, dibenzofurans, and dibenzothiophenes have been reported to be a reliable indicator of source rock sedimentary settings. 203,230The concentrations of the dibenzothiophene series were high in marine carbonate source rocks, while those of uorenes and dibenzofurans series were high in freshwater source rocks. 209,226,227,230As a result, the uorene, dibenzofuran, and dibenzothiophene series can be further used in the oil-source rock correlation study. in crude oils and rock extracts by co-injecting authentic standards and proposed that this ratio may be a potential molecular geochemical parameter to indicate oil migration pathways and distances. 224In the Northern (Poland) and Southern (Argentina), these compounds have been found in bitumen from uvial-deltaic siltstone and charcoal from Jurassic records of wildres. 231,232There is currently no clear understanding of the factors that inuence benzo[b]naphtho [d]furan isomerization, and the source of these compounds is unknown.Such oxygenated chemicals, on the other hand, are thought to come from terrigenous organic matter, which would explain their high abundance in coal and coaly shales. 224Li and Ellis (2015) found BNFs in uids and source rocks from various depositional settings in a more extensive investigation of the BNFs. 224In pyrolysates from a subbituminous coal (random vitrinite reectance Rr = 0.42 percent) and a high volatile bituminous coal (random vitrinite reectance Rr = 0.56%), Vukovic et al. (2016) found a larger abundance of [2,1] and [1,2]BNFs compared to [2,3]BNF.They suggested that the water produced by kerogen, in combination with the clay minerals, undergoes numerous interactions with OM, and that these reactions could be the source of diverse oxygenated PAHs. 233Cesar and Grice (2017), recently reported benzonaphthofurans in crude oils and source rocks from the Dampier sub-basin in Western Australia, noting that clay catalysis appears to impact the formation of [1,2]BNF and that the ratio [2,1]/ [1,2]BNF might be utilized to explain lithofacies.When compared to clay-depleted sediments from marine environments, this ratio was substantially lower in the uvial-deltaic system (carbonate sequences).The authors proposed that the ternary plot of [2,1]-[1,2]- [2,3]BNFs could be used for uid-uid and uid-source rock correlations based on their ndings. 234

Conclusion and outlook
This paper reviews the early and modern applications of biomarkers for paleoenvironmental reconstruction.The oxic and anoxic depositional conditions, and lacustrine and marine depositional environments were compared.The experimental and instrumental methods for analyzing biomarkers in shales were then discussed.Saturated and aromatic biomarkers were discussed as indicators of marine sedimentary depositional environments, uvial/deltaic, freshwater, saline/brackish water depositional environments, and redox conditions.This review showed that biomarkers could be used to establish the sedimentary depositional environments, redox conditions, and organic matter enrichments of shales which are critical to deep energy exploitation.However, because biomarker geochemistry is a rich eld for the paleoenvironmental reconstruction of source rocks and oils, many biomarker classes remain to be discovered and studied for the understanding of paleoenvironmental reconstruction.Hence, abundant opportunities exist for exploring new classes of biomarkers and their paleoenvironmental signicance.Improvements can be made in the chromatographic separation and instrumental analyses of biomarkers by applying some automatic instruments that can separate EOM and oils to SARA and GC × GC-TOFMS which can eliminate the problems of co-elution and poor resolution usually encountered in GC-MS and at times GC-MS/MS.Also, for a more reliable paleoenvironmental reconstructions, there is a need to combine isotopic, elemental and maceral proxies with biomarkers.
Aguiar et al. (2010, 2011) used GC × GC to identify new compounds in crude oils and to evaluate Brazilian petroleum systems. 106,107Ventura et al. (2010) investigated reservoir compartmentalization using GC × GC ngerprinting of several oils.
Silva et al. (2011) offered a detailed biomarker examination of Colombian oils, 106 while Eiserbeck et al. (2011)accurately measured signicant biomarkers using baseline separation in a GC × GC analysis.94Oliveira et al. (2012) used GC × GC to explore branched-cyclic hydrocarbon heterogeneity in crude oil samples from two basins and to describe aromatic steroids and hopanoids from marine and lacustrine crude oils. 36,37Eiserbeck et al. (2012) compared the differences and advantages of various chromatographic separation and detection techniques, such as GC-MS, GC × GC-FID, and GCGC-TOFMS, for biomarker identication. 108The resolution of GC × GC is superior to that of classic 1D GC approaches, according to their research.The GC × GC-TOFMS technique produces a high-resolution separation and complete mass spectra across the whole chromatogram.Silva et al. (2013) and Soares et al.

Fig. 9
Fig.9Formation of pristane and phytane from phytol.140 3 and Es 4 members.b-Carotane concentrations in the Es 4 Member's oils ranged from 105 to 303 mg g −1 oil (averaged at 218 mg g −1 oil), whereas the Es 3 Member's oils had a range of 73-145 mg g −1 oil.In a heavily biodegraded oil.The concentration of -carotane in a heavily biodegraded oil reached 1044 g g −1 oil.Wang et al. (2021) recently discovered b-carotane in a suite of lacustrine oils from the Dongying Depression, Bohai Bay Basin, Eastern China, and classied the oils into two groups based on b-carotane parameters. 158The ratios -carotane/C24 tetracyclic terpane, bcarotane/(C 19 + C 20 ) tricyclic terpanes, and b-carotane/(18(H)-22,29,30-trisnorneohopane+17(H)-22,29,30-trisnorhopane) have been proposed to be useful for distinguishing oils derived from different depositional environments. 158,1595.1.3Tricyclic and tetracyclic terpanes.The tricyclic terpanes (TT) (VII), which are abundant in source rock extracts and crude oils, form a pseudo-homologous series with carbon atoms spanning from C 19 to C 54 .Because the higher members of the family are frequently hidden by hopanes in the m/z 191 mass chromatogram, they are generally recognized up to C 29 compounds. 160Because the formation of C 22 TT and C 27 TT requires the cleavage of two carbon-carbon bonds, their abundance is particularly low. 3 According to Ourisson et al. (1982), the tricyclic terpenes may have been derived from bacterial cell membranes. 161Furthermore, Aquino et al. (1983) claimed that a high abundance of tricyclic terpanes in crude oils could be

3 , 217
According to Zhang et al. (2002) and Mi et al. (2007), C 26 20S, C 26 20R + C 27 20S, and C 27 20R TAS were relatively high in oils derived from Cambrian source rocks, whereas C 28 20S and C 28 20R TAS were relatively high in oils derived from Middle-Upper Ordovician carbonate source rocks in the Lunnan oil eld, Tarim Basin.TAS has also been used to determine the thermal maturity of crude oils and source rocks. 214,216,2185.2.4 Dibenzofurans and benzo[b]naphthofurans.Dibenzofurans (DBFs) (XV), their alkylated homologs, and benzo[b]

Table 1
3,64on terminology describing the redox conditions in sedimentary environments and the metabolism of the corresponding microbial populations (biofacies)3,64