The organic geochemistry of asphaltenes and occluded biomarkers

Accepted Manuscript The organic geochemistry of asphaltenes and occluded biomarkers Lloyd R. Snowdon, John K. Volkman, Zhirong Zhang, Guoliang Tao, Peng Liu PII: DOI: Reference:

S0146-6380(15)00201-6 http://dx.doi.org/10.1016/j.orggeochem.2015.11.005 OG 3334

To appear in:

Organic Geochemistry

Received Date: Revised Date: Accepted Date:

30 June 2015 26 October 2015 4 November 2015

Please cite this article as: Snowdon, L.R., Volkman, J.K., Zhang, Z., Tao, G., Liu, P., The organic geochemistry of asphaltenes and occluded biomarkers, Organic Geochemistry (2015), doi: http://dx.doi.org/10.1016/j.orggeochem. 2015.11.005

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The organic geochemistry of asphaltenes and occluded biomarkers

Lloyd R. Snowdon ∗, John K. Volkman, Zhirong Zhang, Guoliang Tao, Peng Liu

SINOPEC Key Laboratory of Petroleum Accumulation Mechanisms, Wuxi Institute of Petroleum Geology, SINOPEC, 2060 Lihu Road, Wuxi, Jiangsu Province 214126, China

ABSTRACT Asphaltenes are heteroatom rich macromolecules thought to be mainly derived from the early breaking of covalent bonds in kerogen and thus have been considered to be a lower molecular weight analog of the kerogen at the time of early oil generation. This paper reviews information on asphaltene structures and sources derived from a variety of chemical procedures including oxidation, desulfurization and various forms of pyrolysis. An additional focus is on the geochemical information that can be obtained from oxidation of asphaltene with different chemical reagents. This procedure yields not only the chemical units that make up the asphaltene but also the occluded hydrocarbons providing biomarker information on the oil from which the asphaltenes were isolated. The latter is particularly useful when studying biodegraded oils that have received a subsequent charge and where biomarker information on the first, biodegraded phase may have been preserved within the asphaltene structure and/or occlusions. A common observation is that occluded hydrocarbons show distributions of biomarkers having less mature stereochemistry than in the oil maltenes (pentane soluble fraction).

∗

Corresponding author: [email protected], phone +86-185-5201-4310

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Ruthenium ion catalyzed oxidation mineralizes aromatic carbons in asphaltenes leaving a residue of oxidized (acidic) alkyl and alicyclic fragments that show how the aromatic centers were bound together. In addition, occluded hydrocarbons are released with little or no oxidation. Mild oxidation with hydrogen peroxide/acetic acid degrades a limited number of bonds in the asphaltene, releasing occluded or physically trapped species with little contribution from the asphaltene structure itself. Reduction techniques include the use of alkali metal or Raney nickel and generally these are used to desulfurize asphaltenes and release hydrocarbon moieties that were bonded via sulfide linkages. Hydropyrolysis, thermal degradation in the presence of high pressure hydrogen and a Mo catalyst, results in the cleavage of saturated and aromatic asphaltene moieties comprising the asphaltene that are then available for typical biomarker analysis using gas chromatography–mass spectrometry and other approaches. These degradation products include biomarker compounds useful for making genetic correlations among samples, age dating, providing estimates of thermal maturity and deconvoluting mixtures of oils.

Keywords: occlusions, asphaltene moieties, biomarkers, oxidation, pyrolysis, RICO, hydropyrolysis (HyPy), Raney nickel, biodegradation, mixed oils

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1. Introduction Asphaltenes are an operationally defined fraction of crude oil or source rock extract (i.e. bitumen) that is precipitated by low molecular weight (LMW) normal hydrocarbons such as n-pentane or n-heptane, but soluble in aromatic solvents such as toluene (e.g., Semple et al., 1990; Mullins, 2011; ASTM D3279-12; ASTM D6560-12). Unless care is taken, these traditional methods can produce asphaltene fractions that are contaminated with a significant amount of microcrystalline waxes since long chain hydrocarbons are poorly soluble in low boiling n-alkane solvents (e.g., Thanh et al., 1999). Similarly, adsorbed or otherwise trapped maltenes may contaminate poorly prepared asphaltene fractions. Multiple precipitations, solvent washings and/or back extractions are needed to produce uncontaminated asphaltene fractions, particularly for waxy oils. The asphaltene product is solid and typically brown to black in color. Asphaltenes have been studied from at least two important perspectives. One of these is effectively an engineering requirement that relates to understanding their chemistry and solubility or precipitation behavior in crude oils and the impact on fluid properties in reservoirs. Precipitation can also occur during production, mixing of different oils in temporary storage tanks, transportation and refining (Speight and Moschopedis, 1982). Thus one aspect of asphaltene research focuses on the “stability” of crude oils (Carbognani and Espidel, 2003) where reservoirs or production facilities could be plugged by precipitation and deposition of a solid phase in a fluid flow system. A recent review of engineering considerations of asphaltenes has been provided by Adams (2014) so the engineering aspects of asphaltene research are not further pursued here.

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Another application of the study of asphaltenes is to obtain biomarker and other information that is otherwise not available for crude oils because the oils have been altered by biodegradation, high thermal maturation or other geological processes (Rubinstein et al., 1979), or when the source rock is not available for study. These processes may have removed or altered many or all of the biomarkers or geochemical fingerprints of the maltene fraction (n-C5 or n-C7 soluble fraction of oil or bitumen comprising the saturates plus aromatics plus resins or NSOs). These biomarkers often consist of polycyclic saturated or aromatic steroid or terpenoid compounds. They are commonly used to determine genetic affinity (source provenance, including geological age), oil-oil or oil-source correlation and thermal maturity. These applications are fully discussed in a two volume textbook, The Biomarker Guide (Peters et al., 2005). It is this second, geochemical application of the study of asphaltenes that is the focus of this review. The asphaltene fraction is the least altered by biodegradation and other geological processes (Aref'yev et al., 1980) and as such preserves information about the chemical structure of a crude oil even though the saturate and aromatic fractions may have been significantly altered. Because the asphaltenes form at an early stage of petroleum generation and are largely preserved with little alteration from that stage, the biomarker signature incorporated within the asphaltenes represents an early, lower thermal maturity oil (Cassani and Eglinton, 1986 [their Figs. 4 and 5]) and in the case of biodegraded oils, the asphaltenes remain largely unaltered and provide a means of obtaining the biomarker signature of the precursor oil (e.g., Behar and Pelet, 1984 [their Fig. 6]).

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The reason that asphaltenes appear to preserve an early maturity signature has not been extensively discussed in the literature. Protection from biodegradation is most likely due to the low solubility of asphaltene molecules and aggregates and the commensurate lack of bioavailability of this fraction. However, Pan et al. (2013) have demonstrated that asphaltenes are somewhat altered by biodegradation and thus caution must be used when interpreting data derived from that fraction. The apparent reduction in the rate of thermal alteration may have to do with steric restrictions associated with the rigidity of the macromolecular structure, but evidence supporting this contention must be considered speculative. The apparent preservation of biomarker moieties within asphaltenes as they undergo thermal maturation in source rocks and crude oils is somewhat counterintuitive in so far as asphaltenes contain much of the heteroatomic content of bitumen and heteroatomic-carbon covalent bonds tend to be weak. Biomarker moieties could be preserved because of cross linking within asphaltenes that would require the cleavage of two or more bonds to release them to the maltene fraction. However, various degradation processes discussed below indicate that biomarker moieties are bonded through a single, rather specific covalent bond. It could also be speculated that bond cleavage in asphaltene molecules might be partly reversible because the recently cleaved metastable, intermediate species are held in close proximity to each other by the rest of the asphaltene molecule, or in the case of occluded species, the metastable products of a cleavage reaction would presumably remain trapped in the occluding volume and thus be subject to back reaction. The apparently lower thermal maturity of asphaltenes relative to the crude oil or source rock bitumen could also be partially an artefact of the level of thermal maturity of

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the samples that have been selected for study. That is, asphaltene contents of high maturity samples tend to be low and thus these samples may not be well represented within asphaltene research because of the difficulty of obtaining enough material with which to work.

1.1. Asphaltene moieties It has been observed that moieties (small constituent parts) that comprise parts of the asphaltene structure (Aref’yev et al., 1980; Behar and Pelet, 1984; Behar et al., 1984) may preserve intact structures from the original kerogen and/or derived crude oil or bitumen during subsequent biodegradation and thermal alteration (catagenesis). Various strategies have been developed to recover these moieties from the asphaltene and thus provide information on the nature of the oil associated with the asphaltene prior to biodegradation or thermal alteration. These include pyrolysis (Behar and Pelet, 1984; Behar et al., 1984; Cassani and Eglinton, 1986; Jones et al., 1987), chemical degradation of aromatic centers in asphaltenes to CO2 with ruthenium ion catalytic oxidation (RICO) (Strausz et al., 1999a,b; Ma et al., 2008), chemical reduction by metals (Ekweozor, 1984, 1985), hydropyrolysis reduction using a sulfided Mo catalyst (HyPy; Snape et al., 1989; Russell et al., 2004) and sulfide bond cleavage by reagents such as Raney nickel or nickel boride (Schouten et al., 1993, 1995; Peng et al., 1997) or Li/EtNH2 (Hofmann et al., 1992).

1.2. Asphaltene occlusions

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In addition to obtaining information about the chemical composition of the asphaltene macromolecule itself by cleaving and analyzing its constituent moieties, some authors have recognized that the various degradation processes also released compounds that were not covalently bonded to the asphaltene fraction but rather were occluded within it (e.g., Strausz et al., 1999a,b; Russell et al., 2004; Gordadze et al., 2015). The high stability of occlusions in asphaltenes has been documented by Liao and Geng (2002), Liao et al. (2005, 2006a,b,c), Silva et al. (2008) and Tian et al. (2012a,b) who showed that occluded hydrocarbons remain within the asphaltene structure during commonly used extraction and other workup procedures, even after 240 hours of Soxhlet extraction with acetone. Occluded molecules may be trapped within a single covalently bonded asphaltene monomer or within an asphaltene aggregate. Thus the size and shape of asphaltenes and aggregates and their relative stability over geological time has relevance to the information obtained from species occluded within them. The occluded biomarkers of interest include pentacyclic hopanoids, tetracyclic steroids and a range of polyaromatic hydrocarbons and heteroatomic compounds. Not only will the chemical and physical details of the asphaltenes and aggregates control how effective the various thermal and chemical approaches are at recovering information, but these properties will also determine to some extent the timing of the entrapment and hence the interpretation of the biomarker signals recovered from them.

1.3. Asphaltene size and shape

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There has been an extensive debate in the literature regarding the size and shape of both asphaltene monomers and aggregates. This has recently been reviewed by Hosseini-Dastgerdi et al. (2015). Briefly, there are two main models of asphaltene monomers: archipelago and island. The former uses various lines of evidence to support a structure that includes groups of alicyclic and aromatic rings (typically 1–4 rings) bound together with branched and straight chain alkyl moieties extending to C30 and beyond. The typical molecular weight range for these monomers extends up to ~2000 Da (Fig. 1). The island model proposes that asphaltenes are dominantly comprised of larger (6–8 ring) pericondensed aromatic cores with alkyl moieties extending out from this core (Fig. 2). The molecular weight range is indicated to be 750 ± 250 Da (Mullins et al., 2012), although FT–ICR–MS data indicate that the molecular weight range is centered at < 500 Da (Pan et al., 2013), around 500–600 Da (Pomerantz et al., 2009) or 700 Da (Sabbah et al., 2012). Hosseini-Dastgerdi et al. (2015) observed that different analytical approaches supported different models and suggested “that the architecture of asphaltene molecules is in a continuum of island and archipelago types”. The wide range of discrepancies in both observations and interpretations would appear to support this conclusion. From a biomarker perspective, it would seem highly unlikely that an asphaltene monomer with a mass of 750 Da could effectively “occlude” a target compound such as a hopane or sterane with molecular weight in the range of 350–450 Da. Thus it would seem that occluded compounds must be associated only with the relatively small amount proportion of higher molecular weight asphaltenes or the occlusions are trapped in aggregates rather than monomers (Gray et al., 2011; Zhao et al., 2012).

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1.4. Asphaltene aggregates Asphaltene aggregates are held together by forces other than covalent bonds even in solutions of a low concentration (Groenzin and Mullins, 1999; Bergmann et al., 2000; Strausz et al., 2002). These supramolecular entities may also occlude guest molecules and may also have stability over geological time at typical reservoir conditions. The formation of asphaltene aggregates is dependent on concentration (Yarranton et al., 2013). Assuming that most crude oils contain > 0.01% (100 ppm, w/w) asphaltenes, it would appear that asphaltene aggregates would be common and even dominant in crude oils, where the oil is likely to be a less effective solvent than pure toluene. Groenzin and Mullins (1999) presented a review of the determination of asphaltene molecular weight and provided a consideration of the impact of the presence of aggregates during the characterization of an asphaltene fraction. The term “nanoaggregates” was used by Zhang et al. (2003) to describe monolayer asphaltenes that formed at air-water or heptane/toluene (heptol)-water interfaces. These “nanoaggregates” were imaged using atomic force microscopy (AFM) and showed a vertical relief of ~5 nm and aggregate diameters of ~20–200 nm. The same term, nanoaggregates, was subsequently used to describe intermolecular stacking that is largely controlled by π-π or van der Waals interactions between the condensed aromatic cores or asphaltene monomers (Andreatta et al., 2005). These nanoaggregates comprised only ~8 monomers (Betancourt et al., 2009) and were an order of magnitude smaller than the nanoaggregates described by Zhang et al. (2003), whose AFM images could reflect

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“clusters” of the ~8 monomer aggregates or aggregates that are mainly bound by intermolecular forces other than π-π or van der Waals interactions. While occluded compounds could be considered as “absorbed” and/or “adsorbed” onto the surfaces of the asphaltene macromolecule, these surface phenomena are secondary to the cage structure. It is generally presumed that surface “sorbed” material would be removed from the asphaltene during repeated precipitation cycles using solvents such as toluene and heptane and/or recovered during extensive back extraction (e.g., up 240 hours of acetone extraction; Zhao et al., 2010). If asphaltene aggregates generally can be considered as very stable, then occluded species within them will also represent the chemical environment that existed very early in the generation and expulsion history of an oil. On the other hand, if aggregates form and degrade relatively easily in response to PVT conditions and/or changing oil chemistry, the guest hydrocarbons trapped in the aggregates will have a complex origin that will represent a complex evolution. Rayleigh scattering measurements (Derakhshesh et al., 2013b) indicated that asphaltene aggregates in toluene (1 g/l) were in the size range of 2–20 nm and that complete dissociation of the aggregates was not possible using various chemical dispersants, ultrasonic treatment, or heating (200 °C) either alone or in various combinations. These results suggest that once asphaltene aggregates have formed, they will remain relatively stable through geological time under typical reservoir conditions. In contrast to the stability of asphaltene aggregates indicated by the Derakhshesh et al. (2013b) results, Strausz et al. (2002) used gel permeation chromatography (GPC) to document the dissociation of up to ~80% of asphaltene aggregates simply on standing in

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CH2Cl2 solution for up to 14 d. These results emphasize the fact that asphaltene aggregates, like asphaltenes themselves, are polydispersed (wide range of sizes and shapes) and thus likely have a commensurate range of stabilities.

2. Chemical analysis of asphaltene moieties 2.1. Pyrolysis One of the most commonly used methods for characterizing the organic chemistry of the asphaltene fraction of crude oils has been pyrolysis. Various conditions have been used, providing variable results, indicating that considerable care must be used when interpreting the analytical data. Rubinstein et al. (1979) heated an asphaltene fraction at 300 °C for 72 h under vacuum in sealed glass. They observed that at < 300 °C, a similar product pattern was obtained, but with a lower yield. At 400 °C, significant cracking occurred, such that only gases and pyrobitumen were recovered after 72 h. Their 300 °C experiments on a laboratory-biodegraded oil from Prudhoe Bay indicated that > 20% of the initial asphaltene fraction became pentane soluble. Much of the observed product was unsaturated hydrocarbons. Because the asphaltene used in these experiments was laboratory derived and the precursor oil presumably did not contain a significant amount of n-alk-1-enes (hereafter, alkenes), these compounds were most probably derived through C–C bond cleavage. However, recent work by Yang et al. (2009) and Tian et al. (2012a) indicates that at least some of the alkenes could actually represent occluded species from very early in the hydrocarbon generation process.

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Aref’yev et al. (1980) ran similar experiments in sealed glass under vacuum, but at 350 °C for 2–4 h. They recognized the possibility of the existence of occluded hydrocarbons. They indicated that the absence of occluded hydrocarbons was demonstrated using "gas liquid chromatography”, but did not provide details of this procedure. In these experiments, 5–10% of the original asphaltene became hexane soluble, with limited production of alkenes or volatile compounds. Their gas chromatograms do not show any obvious n-alkenes and this could indicate that the products were actually occluded saturated hydrocarbons rather than asphaltene moieties that were cleaved during the pyrolysis. However, the yields appear to be too high to account for this amount of alkane product. Aref’yev et al. (1980) also noted that the pristane/phytane ratio was always higher in the pyrolyzate than in the original oil and that the odd/even n-alkane predominance was always lower in the pyrolyzate than in the oil. Both of these observations are counterintuitive to the presumption that asphaltene moieties (or occluded compounds) represent an early, low maturity stage of the crude oil in which they occur. Pyrolysis experiments were carried out at 450 °C and 550 °C for 30 s in He carrier gas by Behar et al. (1984) and Behar and Pelet (1984), after an initial thermovaporization at 300 °C. These authors used the initial heating at 300 °C and/or liquid chromatography to ensure that the n-C7 asphaltene fraction was “pure”, presumably meaning that no occluded or absorbed hydrocarbon molecules were present within the asphaltene molecules or aggregates. No indication was given in these papers that any of the asphaltenes contained occluded hydrocarbons that were detected with the 300 °C thermovaporization or liquid chromatography. The pyrolysis results indicated that

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after 30 s at 550 °C, 10–50% of the asphaltene was recovered as a pyrolyzate. Gas chromatograms of the products show normal alkene-alkane pairs in sub-equal amounts out to at least C30 (Behar and Pelet, 1984 [their Fig. 5]), although the chromatograms of some samples show an alkene/alkane peak height ratio that is << 1, especially for higher molecular weight compounds. The pyrolysis process should yield about a 50:50 mixture of the saturated versus unsaturated compounds. That is, there should be about equal probability of the electron pair in the cleaved C–C bond jumping to the alkyl fragment or to the macromolecule to which it was attached, giving rise to about 50:50 alkene:alkane product mix. However, the observation of variable ratios of products suggests the presence of occluded saturated compounds being added to the neoformed alkene-alkane cleavage products. Excess n-alkanes could also indicate that the n-alkyl moieties were bonded through a heteroatom. On the other hand, Mushrush and Hazlett (1984) obtained highly irregular distributions of alkene/alkane ratios for various starting compounds pyrolyzed at 450 ºC. They observed a general trend of decreasing alkene/alkane ratio with increasing pyrolysis time from 15 min to 180 min consistent with lower stability of the alkenes. It is not clear that these experiments provide a useful explanation of open system, flash or short duration (< 1 min) pyrolysis results. Fig. 3a shows flash pyrolysis results (610 °C) for a crude oil asphaltene from well TK772 in the Tahe oilfield, China. The mass chromatogram for m/z 55+57 shows that the n-alkane peaks are higher than those of the n-alkenes for all compounds from n-C8–n-C30 (except for C10, presumably due to coelution). In contrast, Fig. 3b shows that the alkene/alkane ratio is > 1 for C10–C30 for the same sample when the m/z 83 and m/z 85 peaks are used to identify the normal hydrocarbons. The apparent compound distribution

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for the different carbon numbers is also different depending on which mass fragment is used. Pyrolysis experiments were carried out on asphaltenes isolated from Venezuelan and Indonesian (lacustrine) source rocks by Strausz et al. (1999b) at 430 °C with a short residence time (30 ml/min N2 flow rate). Straight chain alkene-alkane pairs were among the products recovered, with a decreasing proportion of alkenes with increasing carbon number. Similar pyrolysis results were obtained for residues from a ruthenium ion catalyzed (RICO) decomposition of asphaltenes (see below) demonstrating that n-alkyl chains were coupled with naphthenic as well as aromatic nuclei. In contrast to the results of Behar et al. (1984), Behar and Pelet (1984) and Strausz et al. (1999b), Cassani and Eglinton (1986) reported the recovery of < 5% alkenes based on silver ion thin layer chromatography from the pyrolysis of heavy oil asphaltenes at the much lower temperature range of 300–370 °C (72 h in vacuum). At similar temperatures, Rubinstein and Strausz (1979) also recovered about 5% unsaturated compounds. If all of the Cassani and Eglinton (1986) unsaturated compounds were alk-1enes, these were apparently present in amounts similar to those of the n-alkanes. On the other hand, if the pyrolysis conditions were insufficient to break a significant number of C–C bonds and thus giving rise to much smaller yields of alkenes, much of the product could represent occluded compounds rather than covalently bound asphaltene moieties. It is also possible that the longer reaction time (72 h) resulted in back reactions of alk-1enes with the asphaltene and/or the formation of pyrobitumen. Yields of heptane soluble products increased from about 5% to 18% with increasing temperature, consistent with yields recovered after mild oxidation (Liao and Geng, 2002).

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The principal objective of Cassani and Eglinton (1986) was to recover biomarkers from biodegraded, extra-heavy oils and they were able to demonstrate that the asphaltenes did provide useful information regarding the characteristics of the original oils. Jones et al. (1987) also used moderate temperature pyrolysis (250 °C, 290 °C, 330 °C for 72 h) under either anhydrous (under N2) or hydrous conditions to recover biomarkers from the asphaltene fraction of two oil samples. While prist-1-ene and prist2-ene were reported, they did not recover any n-alkenes. These authors did not discuss the possibility of occluded hydrocarbons within the asphaltene molecules or asphaltene aggregates. Overall, pyrolysis has been mainly applied to thermally cleave covalent bonds within asphaltene (or kerogen) macromolecules and thus characterize the chemical structure of these materials. While the possibility of occluded species has been considered in some of this research, in most cases the resulting products were presumed to mainly represent asphaltene moieties and in some cases, preparative procedures were presumed to preclude occluded compounds. More recent work, however, has suggested that both the quantity and stability of occlusions are greater than was presumed in the earlier experiments. Wilhelms et al. (1993) monitored molecular weights of asphaltenes in real time during ramped heating (50–750 °C at 1 °C/s) under vacuum (10-3 Pa) with a field ionization mass spectrometer as the detector (pyrolysis–FIMS). FIMS is a very soft ionization technique that tends to generate molecular ions rather than fragment ions. While the weight average mass ranged up to ~700 Da, the mass spectra show that most of the products (representing 50–75% of the initial asphaltenes) had apparent masses that

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were < 600 Da. The largest portion of the signal was obtained at temperatures of 250– 600 °C suggesting to those authors that the detected compounds were “pyrolysis products rather than volatilization products”. Molecules or fragments up to 1500 Da were observed in these experiments. Gel permeation chromatography (GPC) on the same samples yielded up to ~40% of the asphaltene with a mass of 5500–10,000 Da. It would appear that this GPC analysis may have yielded masses that reflect asphaltene aggregates rather than monomers. Two more recent pyrolysis techniques (microscale sealed vessel, MSSV) and Mo catalyzed hydropyrolysis (HyPy) have several advantages over older pyrolysis methods. Both techniques provide good yields with minimal structural rearrangement. These methods have been extensively compared and contrasted by Berwick et al. (2010 and references therein). MSSV is typically run in a closed system at a static temperature between about 250 °C and 350 °C for hours to days. HyPy is run as an open system, but under high pressure (> 10 MPa) in a hydrogen gas flow. Fig. 4 shows GC–MS results for the saturate fraction recovered after HyPy of asphaltenes from the oil from well TK772 in the Tahe oilfield, China. The m/z 85 trace shows a wide range of n-alkanes extending at least to C40 while there are no n-alkenes apparent, as would be expected in a catalyzed reducing pyrolysis. The m/z 83 trace does show, however, a wide range of alkylcyclohexanes. These compounds are present in the original maltene fraction of the oil and the asphaltene HyPy products but not in the 610 °C flash pyrolysis products, suggesting that they are formed but subsequently degraded at higher temperatures. Thus it would thus appear to be important to use pyrolysis operating conditions that are consistent with the analytical objectives.

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3.2.2. Ruthenium ion catalytic oxidation (RICO) Ruthenium ion catalyzed oxidation (Strausz et al., 1999a,b) selectively mineralizes aromatic centers in asphaltene or kerogen macromolecules to CO2. Naphthenic (saturated) rings, alkyl side chains and bridges are not degraded. The major products are typically homologous series of n-alkanoic acids (C1–C30+), representing the n-alkyl side chains that were attached to aromatic moieties; α,ω-di-n-alkanoic acids (diacids; C4–C26+), representing polymethylene bridges connecting two aromatic units; and benzenepolycarboxylic acids (known as BPCA) derived from pericondensed polyaromatic systems (e.g., Strausz et al., 1999a,b). The RICO reaction also yields an oxidized residue, which, after pyrolysis and methylation can yield additional series of nalk-1-enes and n-alkanes, n-alkenoic and n-alkanoic acid methyl esters, and free nalkanoic acids with strong even/odd carbon preference (Strausz et al., 1999b). This technique has been used almost exclusively to obtain information about the asphaltene structure itself, rather than being exploited as a tool to degrade the asphaltene sufficiently to release occluded hydrocarbons. Strausz et al. (1999b) used RICO of asphaltenes from marine Boscan and lacustrine Duri oils and analyzed the methyl ester products to C30+ in order to determine the structures of the non-aromatic moieties in the asphaltenes. They recognized that some of the products, such as bicyclic terpenoids, may have been occluded rather than part of the asphaltene structure. Pyrolysis of the residue yielded alk-1-ene/alkane pairs, indicating the presence of alkane chains that were bound to naphthenic rings, but alk-1enes were not recovered from the RICO process directly. The absence of alkenes from

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this oxidation process is in contrast to the n-alkenes recovered by Yang et al. (2009) and Tian et al. (2012a) after mild oxidation (see Section 4). Clearly, additional analytical approaches and/or the analysis of additional samples at different levels of thermal maturity may be required to resolve this discrepancy. Ali et al. (2004) also used RICO to determine the structures of the non-aromatic moieties in both heavy and medium Arabian residual fuel oil asphaltenes. Detailed analyses of the products using FT-IR, 13C NMR, ion chromatography, GPC and GC–MS techniques revealed that n-alkyl chains are important constituents of the chemical structure of asphaltenes from both types of oils. Water soluble RICO products consisted of aliphatic dicarboxylic acids and aromatic polycarboxylic acids with longer alkyl chains. The organic phase products contained large amounts of aliphatic carboxylic acids with long chain alkyl groups. Fig. 5 is an example of GC–MS traces of esterified fatty acids and di-acids recovered from the asphaltene fraction of a crude oil (well AD26, Tahe oilfield, China). The di-acids represent alkyl chains that occurred as bridges between two aromatic centers that were oxidized during the ruthenium ion catalyzed reaction, whereas the fatty acids represent alkyl side chains on aromatic centers. The di-acids show a relative response of ~10% of the mono-acids when monitored using the m/z 98 ion for the di-acid methyl esters and m/z 74 for the mono-acid esters. That is, long chain alkyl bridges are present but not as common as alkyl side chains. Similarly, Ma et al. (2008) used the recovered acidic analogs of the terpanes and steranes from RICO of Tarim Basin crude oil asphaltenes to conclude that Lunnan and Tahe oils represented two charges each, but that both of these were sourced from the

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Upper Ordovician. Neither Strausz et al. (1999a,b) nor Ma et al. (2008) discussed the possibility of occluded compounds, presumably because both reported results for only the derivatized acid fraction prior to instrumental analysis. An advantage of the RICO analysis is that it offers the opportunity to characterize both the covalently bound moieties comprising asphaltenes (in the form of methyl esters or other derivatives) as well as alkyl and naphthenic species occluded within the asphaltenes because the former are recovered as polar, alkanoic acids while the latter can be recovered as hydrocarbons. These compound classes are easily separated and can be analyzed separately. This procedure offers the opportunity to compare and contrast the bound and unbound material associated with asphaltene (or kerogen).

3.2.3. Reduction with metals Limited use has been made of asphaltene reduction using metallic K in tetrahydrofuran and naphthalene (Ignasiak et al., 1977; Ekweozor 1984, 1985). The naphthalide anion selectively cleaves S–C bonds and O–C bonds without significant alteration of C–C bonds. The reduction in average molecular weight of the asphaltenes was used to demonstrate that most of the reactive S was in sulfide rather than thiophenic structures because the latter would result in ring opening with little change in molecular weight (Ignasiak et al., 1977). However, Speight and Moschopedis (1982) pointed out that potassium naphthalide also cleaves some C–C bonds and that only a minority of the sulfur species were liberated during the cleavage reactions, indicating that the dominant form of S was

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not in sulfides, but rather was thiophenic. This observation was amplified by noting that Raney nickel degraded thiophenic moieties react very slowly relative to sulfide species. Bituminous sand samples from Alberta and Nigeria yielded about 20% of the initial asphaltene weight as pentane soluble maltenes after reduction using metallic potassium (Ekweozor, 1984). This was interpreted to result from either direct formation via protonation of cleavage products and/or the release of occluded compounds. The observation of a strong predominance of cheilanthanes (tricyclic terpanes) relative to hopanes in the cleavage products was the opposite of that found in the original maltenes and was interpreted to indicate that the main source of the metallic K reduction was occluded materials rather than cleavage fragments. Similarly, the slight even/odd predominance for some ranges of the recovered n-alkanes supported this interpretation. Ekweozor (1985) suggested that even milder asphaltene degradation conditions (FeCl3/acetic anhydride or methanolic KOH saponification) yielded 7–10% maltenes. The saturate and aromatic fractions of these could be readily characterized using standard GC–MS techniques. The observation of a high cheilanthane/pentacyclic hopane ratio appears to be counterintuitive to the notion that these compounds were occluded. In most cases tricyclic terpanes are in low concentration at low thermal maturity (as was observed for the maltene fraction of the oils). Thus the observation of relatively high concentrations of cheilanthanes in the asphaltene pyrolyzate might suggest that the pentacyclic precursors were not present or incorporated into the asphaltene structure and/or that they were preferentially degraded relative to the more stable cheilanthanes by the naphthalide reagent used to cleave asphaltene bonds. Alternatively, if the cheilanthanes were indeed occluded, the results would suggest that they were occluded

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preferentially to the larger pentacyclic hopanes. Analyses of additional samples using metallic K and/or using additional degradation techniques to compare the products could yield useful insights into the cheilanthane and hopane relationship with asphaltenes.

3.2.4. Hydropyrolysis with sulfided Mo catalyst (HyPy) Love et al. (1995) used hydropyrolysis (HyPy) to recover maltenes from kerogen with nearly 80 wt% conversion, consistent with the value expected from Rock-Eval pyrolysis results for a Type I kerogen. In this method, a sulfided molybdenum catalyst was mixed with an organic substrate (asphaltene or kerogen) under 15 MPa of flowing H2 with the temperature programmed from 100–520 °C at 5 °C/min (84 min). The recovered maltene fraction from the analysis of a low maturity organic rich rock (Type I kerogen) included ββ-hopanes in concentrations that were 3–5 times higher than were present in the original solvent-extracted oil or from maltenes recovered after mild hydrogenation (300 °C) of the extracted rock. The distribution of biomarker products indicated that the HyPy method causes very limited amounts of structural rearrangement and is thus suitable for characterizing the constituent moieties present in geomacromolecules. Although relatively high temperatures are used, this is an open system pyrolysis carried out under high pressure, flowing H2 in which the products are quickly swept out of the reaction chamber, minimizing secondary cracking reactions. Very low thermal maturity Paleogene-Neogene oil shale and coal samples were used to optimize the HyPy reactor variables (Love et al., 1997) and also served to demonstrate that HyPy temperatures ramped at 5 °C/min up to 520 °C did not significantly alter the stereochemical configuration of biomarker compounds. Similarly,

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Liao et al. (2012) recovered biomarkers using HyPy of solid bitumen residues (pyrobitumen) that had been synthetically altered up to the equivalent thermal maturity of 2.8 %Ro (percent white light Reflectance under oil immersion from vitrinite). Although the original soft bitumen had been extensively biodegraded and contained ~60% asphaltenes and no n-alkanes, these compounds were present in the HyPy product along with a range of sterane and terpane biomarkers. HyPy of overmature kerogen generated sufficient biomarkers to permit an oil-source correlation in the Sichuan Basin where the putative Lower Cambrian source is now highly overmature (> 3 %Ro; Wu et al., 2012). Love et al. (1996) used a similar HyPy approach to characterize vitrinite concentrates from coals at different maturities up to 1.32 %Ro. These authors observed that occluded species comprised an increasing proportion of the dichloromethane soluble products for the higher maturity samples. They also noted that yields from the HyPy of the vitrinite were much lower than that for more hydrogen rich kerogens (types I and II) and that the aliphatic products were dominated by low molecular weight compounds (< C12). Russell et al. (2004) applied HyPy to asphaltenes recovered from reservoir cores to reconstruct the migration history or reservoir filling history of a North Sea oil field. The migration direction was inferred by observing the increasing thermal maturity of the released hydrocarbons based on the presumption that the earliest migrated oil would have the lowest maturity signature and asphaltenes adsorbed onto the mineral surfaces would preserve that signature. That is, the “bound” biomarker compounds were not displaced or altered by later generated, higher maturity oil that migrated past the entry point and into the reservoir or by any secondary reservoir processes. The assertion of irreversible

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binding on mineral surfaces appears to presume that the wetting phase of the reservoir minerals is oil rather than water. Bowden et al. (2006) also showed HyPy results for asphaltene (and other fractions) in which the asphaltene products clearly have a lower thermal maturity profile than the oil maltene fraction on the basis of sterane and terpane biomarkers (their Figs. 2 and 4). Similarly, Meredith et al. (2008 [their Fig.1]) used HyPy of the asphaltene fraction of a seep oil sample to recover “pristine biomarkers” from material that was so severely degraded that all recognizable, free biomarkers had been removed. The hydropyrolyzate of this seep sample included several unusual hopenes. HyPy has been shown to provide relatively high yields of GC-amenable compounds from kerogen and asphaltene. However, it is not clear that bound moieties can be readily differentiated from occluded species. Thus the interpretation of the HyPy results must take into account the fact that variable amounts of these two sources may be present in the product maltenes.

3.2.5. Sulfide bond cleavage with Raney nickel, Ni2B or Li/EtNH2 Various chemical treatments have been used to cleave sulfide bonds and desulfurize asphaltenes. These include using Raney Ni, metallic lithium in ethylamine (or N,N-deutero-ethylamine) and nickel boride (Ni2B). In essentially all of these experiments, the intent was to cleave and recover S-bonded moieties for subsequent instrumental analysis in order to determine the structural details of the macromolecules. The initial macromolecules that have been analyzed have almost always been derived from S-rich, low maturity samples and as such, these reduction techniques may not be applicable to samples that are high maturity and/or rather low in S concentration.

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Hofmann et al. (1992) used two S-rich samples (the asphaltene fraction of Rozel Point seep [Utah] and kerogen from the Serpiano shale [Switzerland]) to show that Li/C2H5NH2 provided similar product distributions, but in higher yield, to the traditional Raney nickel method of asphaltene desulfurization. Further, the N,N-deuterated reagent (Li/C2H5ND2) provided information on how the recovered moieties were bound to the asphaltene macromolecule by identifying the position at which the D was incorporated into the product. For example, the Li/C2H5ND2 reduction of 2-hexadecylthiophene yielded n-dodecane with deuterium substitution on carbons 1 through 4. For steranes, a single deuterium was incorporated into the A ring or position 6 of the B ring while the hopanes appeared to be incorporated into the Serpiano kerogen on the extended side chain of the E ring. Schouten et al. (1993, 1995) demonstrated that Ni2 B was also more effective at cleaving S bound moieties from S-rich polar fractions extracted from various sediments and oils than Raney nickel. However, Hartgers et al. (1996) suggested that Ni2B was not suitable for many situations because it is not specific to sulfide bonded moieties. This reagent also yielded hydrocarbons from the direct reduction of “ketones, aldehydes, and α-unsaturated alcohols and esters”. Asphaltenes precipitated from Athabasca oil sand samples were characterized by Peng et al. (1997) who used Ni2B to cleave sulfides. Up to 18% n-C5 soluble material was recovered after Ni2B treatment of asphaltenes. Subsequent steps of basic hydrolysis and BBr3 treatment were used to cleave esters and ethers, respectively, demonstrating that these functionalities were not affected by the Ni2B desulfurization.

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Sulfur specific cleavage using various chemical reagents can provide valuable information regarding the structure of macromolecules. However, these techniques appear to be best suited to sulfur rich samples with a low to moderate level of thermal maturity. Thiophenic S is chemically much more stable than sulfides and interpretations of the cleavage product distributions must take this difference in reactivity into account.

4. Characterization of occluded compounds released by mild oxidation In contrast to many of the pyrolysis and chemical reduction techniques discussed above, experiments using mild oxidation have tended to focus on the occluded compounds within asphaltenes, rather than moieties covalently bonded to and thus part of the asphaltene structure. In general, this chemical treatment is intended to oxidatively cleave sufficient bonds within the asphaltene or kerogen macromolecule to release physically occluded species without significantly contributing neoformed hydrocarbon products from asphaltene (or kerogen) breakdown. Zhao et al. (2012) carried out confined (gold tube) heating of a mixture of asphaltene + deuterated n-C20 (with and without added H2O and various chloride salts) at 240 °C, 270 °C or 290 °C for 3, 2 or 1 d, respectively, to observe the synthetic sorption/entrapment of n-C20. Compounds that were not Soxhlet extractable (hexane for 48 h followed by acetone for 72 h) after the heating cycle were considered to be occluded in either asphaltenes or asphaltene aggregates (not specified). Experiments with and without added water, and with water plus NaCl yielded significant amounts of occluded C20D42. The occluded deuterated hydrocarbon may indicate that heating resulted in the neoformation of asphaltene monomers and/or the formation of stable asphaltene

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aggregates that trapped the added deuterated hydrocarbon. Alternatively, Derakhshesh et al. (2013a) suggested that the asphaltene aggregates form nanoporous networks into which tracer molecules may migrate, even at room temperature. If this model is correct, the Zhao et al. (2012) results suggest that once the hydrocarbons have migrated into the nanoporous network, they cannot be easily removed by solvent extraction. Experiments with CaCl2 and AlCl3 (Zhao et al., 2012) did not result in significant amounts of occlusion, presumably because these salts resulted in the cracking of the asphaltenes and/or the blocking of the development of aggregates or possibly (as indicated by the authors) because of the thermal cracking of the deuterated hydrocarbon tracer compound. These experiments suggest that new asphaltene or asphaltene aggregate occlusions form and/or that occluded compounds may be very rapidly (days) exchanged at temperatures at least as low as 240 °C. Lower temperature experiments (closer to realistic reservoir temperatures), might provide more constraint on the possible timing of trapping or exchange of occluded hydrocarbons. It should be noted that the Zhao et al. (2012) starting material was asphaltene from a low maturity oil and thus the results may not be generally applicable to asphaltenes that have been recovered from oils or source rock bitumens that are at moderate or high levels of thermal maturity. However, our recent unpublished results have shown good recoveries of occluded compounds from at least one high thermal maturity oil.

4.1. Mild oxidation using H2O2:CH3COOH

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A large number of papers have been published by the Chinese Academy of Sciences group in Guangzhou (Liao and Geng, 2002; Liao et al., 2005, 2006a,b,c; Yang et al., 2009; Zhao et al., 2010, 2012; Tian et al., 2012a,b; Cheng et al., 2014) in which H2O2 (30%)/CH3COOH or NaIO4/NaH2PO4 have been used as mild oxidants to release occluded compounds from isolated asphaltenes. While both of these reagent systems were tested initially, subsequent work used only the former even though the latter resulted in a higher maltene yield (Liao and Geng, 2002). [An anonymous reviewer of the first draft of this submission has indicated that the latter reagent system yielded I2 that compromised subsequent workup of the products.] Reactions were run in stirred benzene at room temperature over 12 h for both reagent systems. In the initial experiments, Liao and Geng (2002) used 1:10 peroxide (30% H2O2): CH3COOH (v:v). Subsequently, Zhao et al. (2010) showed that in a sequence of increasing proportions of peroxide, a 3:4 v:v ratio of H2O2 (30%)/CH3COOH yielded longer average chain length alkane products than experiments run at both lower ratios of 1:5 and 2:5 and also experiments run at the higher ratio of 5:4. Clearly, mild oxidation results have yielded occluded hydrocarbons that appear to represent a less thermally mature compound distribution from earlier in the catagenetic history of an oil. For example, Tian et al. (2012a [their Fig. 3]) show an extremely high even/odd predominance in bitumen released from asphaltenes from four Tarim Basin oil samples. Tian et al. (2012a) also tabulated several systematic changes in various biomarker ratios among the whole oil, asphaltene adsorbed fraction and the asphaltene occluded compounds.

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Comparison of the sterane distributions of the AD26 oil from Tahe oilfield with the mild oxidation product of asphaltene recovered from that oil (Fig. 6) shows that αααC2920S/(20S+20R) ratio is 0.52 for the former but only 0.49 for the latter. Similarly, the C29αββ/(ααα+αββ) ratio of the oil is 0.57 while for the mild oxidation product the ratio is 0.46. Synthetic “occlusion” experiments were also carried out by Derakhshesh et al. (2013a). These authors suggested that PAH (phenanthrene and pyrene) were occluded in asphaltene aggregates in a toluene solution with 48 hours of stirring at room temperature. However, the “control” experiment used to differentiate simple adsorption from occlusion does not appear to have been very rigorous. The precipitated asphaltenes in both the “occlusion” and “adsorption” tests were recovered in a single precipitation step and then subjected to “vacuum filtration and overnight air-drying” but without any indication that the asphaltenes were back extracted with even the non-polar precipitation solvent and certainly not with a solvent such as acetone. Asphaltene aggregate occlusions have been observed to be quite stable and thus it is not clear what might be the mechanism whereby simulated distillation (SimDis, used by Derakhshesh et al., 2013a) with a heating rate of 20 °C/min releases the target compounds, if they are indeed occluded rather than adsorbed. It would seem that pure volatilization associated with SimDis would not be effective for a truly occluded species. The experimental design of Derakhshesh et al. (2013a) provides little information on the relative stability of asphaltene aggregates. Mild oxidation yielded about 2 wt% saturated hydrocarbons (relative to the original asphaltene weight) for a Silurian age oil sand from the Tarim Basin and also for a

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Triassic oil from the Songliao Basin (Liao and Geng, 2002). Aromatic fraction yields were reported to be about double that of the saturates. Similar experiments on oil samples from the Congo and Venezuela (Liao et al., 2005) yielded about 0.5% saturated hydrocarbons. Molecular and isotopic distributions of the Tarim and Songliao saturate fractions were subsequently used to demonstrate the efficacy of the mild oxidation technique. The oxidation products of the Tarim sample yielded higher relative concentrations of cheilanthanes and isotopically less depleted n-alkanes than the maltene of the oil sand itself (Liao and Geng, 2002). In general, cheilanthane concentrations increase and C isotopes become less depleted with increasing thermal maturity and thus both of these observations indicate that the occluded material released by mild oxidation has a higher maturity than the maltene fraction. This is thus inconsistent with the occluded fraction representing a lower thermal maturity, geologically younger material than the bulk oil, but consistent with an interpretation of a different source for the occlusion oil. The observation of elevated cheilanthanes in the recovered hydrocarbons from Tarim Basin (Liao and Geng, 2002) is similar to the results of the metallic K reduction experiments of Ekweozor (1984) for Athabasca and Nigerian oil sands. The initial Tarim oil, represented by the oxidation products, could have either been extensively altered through biodegradation (relative enrichment of the cheilanthane concentrations and shift to less depleted C isotopes) or volumetrically overwhelmed by the second oil charge, which dominates the chemical character of the current oil sand. An additional Tarim Basin sample has been examined using mild oxidation (Zhao et al., 2010) who used the heavy isotopic signature of compounds in the maltene fraction released through asphaltene

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oxidation of a crude oil (Ordovician reservoir in well ZG31) to conclude that the reservoir was initially charged from a Cambrian source rock. The isotopic evidence was also corroborated by the presence of gammacerane in the hydrocarbons released through mild oxidation which was notably absent in the maltene fraction of the ZG31 oil. Tian et al. (2012a,b) also used mild oxidation to determine that Tarim Basin reservoirs in the Tazhong, Tabei, Lunnan and Halahatang areas represented two oil charges, one from a Cambrian-Lower Ordovician source and another from a MiddleUpper Ordovician source. This interpretation was also based on terpane and sterane biomarker signatures along with stable carbon isotopes of the recovered n-alkanes. Experiments in which isotopically labelled material was combined with asphaltenes (Liao et al., 2005) show that the timing of the entrapment of occluded molecules must be essentially at the same time as the formation of the asphaltene molecule itself. While the added labelled compound, C20D42, persisted through several precipitations and indicated reasonably strong adsorption characteristics, none of the labelled material was observed in the maltene fraction that was released by mild oxidation of the exhaustively extracted asphaltene. This interpretation is consistent with additional observations (Liao et al., 2006a,c) in which occluded material from related biodegraded and non-degraded oils were compared. Occlusion of labelled material was achieved, however, by Zhao et al. (2012) during gold tube pyrolysis (240–290 °C) of asphaltenes of low maturity with and without various salt additives. Even if the original occluded species were essentially unaffected by the experimental conditions, these experiments suggest that processes resulting in the neoformation of asphaltenes could add to the cumulative character of the occluded material.

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Yang et al. (2009) and Cheng et al. (2014) have reported recovering significant amounts of occluded alk-1-enes from low maturity asphaltene, kerogen and high maturity solid bitumen samples (%VRo = 2.3–2.5) from Sichuan Basin drill core by using mild oxidation with H2O2 (30%)/CH3COOH. In addition, various triterpenes were also recovered and characterized by GC–MS. Azevedo et al. (2009) studied biodegraded crude oils from Brazil that are depleted in n-alkanes and noted that the released products from the corresponding asphaltenes had n-alkane distributions from n-C10 to n-C40, suggesting that these occluded hydrocarbons were protected from biodegradation and thus representative of the original oil. They also observed that the tricyclic/pentacyclic terpane ratio was lower in the occluded compounds relative to the oils. This is opposite to what was observed by Ekweozor (1984, 1985) and Liao and Geng (2002) and was attributed by Azevedo et al. (2009) to the selective preservation of the cheilanthanes during biodegradation and the preservation of the original, pre-biodegradation oil signature in the occluded compounds.

4.2. Mild oxidation using other reagents In addition to using metallic K, Ekweozor (1985) used FeCl3 + acetic anhydride as a mild oxidant and KOH and benzene to carry out saponification of crude oil asphaltenes. These afforded GC amenable maltenes that were considered to have only a very limited contribution of material cleaved from the asphaltene molecules. The products were considered to be representative of the “original oil” prior to severe biodegradation.

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Silva et al. (2008) used NaIO4/NaH2PO4 following the procedure of Liao and Geng (2002) to oxidize asphaltenes from four variably biodegraded Brazilian crude oils. About 1.5% of the asphaltene was recovered as a maltene fraction. About 30% of the maltene was saturated hydrocarbons and this fraction was analyzed using GC–MS. Three of the four samples showed higher cheilanthane/hopane ratios in the original oils relative to the occluded oils, consistent with a lower degree of susceptibility of the tricyclic compounds to biodegradation. That is, biodegradation selectively reduced the hopane concentrations in the free phase oil, but the terpane fraction in the occluded oil represented the distribution of compounds in the non-degraded oil.

5. Asphaltene adsorbed compounds In addition to considering asphaltene moieties and compounds occluded within asphaltenes, various studies have been undertaken to consider “sorption” (adsorption and/or absorption) interactions of LMW hydrocarbon fractions with asphaltene molecules and/or aggregates. For example, Pan et al. (2002) used sequential precipitation with petroleum ether to recover materials that were increasingly strongly adsorbed onto asphaltenes. Strausz et al. (2006) used the partitioning of alkanes between solvent and asphaltene fraction to elucidate the asphaltene adsorption phenomenon. Clearly, the study of compounds occluded in asphaltenes as well as the study of moieties covalently bound to the asphaltene macromolecule requires very careful preparation in order to avoid the incorporation of materials that are simply adsorbed onto the asphaltene surfaces, and presumably readily exchangeable during the evolution of a petroleum system. For example, Strausz et al. (1999a) extracted asphaltenes with acetone

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for 68 hours while Zhao et al. (2010) removed “sorbed” materials by extracting the asphaltene fraction in a Soxhlet apparatus with acetone for 240 hours. Strausz et al. (2002) observed the breakdown of asphaltene aggregates in dichloromethane after several days, potentially accompanied by loosely occluded hydrocarbons. Occluded compounds and asphaltene moieties themselves preserve information about the early history of oil generation, whereas adsorbed materials reflect a much more complex, dynamic and cumulative history of the chemical nature of a crude oil.

6. Summary Asphaltenes and asphaltene aggregates comprise chemical moieties that can be recovered from the parent molecule by various pyrolysis and chemical degradation techniques in which covalent bonds are broken. Experimental results to date indicate that these fragments can provide biomarker information that generally reflects the character of the source organic matter at an early stage of its thermal evolution and also prior to most secondary alteration processes such as biodegradation. Similarly, asphaltenes and asphaltene aggregates contain small amounts of occluded compounds that are trapped in the macromolecular structure but not covalently bound to the asphaltenes or asphaltene aggregates. These compounds represent a biomarker signature that was also present at the early stage of the generation of the asphaltene molecules as they were cleaved from their kerogen precursor. While species occluded in monomer asphaltenes would appear to be necessarily representative of very early maturity, asphaltene aggregates may apparently form at a later stage in the evolution of petroleum and thus carry a signature of a different stage of the geological

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evolution of the asphaltene fraction. In general, if the asphaltene aggregates are carefully recovered to avoid inclusion of adsorbed or co-precipitated species, the biomarker signal will be dominated by early processes. Asphaltene aggregates dissociate to a great extent in dilute dichloromethane solutions, but the stability of aggregates in normal crude oil at typical concentrations and typical reservoir temperatures appears to be quite high, generally precluding the exchange of occluded species. Many analytical approaches have been used to characterize asphaltenes and asphaltene aggregates including both instrumental and chemical techniques. Open system pyrolysis techniques include flash pyrolysis (heating to high temperatures in a few seconds or less), ramped pyrolysis (heating rates of a few degrees per minute) and HyPy (hydropyrolysis) in which the products are typically trapped and analyzed offline. Closed system pyrolysis approaches include heating in a glass tube under vacuum, in a gold tube under argon, MSSV (microscale sealed vessel under inert gas), and hydrous pyrolysis (under liquid water). Other degradation techniques rely on chemical reagents and have included the use of metallic K (naphthalide) and reduction using Raney Ni, Ni2B, Li/EtNH2 or Li/EtND2. All of these foregoing tools provide moieties derived from the asphaltene molecule or aggregate that generally have not been separated or differentiated from occluded compounds. Ruthenium ion catalyzed oxidation is a method that provides aliphatic and alicyclic moieties of the asphaltene that are oxidized plus occluded compounds as hydrocarbons. These fractions could, in principal, be readily separated and analyzed offline. Mild oxidation (typically using peroxide and acetic acid) yields mainly occluded compounds rather than asphaltene moieties.

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The information obtained from each of these must be carefully considered because the analytical approach itself will have inherent limitations and biases and the molecular behaviour of asphaltenes and asphaltene aggregates depends on the physical conditions of the sample (e.g., temperature and especially concentration in either solvent or crude oil solution). Interpretation or speculation as to why asphaltenes are sheltered from thermal evolution has been very limited. It is possible that purely organic systems preclude organic-inorganic interactions and these are key to the geothermal evolution of asphaltenes. That is, the asphaltene molecules are dissolved or suspended in and comprise part of the bitumen or oil phase that has limited direct contact with the water wet surfaces of minerals. The macromolecular structure of asphaltene molecules and aggregates may also impart a steric resistance to some reactions resulting in bond cleavage. Opportunities appear to exist for making significant advances in understanding asphaltene chemistry and more generally the organic geochemistry of petroleum. Future research in which carefully isolated asphaltene fractions from a wide range of source rock bitumen and crude oil types at different levels of thermal maturity and using a wide array of analytical techniques may help to resolve some of the apparent discrepancies that have been documented in the asphaltene literature. Additionally, it would appear that experiments in which both asphaltene moieties and occluded species are obtained and characterized separately for the same sample should provide significant new insights into the provenance and history of complex crude oil systems.

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Acknowledgments

This review received funding support from Sinopec project contract P14051. Two anonymous reviewers and the Co-Chief Editor, Erdem Idiz, made many suggestions to improve this manuscript and we gratefully acknowledge the time and effort that they contributed.

Associate Editor – Clifford Walters

References

(ASTM), A.S.f.T.a.M., 2012. ASTM D3279-12 Standard Test Method for n-Heptane Insolubles. ASTM International, www.astm.org, West Conshohocken, PA. (ASTM), A.S.f.T.a.M., 2012. ASTM D6560 - 12 Standard Test Method for Determination of Asphaltenes (Heptane Insolubles) in Crude Petroleum and Petroleum Products. ASTM International, www.astm.org, West Conshohocken, PA. Adams, J.J., 2014. Asphaltene adsorption, a literature review. Energy & Fuels 28, 2831−2856. Andreatta, G., Goncalves, C.C., Buffin, G., Bostrom, N., Quintella, C.M., Arteaga-Larios, F., Pérez, E., Mullins, O.C., 2005. Nanoaggregates and structure−function relations in asphaltenes. Energy & Fuels 19, 1282–1289. Aref'yev, O.A., Makushina, V.M., Petrov, A.A., 1980. Asphaltenes as indicators of the

- 36 -

geochemical history of oil. International Geology Review 24, 723–728. Azevedo, D. de A., da Silva, T.F., da Silva, D.B., 2009. Geochemical evaluation of occluded biomarkers in asphaltenic structures. Quimica Nova 32, 1770–1776 (in Portuguese with English abstract). Behar, F., Pelet, R., 1984. Characterization of asphaltenes by pyrolysis and chromatography. Journal of Analytical and Applied Pyrolysis 7, 121–135. Behar, F., Pelet, R., Roucache, J., 1984. Geochemistry of asphaltenes. Organic Geochemistry 6, 587–595. Bergmann, U., Mullins, O.C., Cramer, S.P., 2000. X-ray Raman spectroscopy of carbon in asphaltene:  Light element characterization with bulk sensitivity. Analytical Chemistry 72, 2609–2612. Berwick, L.J., Greenwood, P.F., Meredith, W., Snape, C.E., Talbot, H.M., 2010. Comparison of microscale sealed vessel pyrolysis (MSSVpy) and hydropyrolysis (Hypy) for the characterisation of extant and sedimentary organic matter. Journal of Analytical and Applied Pyrolysis 87, 108–116. Betancourt, S.S., Ventura, G.T., Pomerantz, A.E., Viloria, O., Dubost, F.X., Zuo, J., Monson, G., Bustamante, D., Purcell, J.M., Nelson, R.K., Rodgers, R.P., Reddy, C.M., Marshall, A.G., Mullins, O.C., 2009. Nanoaggregates of asphaltenes in a reservoir crude oil and reservoir connectivity. Energy & Fuels 23, 1178–1188. Bowden, S.A., Farrimond, P., Snape, C.E., Love, G.D., 2006. Compositional differences in biomarker constituents of the hydrocarbon, resin, asphaltene and kerogen fractions: An example from the Jet Rock (Yorkshire, UK). Organic Geochemistry 37, 369–383.

- 37 -

Carbognani, L., Espidel, J., 2003. Preparative subfractionation of petroleum resins and asphaltenes. II. Characterization of size exclusion chromatography isolated fractions. Petroleum Science and Technology 1, 1705–1720. Cassani, F., Eglinton, G., 1986. Organic geochemistry of Venezuelan extra-heavy oils 1. Pyrolysis of asphaltenes: A technique for the correlation and maturity evaluation of crude oils. Chemical Geology 56, 167–183. Cheng, B., Yang, C., Du, J., Zhao, J., Liao, Z., 2014. Determination of the series of even carbon numbered n-alk-1-enes trapped inside geomacromolecules. Marine and Petroleum Geology 51, 49–51. Derakhshesh, M., Bergmann, A., Gray, M.R., 2013a. Occlusion of polyaromatic compounds in asphaltene precipitates suggests porous nanoaggregates. Energy & Fuels 27, 1748−1751. Derakhshesh, M., Gray, M.R., Dechaine, G.P., 2013b. Dispersion of asphaltene nanoaggregates and the role of Rayleigh scattering in the absorption of visible electromagnetic radiation by these nanoaggregates. Energy & Fuels 27, 680−693. Ekweozor, C.M., 1984. Tricyclic terpenoid derivatives from chemical degradation reactions of asphaltenes. Organic Geochemistry 6, 51–61. Ekweozor, C.M., 1985. Characterisation of the non-asphaltene products of mild chemical degradation of asphaltenes. Organic Geochemistry 10, 1053–1058. Gordadze, G.N., Giruts, M.V., Koshelev, V.N., Yusupova, T.N., 2015. Distribution features of biomarker hydrocarbons in asphaltene thermolysis products of different fractional compositions (using as an example oils from carbonate deposits of Tatarstan oilfields). Petroleum Chemistry 55, 22–31.

- 38 -

Gray, M.R., Tykwinski, R.R., Stryker, J.M., Tan, X., 2011. Supramolecular assembly model for aggregation of petroleum asphaltenes. Energy & Fuels 25, 3125–3134. Groenzin, H., Mullins, O.C., 1999. Asphaltene molecular size and structure. The Journal of Physical Chemistry A 103, 11237–11245. Hartgers, W.A., Lopez, J.F., De las Heras, F.X.C., Grimalt, J.O., 1996. Sulphur-binding in recent environments. I. Lipid by-products from Ni2B desulphurization. Organic Geochemistry 25, 353–365. Hofmann, I.C., Hutchison, J., Robson, J.N., Chicarelli, M.I., Maxwell, J.R., 1992. Evidence for sulphide links in a crude oil asphaltene and kerogens from reductive cleavage by lithium in ethylamine. Organic Geochemistry 19, 371–387. Hosseini-Dastgerdi, Z., Tabatabaei-Nejad, S.A.R., Khodapanah, E., Sahraei, E., 2015. A comprehensive study on mechanism of formation and techniques to diagnose asphaltene structure; molecular and aggregates: A review. Asia-Pacific Journal of Chemical Engineering 10, 1–14. Ignasiak, T., Kemp-Jones, A.V., Strausz, O.P., 1977. The molecular structure of Athabasca asphaltene. Cleavage of the carbon-sulfur bonds by radical ion electron transfer reactions. The Journal of Organic Chemistry 42, 312–320. Jones, D.M., Douglas, A.G., Connan, J., 1987. Hydrocarbon distributions in crude oil asphaltene pyrolyzates. 1. Aliphatic compounds. Energy & Fuels 1, 468–476. Liao, Z., Geng, A., 2002. Characterization of nC7-soluble fractions of the products from mild oxidation of asphaltenes. Organic Geochemistry 33, 1477–1486. Liao, Z., Geng, A., Graciaa, A., Creux, P., Chrostowska, A., Zhang, Y., 2006a. Different adsorption/occlusion properties of asphaltenes associated with their secondary

- 39 -

evolution processes in oil reservoirs. Energy & Fuels 20, 1131–1136. Liao, Z., Geng, A., Graciaa, A., Creux, P., Chrostowska, A., Zhang, Y., 2006b. Saturated hydrocarbons occluded inside asphaltene structures and their geochemical significance, as exemplified by two Venezuelan oils. Organic Geochemistry 37, 291–303. Liao, Z., Graciaa, A., Geng, A., Chrostowska, A., Creux, P., 2006c. A new lowinterference characterization method for hydrocarbons occluded inside asphaltene structures. Applied Geochemistry 21, 833–838. Liao, Z., Zhou, H., Graciaa, A., Chrostowska, A., Creux, P., Geng, A., 2005. Adsorption/occlusion characteristics of asphaltenes: some implication for asphaltene structural features. Energy & Fuels 19, 180–186. Liao, Y., Fang, Y., Wu, L., Geng, A., Hsu, C.S., 2012. The characteristics of the biomarkers and δ13C of n-alkanes released from thermally altered solid bitumens at various maturities by catalytic hydropyrolysis. Organic Geochemistry 46, 56– 65. Love, G.D., Snape, C.E., Carr, A.D., Houghton, R.C., 1995. Release of covalently-bound alkane biomarkers in high yields from kerogen via catalytic hydropyrolysis. Organic Geochemistry 23, 981–986. Love, G.D., Snape, C.E., Carr, A.D., Houghton, R.C., 1996. Changes in molecular biomarker and bulk carbon skeletal parameters of vitrinite concentrates as a function of rank. Energy & Fuels 1996 10, 149–157. Love, G.D., McAulay, A., Snape, C.E., 1997. Effect of process variables in catalytic hydropyrolysis on the release of covalently bound aliphatic hydrocarbons from

- 40 -

sedimentary organic matter. Energy & Fuels 11, 522–531. Ma, A., Zhang, S., Zhang, D., 2008. Ruthenium-ion-catalyzed oxidation of asphaltenes of heavy oils in Lunnan and Tahe oilfields in Tarim Basin, NW China. Organic Geochemistry 39, 1502–1511. Meredith, W., Snape, C.E., Carr, A.D., Nytoft, H.P., Love, G.D., 2008. The occurrence of unusual hopenes in hydropyrolysates generated from severely biodegraded oil seep asphaltenes. Organic Geochemistry 39, 1243–1248. Mullins, O.C., 2011. The asphaltenes. In: Cooks, R.G., Yeung, E.S. (Eds.), Annual Review of Analytical Chemistry, Vol. 4, pp. 393–418. Mullins, O.C., Sabbah, H., Eyssautier, J., Pomerantz, A.E., Barré, L., Andrews, A.B., Ruiz-Morales, Y., Mostowfi, F., McFarlane, R., Goual, L., Lepkowicz, R., Cooper, T.M., Orbulescu, J., Leblanc, R.M., Edwards, J.C., Zare, R.N., 2012. Advances in asphaltene science and the Yen-Mullins model. Energy & Fuels 26, 3986–4003. Murgich, J., Abanero, J.A., Strausz, O.P., 1999. Molecular recognition in aggregates formed by asphaltene and resin molecules from the Athabasca oil sand. Energy & Fuels 13, 278–286. Mushrush, G.W., Hazlett, R.N., 1984. Pyrolysis of organic compounds containing long unbranched alkyl groups. Industrial & Engineering Chemistry Fundamentals 23, 288–294. Pan, C., Geng, A., Liao, Z., Xiong, Y., Fu, J., Sheng, G., 2002. Geochemical characterization of free versus asphaltene-sorbed hydrocarbons in crude oils: Implications for migration-related compositional fractionations. Marine and Petroleum Geology 19, 619–632.

- 41 -

Pan, Y., Liao, Y., Shi, Q., Hsu, C.S., 2013. Acidic and neutral polar NSO compounds in heavily biodegraded oils characterized by negative-ion ESI FT-ICR MS. Energy & Fuels 27, 2960–2973. Peng, P.a., Morales-Izquierdo, A., Hogg, A., Strausz, O.P., 1997. Molecular structure of Athabasca asphaltene:  Sulfide, ether, and ester linkages. Energy & Fuels 11, 1171–1187. Peters, K.E., Walters, C.C., Moldowan, J.M., 2005. The Biomarker Guide 1. Biomarkers and Isotopes in the Environment and Human History. Cambridge University Press, New York. Pomerantz, A.E., Hammond, M.R., Morrow, A.L., Mullins, O.C., Zare, R.N., 2009. Asphaltene molecular-mass distribution determined by two-step laser mass spectrometry. Energy & Fuels 23, 1162–1168. Rubinstein, I., Strausz, O.P., 1979. Thermal treatment of the Athabasca oil sand bitumen and its component parts. Geochimica et Cosmochimica Acta 43, 1887–1893. Rubinstein, I., Spyckerelle, C., Strausz, O.P., 1979. Pyrolysis of asphaltenes: A source of geochemical information. Geochimica et Cosmochimica Acta 43, 1–6. Russell, C.A., Snape, C.E., Meredith, W., Love, G.D., Clarke, E., Moffatt, B., 2004. The potential of bound biomarker profiles released via catalytic hydropyrolysis to reconstruct basin charging history for oils. Organic Geochemistry 35, 1441–1459. Sabbah, H., Pomerantz, A.E., Wagner, M., Müllen, K., Zare, R.N., 2012. Laser desorption single-photon ionization of asphaltenes: Mass range, compound sensitivity, and matrix effects. Energy & Fuels 26, 3521-3526. Schouten, S., Pavlović, D., Sinninghe Damsté, J.S., de Leeuw, J.W., 1993. Nickel boride:

- 42 -

An improved desulphurizing agent for sulphur-rich geomacromolecules in polar and asphaltene fractions. Organic Geochemistry 20, 901–909. Schouten, S., Sinninghe Damsté, J.S., Baas, M., Kock-Van Dalen, A.C., Kohnen, M.E.L., de Leeuw, J.W., 1995. Quantitative assessment of mono- and polysulphide-linked carbon skeletons in sulphur-rich macromolecular aggregates present in bitumens and oils. Organic Geochemistry 23, 765–775. Semple, K.M., Cyr, N., Fedorak, P.M., Westlake, D.W.S., 1990. Characterization of asphaltenes from Cold Lake heavy oil: Variations in chemical structure and composition with molecular size. Canadian Journal of Chemistry 68, 1092–1099. Silva, T.F., Azevedo, D.A., Rangel, M.D., Fontes, R.A., Aquino Neto, F.R., 2008. Effect of biodegradation on biomarkers released from asphaltenes. Organic Geochemistry 39, 1249–1257. Snape, C.E., Bolton, C., Dosch, R.G., Stephens, H.P., 1989. High liquid yields from bituminous coal via hydropyrolysis with dispersed catalysts. Energy & Fuels 3, 421–425. Speight, J.G., Moschopedis, S.E., 1982. On the molecular nature of petroleum asphaltenes. In: Bunger, J.W., Li, N.C. (Eds.), Chemistry of Asphaltenes. American Chemical Society, Washington, DC, pp. 1–15. Strausz, O.P., Mojelsky, T.W., Faraji, F., Lown, E.M., Peng, P., 1999a. Additional structural details on Athabasca asphaltene and their ramifications. Energy & Fuels 13, 207–227. Strausz, O.P., Mojelsky, T.W., Lown, E.M., Kowalewski, I., Behar, F., 1999b. Structural features of Boscan and Duri asphaltenes. Energy & Fuels 13, 228–247.

- 43 -

Strausz, O.P., Peng, P.A., Murgich, J., 2002. About the colloidal nature of asphaltenes and the MW of covalent monomeric units. Energy & Fuels 16, 809–822. Strausz, O.P., Torres, M., Lown, E.M., Safarik, I., Murgich, J., 2006. Equipartitioning of precipitant solubles between the solution phase and precipitated asphaltene in the precipitation of asphaltene. Energy & Fuels 20, 2013–2021. Thanh, N.X., Hsieh, M., Philp, R.P., 1999. Waxes and asphaltenes in crude oils. Organic Geochemistry 30, 119–132. Tian, Y., Yang, C., Liao, Z., Zhang, H., 2012a. Geochemical quantification of mixed marine oils from Tazhong area of Tarim Basin, NW China. Journal of Petroleum Science and Engineering 90–91, 96–106. Tian, Y., Zhao, J., Yang, C., Liao, Z., Zhang, L., Zhang, H., 2012b. Multiple-sourced features of marine oils in the Tarim Basin, NW China – Geochemical evidence from occluded hydrocarbons inside asphaltenes. Journal of Asian Earth Sciences 54–55, 174–181. Wilhelms, A., Larter, S.R., Schulten, H.R., 1993. Characterization of asphaltenes by pyrolysis-field ionization mass spectrometry – some observations. Organic Geochemistry 20, 1049–1062. Wu, L., Liao, Y., Fang, Y., Geng, A., 2012. The study on the source of the oil seeps and bitumens in the Tianjingshan structure of the northern Longmen Mountain structure of Sichuan Basin, China. Marine and Petroleum Geology 37, 147–161. Yang, C., Liao, Z., Zhang, L., Creux, P., 2009. Some biogenic-related compounds occluded inside asphaltene aggregates. Energy & Fuels 23, 820–827. Yarranton, H.W., Ortiz, D.P., Barrera, D.M., Baydak, E.N., Barré, L., Frot, D., Eyssautier,

- 44 -

J., Zeng, H., Xu, Z., Dechaine, G., Becerra, M., Shaw, J.M., McKenna, A.M., Mapolelo, M.M., Bohne, C., Yang, Z., Oake, J., 2013. On the size distribution of self-associated asphaltenes. Energy & Fuels 27, 5083–5106. Yen, T.F., Erdman, J.G., Pollack, S.S., 1961. Investigation of the structure of petroleum asphaltenes by X-Ray diffraction. Analytical Chemistry 33, 1587–1594. Zhang, L.Y., Xu, Z., Masliyah, J.H., 2003. Langmuir and Langmuir-Blodgett films of mixed asphaltene and a demulsifier. Langmuir 19, 9730–9741. Zhao, J., Liao, Z., Zhang, L., Creux, P., Yang, C., Chrostowska, A., Zhang, H., Graciaa, A., 2010. Comparative studies on compounds occluded inside asphaltenes hierarchically released by increasing amounts of H2O2/CH3COOH. Applied Geochemistry 25, 1330–1338. Zhao, J., Liao, Z., Chrostowska, A., Liu, Q., Zhang, L., Graciaa, A., Creux, P., 2012. Experimental studies on the adsorption/occlusion phenomena inside the macromolecular structures of asphaltenes. Energy & Fuels 26, 1746–1755.

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Figure Captions

Fig. 1. 2D cartoon of covalently bound moieties comprising an asphaltene monomer (modified after Murgich et al., 1999), showing potential locations for adsorbed and occluded molecules that are not covalently bound but rather adsorbed onto molecular surfaces or physically trapped (inside 3D cage structures). This drawing is highly schematic showing one hopanoid, one steroid, one porphyrin and one large pericondensed aromatic moiety along with several other alkyl and cyclic structures along with two potential occlusion locations large enough to accommodate a steroid or terpenoid biomarker. The molecular weight is > 5000 Da, the atomic sulfur/nitrogen ratio is about 2 and the saturate/aromatic carbon ratio is about 1.

Fig. 2. Tetracosyldodecyl-benzo[ghi]perylene (C58H84; 780.7 Da, 6 pericondensed rings). The aromatic carbons comprise ~38% of the total carbons. The n-C24 and n-C12 side chains reflect typical flash pyrolysis results shown in Fig. 3, but the RICO degradation results shown in Fig. 4 suggest that about 10% of the n-alkyl side chains should be bound to the aromatic core at two places in order to yield di-acid products after oxidation. It would appear the occlusion of a pentacyclic, saturated hydrocarbon such as hopane (MW 412 Da) within an asphaltene monomer of the “island” type with a molecular weight in the range of 500–1000 Da would not be physically very likely or even possible.

Fig. 3. Mass chromatogram of the flash pyrolysis (610 °C) of crude oil asphaltene from well TK772 (Tahe oilfield, Tarim Basin, China) showing straight chain alkene and alkane

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peaks; (a) sum of m/z 55+57 (b) sum of m/z 83+85. Carbon numbers are annotated and n-alkenes are partially resolved from the later eluting n-alkanes up to n-C30. The alkene/alkane ratios are significantly lower in the m/z 55+57 mass chromatogram.

Fig. 4. Mass chromatogram of the saturate fraction recovered from hydropyrolysis (HyPy) of the crude oil asphaltenes from well TK772. No n-alkenes are apparent in the m/z 83 trace, but significant amounts of alkylcyclohexanes are present (marked with arrows). The other significant peaks in the m/z 83 trace are minor ions from the n-alkanes shown in the m/z 85 trace.

Fig. 5. Fatty acid methyl esters (FAME) of (a) acids and (b) α,ω-di-acids (DFAME) from the RICO (ruthenium ion catalyzed oxidation) decomposition of asphaltene from the Tahe oilfield crude oil AD26, China. The relative response for the FAME and DFAME compounds suggests that about 10% of the alkyl moieties in the precursor asphaltenes were bonded to aromatic nuclei at both ends. Double ended bonding is not consistent with an “island” structure for the asphaltene, but rather were more likely derived from asphaltene with an “archipelago” structure.

Fig. 6. Partial m/z 217 ion chromatograms of the saturate fraction of a crude oil and the mild oxidation product of asphaltene from the same oil. The distribution of the C29 sterane isomers indicates that the asphaltene fraction contains occluded sterane molecules that have a lower thermal maturity than the free sterane molecules within the bulk oil itself.

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Highlights: The organic geochemistry of asphaltenes and occluded biomarkers Asphaltenes incorporate chemical structures useful for biomarker correlation Asphaltenes host occluded hydrocarbons that represent the original source character Asphaltene moieties and occlusions may be obtained independently and unambiguously Analytical approaches include oxidation, reduction and pyrolysis Analytical methods may constrain the elucidation of asphaltene properties

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covalently bound alkyl and alicyclic moieties

=

O

O

S S

adsorbed hydrocarbon N

pericondensed aromatic centers

occluded hydrocarbon

N S

S

S

O

S

S

N

S S

S S

O

S

=

S

OH

O S

occluded hydrocarbon

O= COH

NH

N

N

HN

S =

O O

S

adsorbed hydrocarbon

Fig. 1. 2D cartoon of covalently bound moieties comprising an asphaltene monomer (modified after Murgich et al., 1999), showing potential locations for adsorbed and occluded molecules that are not covalently bound but rather adsorbed onto molecular surfaces or physically trapped (inside 3D cage structures). This drawing is highly schematic showing one hopanoid, one steroid, one porphyrin and one large pericondensed aromatic moiety along with several other alkyl and cyclic structures along with two potential occlusion locations large enough to accommodate a steroid or terpenoid biomarker. The molecular weight is > 5000 Da, the atomic sulfur/nitrogen ratio is about 2 and the saturate/aromatic carbon ratio is about 1.

Fig. 2. Tetracosyldodecyl-benzo[ghi]perylene (C58H84; 780.7 Da, 6 pericondensed rings). The aromatic carbons to comprise ~38% of the total carbons. The n-C24 and n-C12 side chains reflect the flash pyrolysis results shown in Fig. 3 but the RICO degradation results shown in Fig. 4 suggest that about 10% of the nalkyl side chains should be bound to the aromatic core at two places in order to yield di-acid products. It would appear the occlusion of a pentacyclic, saturated hydrocarbon such as hopane (MW 412 Da) within an asphaltene monomer of the “island” type and this mass would not be physically very likely or even possible.

a o

TK772 asphaltene 610 C flash pyrolysis

11

1200000

m/z 55+57 1000000

Abundance

800000

15 600000

400000

20

200000

25

0 Minutes-->

10

20

30

40

50

60

70

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90

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Fig. 3. Mass chromatogram of the flash pyrolysis (610 °C) of crude oil asphaltene from well TK772 (Tahe field, Tarim Basin, China) showing straight chain alkene and alkane peaks; (a) sum of m/z 55+57 (b) sum of m/z 83+85. Carbon numbers are annotated and n-alkenes are partially resolved from the later eluting n-alkanes up to n-C30. The alkene/alkane ratios are significantly lower in the m/z 55+57 mass chromatogram.

b o

TK772 asphaltene 610 C flash pyrolysis

1200000

m/z 83+85 1000000

800000 Abundance

11

600000

15

400000

20

200000

25

0 Minutes-->

10

20

30

40

50

60

70

80

90

100

Fig. 3. Mass chromatogram of the flash pyrolysis (610 °C) of crude oil asphaltene from well TK772 (Tahe field, Tarim Basin, China) showing straight chain alkene and alkane peaks; (a) sum of m/z 55+57 (b) sum of m/z 83+85. Carbon numbers are annotated and n-alkenes are partially resolved from the later eluting n-alkanes up to n-C30. The alkene/alkane ratios are significantly lower in the m/z 55+57 mass chromatogram.

Abundance 100000

TK772 asphaltene HyPy saturates

90000 80000

m/z 83

70000 60000 50000 40000 30000 20000 10000 Minutes-->

10

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80

90

70

80

90

350000

m/z 85

300000 250000 200000 150000 100000 50000

Minutes-->

10

20

30

40

50

60

Fig. 4. Mass chromatogram of the saturate fraction recovered from hydropyrolysis (HyPy) of the crude oil asphaltenes from well TK772. No n-alkenes are apparent in the m/z 83 trace, but significant amounts of alkylcyclohexanes are present (marked with arrows). The other significant peaks in the m/z 83 trace are minor ions from the n-alkanes shown in the m/z 85 trace.

350000

(a) FAME (m/z 74) 300000

C12:0

250000

C16:0

200000

C20:0

150000

100000

50000

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35000

20

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(b) DFAME (m/z 98)

50 min

C14:0

30000 25000 20000 15000 10000 5000 0 10

20

30

40

50 min

Fig. 5. Fatty acid methyl esters (FAME) of (a) acids and (b) α,ω-di-acids (DFAME) from the RICO (ruthenium ion catalyzed oxidation) decomposition of asphaltene from the Tahe Basin crude oil AD26, China. The relative response for the FAME and DFAME compounds suggests that about 10% of the alkyl moieties in the precursor asphaltenes were bonded to aromatic nuclei at both ends. Double ended bonding is not consistent with an “island” structure for the asphaltene, but rather were more likely derived from asphaltene with an “archipelago” structure.

AD26 oil saturates

20R

AD26 saturates from mild oxidation of asphaltenes 20S

20S 20R

Fig. 6. Partial m/z 217 ion chromatograms of the saturate fraction of a crude oil and the mild oxidation product of asphaltene from the same oil. The thermal maturity pattern for the distribution of the C29 steranes indicates that the asphaltene fraction contains occluded sterane molecules that have a lower thermal maturity than the free sterane molecules within the oil itself.