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Journal of Natural Gas Geoscience 1 (2016) 257e265http://www.keaipublishing.com/jnggs

Original research paper

Advancement in application of diamondoids on organic geochemistry*

Anlai Ma

Exploration & Production Research Institute, SINOPEC, Beijing 100083, China

Received 19 July 2016; revised 10 August 2016

Available online 27 September 2016

Abstract

Diamondoids occur in all kinds of fossil fuels. Due to peculiar cage molecular structures, diamondoids have been widely used in the maturityassessment of high mature to over-mature oils as well as source rocks since the 1990s. New advancements in maturity, oil-cracking, oil mixing,oil biodegradation, organic facies, TSR, gas washing, migration, and oil spill identification using diamondoids during the 21st century will befurther discussed in this paper; the origin and possible forming mechanisms of diamondoids are also explained. Owing to the vagueness of theorigin of diamondoid, the results of the maturity and oil cracking among researchers brought about great differences. It is suggested that theresearch of the evolution of diamondoid in different type oils and source rocks are beneficial when applied in organic geochemistry, especiallyfor the depth limits for the deep reservoirs.Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute ofPetroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This isan open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Diamondoids; Adamantane; Diamantane; Maturity; Oil-cracking; Biodegraded; Organic facies; Thermochemical sulfate reduction (TSR)

1. Introduction

Diamondoids, a cage hydrocarbon with a molecular for-mula of C4nþ6H4nþ12, widely occur in fossil fuels [1]. Thebasic repeated unit of diamondoids is adamantane [2,3](Fig. 1), a four ring cage system containing 10 carbons. Thescientific name of adamantane is tricyclo [3,3,1,1,(3,7)]decane, which can be regarded as composed by three rigidlyfused cyclohexane rings in an all-chair configuration. Thepolyhomologous members of adamantanes include dia-mantanes, triamantanes, tetramantanes, pentamantanes, andhexamantanes, in which adamantanes, diamantane, and tri-amantanes belong to lower diamondoids, whereas repeatedunits reaching over 4 belong to higher diamondoids. Lower

* This is English translational work of an article originally published in

Natural Gas Geoscience (in Chinese).The original article can be found at:10.

11764/j.issn.1672-1926.2016.05.0851.

E-mail address: maal.syky@sinopec.com.

Peer review under responsibility of Editorial Office of Journal of Natural Gas

Geoscience.

http://dx.doi.org/10.1016/j.jnggs.2016.09.001

2468-256X/Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Science

China. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open

diamondoids only contain one isomer, while polymantanescontain 4, 5, and 6 repeated units, the isomer numbers are 3, 6,and 17, respectively [3].

Diamondoids play a vital role by applying it in the field ofpetroleum geochemistry, medicine, chemistry, and nanometernew material [3] since Landa et al. [1] discovered adamantanein the oil from Hodonin oilfield within Czechoslovakia. Linet al. [4] reported higher diamondoids namely tetramantane,pentamantane, and hexamantane in the condensate oil pro-duced from the Jurassic Norphlet Formation sandstone in itsdepth of 6800 m within the U.S. Gulf of Mexico. Usingreversed-phased high-performance liquid chromatography(HPLC), Dahl et al. [5,6] isolated the crystal of hexamantaneor cyclohexamantane from distillate fractions from oil anddetermined the structure of C26H30 by means of X-ray diffu-sion, and GCeMS, 1H, 13C NMR methods.

In fossil fuels, the abundance of adamantanes is the highestsequence. Assuming that the abundance of adamantanes is100, the concentration of diamantanes, triadamantanes, tetra-mantanes, pentamantanes, and hexamantanes are 50, 15, 5, 1,and 0.1, respectively [4].

s AND Langfang Branch of Research Institute of Petroleum Exploration and Development, Petro-

access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Fig. 1. Diamondoid structure (after Ref. [3]).

Table 1

Relationship between the diamondoids' parameters and vitrinite reflectance

(after Ref. [18]).

Index I: MAI/% Index II:MDI/% Ro/%

50e70 30e40 1.1e1.370e80 40e50 1.3e1.6

80e90 40e60 1.6e1.9

>90 >60 >1.9

258 A. Ma / Journal of Natural Gas Geoscience 1 (2016) 257e265

2. Diamondoid detection method, internal standards, andpretreatment

Wingert [7] used MIDeGCeMS to identify the dia-mondoids in petroleum. In mainland China, Zhao et al. [8]adapted MIDeGCeMS to detect diamondoids in marginalmature source rock, in which it can reach the triamantaneseries. Both the GCeMS and GCeMSeMS methods wereapplied to detect the diamondoids in the Molecular OrganicGeochemistry Laboratory at Standford University [9]. Lianget al. [10,11] proposed that the sensitivity of theGCeMSeMS is about 4e66 times higher than that of theGCeMS, considering that the former has a better recovery.Through the development of technology, the comprehensivetwo-dimensional gas chromatography coupled to time-of-flight mass spectrometry, this was adapted to assess the dia-mondoids in crude oil. Using GC/GC-TOFMS, over 100compounds of adamantane, diamantane, and triamantane canbe identified [12,13].

The Molecular Organic Geochemistry Laboratory super-vised by K. E. Moldowan at the Standford University has themost advanced technology of determining diamondoid con-centrations by using six internal standards including D4-ada-mantane, D3-1-methyladamantane, D3-1-methyldiamantane,D4-diamantane, D4-ethyldiamantane, and D4-triamantane.Quantitation of alkyl-adamantanes, diamantanes, and tri-amantanes were achieved by integrating peak heights withrespect to corresponding standards: D3-1-methyladamantane,D3-1-methyldiamantane, and D4-triamantane, respectively.D4-adamantane, D4-diamantane, and D4-triamantane wereused to quantify adamantane, diamantane, triamantane, andtetramantane, respectively. Meanwhile, D5-1-ethyldiamantanewas used to quantify ethyl diamantanes. Their standard devi-ation was less than 8% [9]. Inland, Ma et al. [14] used twointernal standards (D16-adamantane and D3-1-methyldiamantane) to quantify the concentration of dia-mondoids in oils. Azevode et al. [15], Spriner et al. [16], andLiang et al. [10,11] adapted the diamondoid kits from theChrion AS Company of Norway as the calibration solution andused the n-C12D26, n-C16D34 as the internal standards toquantify the adamantanes and diamantanes, respectively.

In the pretreatment of compound-specific isotope analysisof diamondoids, Huang et al. [17] used the b-cyclodextin (b-

CD) to enrich diamondoids. This method has a high recoveryof the diamondoids in the oils and can satisfy the compound-specific isotope analysis (CSIA).

3. Application of diamondoids in organic geochemistry

3.1. Maturity determination of high mature toovermature oils and source rocks

The stability of diamondoids relies on the methyl substi-tution position. For the reason that methyl substitutionattached to a bridgehead position, 1-methyl adamantane, and4-methyl diamantane showed more stability than that ofdiamondoids with methyl substitution positioned at the qua-ternary carbon. Based on the aforementioned mechanism,Chen et al. [18e20] proposed two diamondoid maturityratios:

Methyl adamantane index I, MAI ¼ 1-MA/(1-MA þ 2-MA)Methyl diamantane index II, MDI ¼ 4-MD/(1-MD þ 3-MD þ 4-MD)

Both MAI and MDI, showing good correlation with vitri-nite reflectance (Table 1), were widely used as maturity in-dictors for high mature oils and condensate in China [20e29].In addition, Fu et al. [30] made use of the preceding ratios todetermine the maturity of gas from the southern QiongdongBasin in China. The determined result is consistent to that ofthe maturity obtained by carbon isotope of methane from thegas.

Some other diamondoids maturity ratios are as follows[31�35]:

DMDI-1(dimethyldiamantane index 1) ¼ 4,9-DMD/(4,9-DMD þ 3,4-DMD)DMDI-2 (dimethyldiamantane index 2) ¼ 4,9-DMD/(4,9-DMD þ 4,8-DMD)EAI (ethyladamantane index) ¼ 1-EA/(1-EA þ 2-EA)DMAI-1(dimethyladamantane index 1) ¼ 1,3-DMA/(1,3-DMA þ 1,2-DMA)DMAI-2(dimethyladamantane index 2) ¼ 1,3-DMA/(1,3-DMA þ 1,4-DMA)TMAI-1(trimethyladamantane index 1) ¼ 1,3,5-TMA/(1,3,5-TMA þ 1,2,4-TMA)TMAI-2(trimethyladamantane index 2) ¼ 1,3,5-TMA/(1,3,5-TMA þ 1,3,6-TMA)

Fig. 2. Cracking and mixed oil characteristics utilizing the concentration of

stigmastane and methyldiamantes (after Ref. [39]).

259A. Ma / Journal of Natural Gas Geoscience 1 (2016) 257e265

The research sample covering more lithology and maturitycaused the dispute of the range application of the MAI andMDI to be stimulated [31e34,36]. Some authors suggestedthat the application range of MAI and MDI used as maturitywas narrow. For example, Schulz et al. [31] argued that MAIand MDI only worked for samples with maturity above 1.3%.The result of thermal simulation experiment by Wei et al. [32]supported the conclusion of Schulz. Li et al. [33,34] suggestedthat MDI can be used as a maturity indicator for carbonaterock in high mature stage (Ro ¼ 0.9%e2.0%), and MDI is nota valid maturity indicator when the carbonate rock is over-mature (Ro > 2.0%).

Based on the golden tube pyrolysis results of the oil fromTarim Basin, Fang et al. [35,37] proposed that theMAI andMDIwere not suitable for the maturity index. Some diamondoidsratios, such as DMAI-1, TMAI-1, EAI, and DMDI-2 showed alinear correlation with Easy Ro% within the range of 1.0%e3.0%, 1.5%e3.5%, 1.0%e2.5%, and 2.5%e3.5%, respectively.Fang et al. [35] suggested that MAI, EAI, DMAI-1, DMAI-2,TMAI-1, and TMAI-2 are good maturity parameters beingwithin the range of 2.0%e3.0%, 1.6%e2.7%, 2.0%e3.5%,2.3%e3.5%, and 2.3%e3.5% Easy Ro, respectively. However,through the golden tube pyrolysis results of mudstones from thecoal measure, Fang et al. [38] argued that the MAI, EAI, andTMA-1 showed correlation with Easy Ro within the range of1.3%e2.5%, 1.0%e2.5%, 1.5%e2.5%, respectively.

From the results above, an obvious difference exists in thesuitable range for diamondoids maturity index from thedifferent types of organic matter.

3.2. Cracking and mixing oil

Based on the method of determining the extent of evapo-ration of salt water by analyzing salinity, Dahl et al. [39]proposed the method of determining the oil-cracking valueby using the “3-methyl-þ4-methyl-diamantane” and stigmas-tane concentration. The key to the method is that diamondoidsare neither destroyed nor created. The concentration increasesit's diamondoids as a result of the cracking of other liquidhydrocarbons. Thus, diamondoids can be considered as an“internal standard” by which the extent of cracking can bedetermined [39]. The cracking oil usually has high contents ofdiamondoids and low content of C29 steranes. If both highcontents of diamondoids and C29 steranes were observed in anoil, the oil may be a mixed oil derived from both a lowmaturity, biomarker-rich source, and a high-maturity, highlycracked, and diamondoid-rich source (Fig. 2).

The equation of calculating the extent of oil cracking usingthe methyldiamantanes in the oil is:

CR¼ (1�Co/Cc)*100%

Where CR is the cracked ratio, Co is the concentration ofmethyldiamantanes in the uncracked oil samples (termed asdiamondoid baseline), and Cc is the methyldiamantanes con-centration of any cracked oil sampled sourced from the samestarting oil.

Using this method, Schoell et al. [2] suggested that thereserve of the oil in the Gulf of Mexico should be restricted inthe Jurassic strata which depths ranged from 5000 m to6000 m. Later on, this method was used to assess the extent ofoil cracking in the Tarim Basin, China [40], as well as thebasins from Brazil [15]. Although Dahl et al. [39] proposedthat this method overestimated the extent of oil cracking, it ismore accurate than that of the GOR, because GOR can beinfluenced by other factors such as migration fractionation, gasdiffusion processes, and biodegradation. Accurate predictionof the extent of oil cracking relies heavily on the determinationof the diamondoid baseline due to the different basin and theirbaseline. Moldowan et al. [41] proposed that the diamondoidbaseline is within the range of 4e5 ppm and it is unusual tofind a diamondoid baseline greater than 8 ppm. He suggestedthat the diamondoid baseline of an unknown source rock isless than 10 ppm and probably less than 8 ppm [41].

Nevertheless, Fang et al. [37] found that in the oil pyrolysisresults, the concentration of 3-þ4-methyldiamantane barelyvaried in the range of 0.8%e2.0% Easy Ro. The yield grad-ually increased in the range of 2.0%e2.7% Easy Ro, and thenrapidly decreased to less than 2.7% Easy Ro. Fang et al. [37]suggested that the method of determining the extent of oilcracking using the concentration of diamondoid and steraneproposed by Dahl et al. [39] may be not valid for the highmature oil and it's only useful for the oil with a maturity that islower than 2.0% Easy Ro [37].

3.3. Organic facies

Three ratios were proposed by Schulz et al. [31] todistinguish between Type II carbonate, Type II marine sili-clastic, and Type III organic facies. The ratios are as follows:

Dimethyl diamantane index 1(DMDI-1) ¼ 3,4-DMD/(3,4-DMD þ 4,9-DMD)Dimethyl diamantane index 2(DMDI-2) ¼ 4,8-DMD/(4,8-DMD þ 4,9-DMD)Ethyl adamantane index (EAI) ¼ 2-EA/(2-EA þ 1-EA)

Fig. 4. Relationship between EAI and DMDI-1 of marine and lacustrine oils in

the Tarim Basin, China.

Fig. 5. Adamantanes and steranes in crude oils of various origin (a) and ages

(b) (after Ref. [42]).

260 A. Ma / Journal of Natural Gas Geoscience 1 (2016) 257e265

The DMDI-1 values for the Type II marine source rocksand Type III coal samples were 65% and 85% (Fig. 3),respectively. The DMDI-I value of carbonate rocks slightlyincreases with the increasing maturity in the beginning of theoil windows. However, the DMDI-2 value showed to be verystable in the carbonate rocks. The DMDI-2 values for otherfacies types also showed relatively stable values withincreasing maturity, and the difference between them was notsignificant to distinguish between the different facies. In thedimethyl diamantane composition, the relative distributionbetween 4,9-DMD, 4,8-DMD, and 3,4-DMD can distinguishthe different organic facies [31].

EAI ratio can reflect the facies of the source rock, but theratio is obviously affected by maturity. The EAI ratio of TypeII source rocks is about 70%, whereas in the Type III coalextract the value is greater than 85%. The EAI ratio of Type IIsource rocks increases with the increasing maturity, whereasthose with the maturity greater than 1.3% Ro, the ratio of TypeIII samples would decrease rapidly.

Both EAI and DMDI-1 ratios showed dissimilarities for themarine oils and lacustrine oils in the Tarim Basin, China. EAIand DMDI-1 ratios are lower than 65% for the marine oilsfrom the Tahe Oilfield, whereas the two ratios are greater than65% for the lacustrine oils from the Yingmali Oilfield (Fig. 4),thus, this backs that the ratios can be used to classify themarine and lacustrine oil.

Based on the distribution of diamondoids from 11 oilsamples, Gordadze [42] pointed out that the relative contentof adamantane with varying alkyl substituted homologues canbe used to distinguish marine oils and terrestrial oils as wellas the times of the oils. Marine oils usually have a low contentof C13 adamantane, in which relative content is lower than25%; whereas terrestrial oils have a low content of C11 ada-mantene, about 15%, and high content of C12 adamantane,about 47%e48%. Proterozoic oils contain a high amount ofC29 sterane among all the sterane (85%), low amount of C13

adamantanes (23%), and high amount of C12 adamantane(58%) (Fig. 5).

Fig. 3. Triangular plot of the relative distribution of 4,9-DMD, 4,8-DMD, and

3,4-DMD presenting the different organic facies (after Ref. [31]).

3.4. Biodegradation

Diamondoids have a strong resistance to biodegradation[43]. Williams et al. [44] found that the alkyl adamantane keptconstant in biodegradation oils. Even in the oil whosebiodegradation level has reached 7, an abundant unaltered C2-adamantanes were found. Since biodegradation prefers toconsume n-alkane, the ratio of diamondoid/n-alkane can beused to assess biodegradation level. Wingert [7] suggested thatadamantane/n-alkane is an indicator of biodegradation, butonly limited to low biodegradation level. Grice et al. [45]proposed that with the increasing biodegradation, the ratio ofMA/nC11 increased from 0.04 (biodegradation level is 0) to4.9 (biodegradation level is 3e4), whereas the ratio of MA/Aincreased from 5.3 (non-biodegraded oil) to 15.1 (seriouslybiodegraded oils); MA/A has obvious changes only when theoil were biodegraded heavily. In addition, the combined the

261A. Ma / Journal of Natural Gas Geoscience 1 (2016) 257e265

ratios of MA/A and MA/nC11 can describe the percentage ofthe biodegraded oil in the mixed oil. Although adamantaneand diamantane were affected by biodegradation, thementioned ratios were not significantly affected by any dif-ferences in maturity. By way of increasing biodegradation, theratio of alkyladamantane/adamantane increases, whereas theratio of alkyldiamantane/diamantane stays constant [45].

Wei et al. [46] found that the concentration of adamantanenoticeably decreases whenever the biodegradation level hasreached 3 e 4, whereas the concentration of diamantane andtriadamantane only show a slight change when biodegradationhas reached level 8. In oil samples, the diamondoids areresistant to biodegrade like the C19eC24 tricyclic terpanes,gammacerane, oleananes, diasterane, Ts, Tm C29 Ts, and 25-norhopane.

Due to the strong resistance to biodegradation, theimpression of diamondoids can be to correlate the biodegra-dation oil and non-biodegradation oil, as well as the black oiland condensates [31].

3.5. TSR

Thermochemical sulfate reduction (TSR) is an organic-inorganic effect occurring in the deep hot carbonate reser-voir in which the organic matter from petroleum is oxidized bysulfate (anhydrite) from the reservoir rocks which in turn aretransformed to hydrogen sulfide, carbon dioxide, mercaptan,elemental sulfur, and pyrobitumen [47e50]. Many reservoirsin the world have recorded their corresponding TSR. In ahandful of deep hot natural reservoir, TSR can form significantsour H2S gas, in which its content can reach as much as 10%e80%. As a result, abundant concentration of diamondoids andcompounds containing sulfur, such as thiadiamondoids andadamantanethiols can be detected in the oils and condensatesaltered by TSR [51e53]. In TSR altered oils, the concentrationof the adamantanethiols ranges from 1 to 100 ppm, whereasthe concentration of thiadiamondoids changes greatly, rangingfrom several ppm to several thousand ppm. Such compounds,especially 2-thiaadamantanes, can be used as molecular in-dicators for the occurrence of TSR, and to predict the sour gascontent in the petroleum exploration and development [54].

3.6. Migration

According to the pool-forming theory of reservoirgeochemistry, the petroleum migration/filling history has arelatively long geological process, in which the latter oilshaving higher maturity drive the former oils with lowermaturity in the form of a wave front, thus, keeping migration/filling forward until the filling process finished. Acertainingchange in the maturity of oils in the reservoir internal andheterogeneity in the chemical composition and physicalproperty would be produced [55].

Sassen et al. [56] detected the abundance of diamondoids(4742 ppm) from condensate oils with heavy carbon isotope(�23.7‰~24.5‰) from the Upper Jurassic and LowerCretaceous reservoirs in the Hudson Canyon area of the

Baltimore Canyon Trough, US Atlantic. Due to the lowermaturity of the known source rocks developed in the Atlantic,the concentration of diamondoids and the carbon isotope ofcondensate, mismatching to that of the known source rock, aresimilar to condensate generated by the Upper JurassicSmackover Formation of the US Gulf Coast, a laminated limemudstone with shale facies. Sassen et al. [56] suggested thatthe oil originated from the Smackover laminated limemudstone source rocks that were deposited during the Jurassicrifting of the Pangaean supercontinent. Using MDI, Duan et al.[57] proposed that two oil charging direction exited in theLower Ordovician oil from Tahe Oilfield. One is from thesouth to north and the maturity is relatively lower, probablyrepresenting the earlier petroleum migration, and the other isfrom the east to west with the maturity being relatively higher,probably representing the latter petroleum migration from 9blocks of east to 2 blocks of the west. Using a series param-eters calculated by the diamondoids eluting earlier versus tothat of the eluting later in the chromatograph, the migrationfractionation of the oil from the Dawaqi Oilfield. Tarim Basin,China, was analyzed by Ren et al. [58]. Generally speaking,the oils mainly show the vertical migration process, but in thezones developing a deep large fault, the parameters showed alow-value zone. Adjacent, the migration direction of the oil inthe Dawanqi Oilfield is from south to north [58].

3.7. Gas washing and phase fractionation

Great difference lies in the solubility of different compo-nents in the natural gas when oil has suffered gas-washing. n-alkanes of light carbons are likely to solute in the dry gas andescape from the original oil reservoir accompanying migrationwith natural gas, this reduces the light component of theoriginal reservoir. In a natural gas pool-forming of Kela-2Gasfield, Tarim Basin, China, Zhang et al. [59] suggestedthat gas washing reduces the n-alkane in the oil, this phe-nomenon greatly enriches the diamondoids and polycyclicaromatic hydrocarbons whose solubility are low in the naturalgas. The content of light aromatic hydrocarbons would alsoincrease resulting in an increase in aromaticity and a reductionin paraffinity. Gas-washing can change the relative mass per-centage of the diamondoids [59]. Wu et al. [60] proposed thatthe oils that suffered gas washing is characterized by 1-methyladamantane enrichment and depletion of 1,3-dimethyladamantane, 1,4-dimethyladamantane (trans), 1,4-dimethyladamantane (cis), 3-methyl-1-ethyladamantane, and1,2-dimethyladamantane.

3.8. Source identification of the spilled oil of the marine

In the progressive development of the offshore petroleumexploration and transportation, oil spill often occurs and makeserious damages in the marine ecology. In the incident of spillaccidents, spill identification in time is the important base forascertaining the accident responsible party, settling suitablearrangement, and evaluating the accident results [61,62].Wang et al. [63] used concentration of diamondoids and

262 A. Ma / Journal of Natural Gas Geoscience 1 (2016) 257e265

diagnosing ratio to distinguish the spilled oil and refined oilproduct. Zhang et al. [64] used semi-quantitation methods ofdiamondoids impressions to screen for large sets of samplesquickly and reduce the zone of the questionable source of thespill. Based on the characteristic ratios of diamondoids in theoil, Yang et al. [65] realized the rapid separation of the oilswith the evident dissimilarity using principle componentanalysis method (PCA).

4. The origin of diamondoids in oils

4.1. Complex analyzation of diamondoids' oil origin

Fig. 6. Relationship between the concentration of 3-þ4-methyl diamantanes

and the Ro from coals and source rock extracts (after Ref. [74]).

Although it has been over eighty years since diamondoidswere discovered in oils, its origin is still unclear. It is generallyaccepted that diamondoid is the product of polycyclic hydro-carbon polymerization catalyzed by strong Lewis acid underhigh-temperature thermal condition [7]. Pyrolysis of highmolecular mass paraffin and cycloparaffin fractions (bp above350 �C) catalyzed by Bronsted and Lewis acids at 190e286 �Ctemperature yield newly formed adamantane and diamantane[66e69]. The distribution of C10eC13 adamantanes in crudeoils correlates well with that of the products of thermalcracking of high molecular mass saturated fraction. Pyrolysisof non-hydrocarbon and asphaltenes fractions of oil generatedboth the adamantane and diamantane. The distribution ofadamantanes in oils is the same as that of the high molecularmass saturated fraction (>350 �C), nonhydrocarbon, andasphaltenes, suggesting that low-molecular-mass petroleumadamantanes, high-molecular-mass saturated fraction, andnon-hydrocarbon, asphaltenes contain the adamantane moietyin their structure. In addition, the distribution of diamantanesof the pyrolysis of non-hydrocarbon and asphaltenes fractionis the same as that of the pyrolysis of high molecular masssaturated fraction differing to that of the oil.

The result of pyrolysis experiment showed that lower dia-mondoids can be formed by immature sediments, peat,kerogen, oil, and other components including saturated frac-tion, polar fraction, and n-alkane compounds, such as C16, C19,C22, C34, C36, and b-ionone. b-ionone can form diamondoidsthrough acted with activated carbon. b-ionone with the alkanecyclohexane hydrocarbon skeleton, occurring in the sedi-ments, can form tricyclic adamantane structure through 1,3-dehydrocyclisation reaction. In addition, the model com-pound of adamantane heated with the activated carbon in aclosed system, hydrogen exchange, demethylation, methyltransfer, and isomerization reactions were observed [70].Dehydrogenation, cyclization, and aromatization reaction of n-alkane on the surface of activated carbon through hydrogenexchange were well documented in the study [71,72].

It is relatively easy to synthesize adamantanes from otherhydrocarbons in the laboratory. Burn et al. [73] made a highermember of diamondoid family by adding two new rings withfour carbon atoms (single homologation) or a second highermember by adding four new rings utilizing eight carbon atoms(double homologation). Using this method, Burn et al. [73]synthesized anti-tetramantane successfully. It is difficult to

synthesize higher diamondoids up to now; the only highestdiamondoids synthesized is the tetramantanes. Meanwhile, thefluid from deep oil reservoir contains tetramantanes, pentam-antanes, and hexamantanes [4,6]. Moreover, the oils from thecarbonate strata have Lewis acids that are either absent orpresent minor, causing the origin of diamondoids to becomplex.

4.2. The process of generation, enrichment, anddestruction of diamondoids

Recent research found that during maturation evolution,diamondoids undergoes generation, enrichment, and destruc-tion. Through analyzing coals and rocks from natural evolutionwith different maturity and different lithology, Wei et al. [74]suggested three main phases of diamondoid life: diamondoidgeneration (Ro < 1.1%), enrichment (1.1% < Ro < 4.0%), anddestruction (Ro > 4.0%) (Fig. 6). Pyrolysis experiment on thediamantane showed that diamantane stays stable up to 550 �C.At 600 �C, the pyrolysis of diamantane yield 15% aromaticsand 83% gas and other products (e.g. pyrobitumen). However,only a small amount of diamantanes remains. Based on thecarbon isotope analyses, Schoell et al. [39] proposed that somediamondoids are accountable for cracking at high-temperatures.

4.3. Possible origin of diamondoids in oils

Based on the results of the closed system of golden tubepyrolysis experiments of oils, Fang et al. [37] suggested thatthe diamondoids in the oil have three different origins: (1) Freediamondoids that probably formed during the generation of theoils, this component is in the Tarim oil, which can be deter-mined in the original oil, comprises 359 mg/g of adamantanesand 79.8 mg/g of diamantanes. (2) Diamondoids preserved inoil and in other forms. Details on the structures of thiscomponent are not clear, however, they are released or trans-formed at relatively low level of maturity. (3) Diamondoidsformed during oil cracking, a certain proportion of diamond-oids are present in the oils, the formation of abundant

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diamondoids occurs at relatively high levels of maturity.Earlier formed free adamantanes and diamantanes account for29.5% and 36.2% of the maximum yields of adamantanes anddiamantanes, respectively. Late-formed adamantanes anddiamantanes are predominantly generated within the maturityrange 1.0%e2.3% and 1.6%e2.5%, respectively, and they aredominantly derived from the saturated fraction, which ac-counts for 58%e72% of total adamantanes and 25%e62% oftotal diamantanes generated in this stage. The next mostabundant sources of late-formed adamantanes and dia-mantanes are the resin and aromatic fractions, whereas con-tributions from the asphaltene fraction are negligible.

5. Conclusions

(1) The evolution of diamondoids from different types oforganic matters should be carried out. The evolution ofcoal diamondoids and mudstone diamondoids wereresearched in the reference despite lacking the evolutioncomparison among different sediment environments anddifferent types of organic matters. Researching on thegeneration, enrichment and destruction of diamondoidfrom different organic matters would be meaningful inunderstanding the generation mechanism.

(2) The evolution of diamondoids from different oil typesshould be carried out. The generation and concentrationsof diamondoids from a single oil type and four compo-nents of oil (i.e, saturated, aromatic, non-hydrocarbon, andasphaltene fractions) were studied in the reference, lackingevolution comparison between the terrestrial oils (such ashypersaline, brackish, and freshwater environment) andmarine oils. Researching on the evolution and baseline ofdiamondoids of different types of oils would be beneficialto understanding the depth limits of the deep oil reservoir.

Foundation item

Supported by China State Fundamental Research Program(2012CB214800, 2005CB422108); Scientific and TechnicalResearch Project of SINOPEC (P07021); China NationalScientific & Technical Major Project (2011ZX05005,2008ZX05005).

Conflict of interest

The author declares no conflict of interest.

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