A novel biodegradation parameter derived from bicyclic sesquiterpanes for assessing moderate levels of petroleum biodegradation

doi:10.1016/j.orggeochem.2020.104049

Organic Geochemistry

Volume 147, September 2020, 104049

https://doi.org/10.1016/j.orggeochem.2020.104049 Get rights and content

Highlights

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A C₁₅ bicyclane specific to southeastern Mexico oils is biodegradation resistant.
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A new C₁₅ bicyclane biodegradation parameter is proposed to indicate PM 4–5.
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Biodegradation ratios based on hydrocarbons and non-hydrocarbons correlate well.
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Non-hydrocarbon compounds are valuable for assessing heavy biodegradation.

Abstract

Gas chromatography–mass spectrometry (GC–MS) and Fourier transform ion cyclotron resonance–mass spectrometry (FTICR–MS) analyses were performed on 39 heavy oils from an onshore basin in southeastern Mexico to investigate the influence of biodegradation on whole oil compositions. Among the biodegradation resistant C₁₄–C₁₆ bicyclic sesquiterpanes (bicyclanes), a rearranged C₁₅ isomer (compound X) becomes relatively enriched with increasing biodegradation levels, contrasting with a reduction in the summed C₁₄–C₁₆ bicyclanes concentration but consistent with an enrichment of the bicyclic naphthenic acids revealed by FTICR–MS data. The concentration ratio of compound X over all the identified C₁₅ bicyclane isomers (DMHD ratio) is proposed as a new biodegradation parameter. This ratio correlates well with the biodegradation ratios of 3-methylchrysene/methylchrysenes, benzonaphtho[1,2-d]thiophene/benzonaphtho[2,1-d]thiophene and those based on NSO compounds. Compound X has an additional ethyl group but one less methyl group compared to the drimane structure. A lessening of steric effects through demethylation can explain the refractory nature of compound X, and this forms the basis on which possible origins of compound X are discussed. Extensive biodegradation of the studied oil samples is implied by the alteration of polyaromatic hydrocarbons and NSO compounds. This suggests the relative enrichment of compound X can be used as a new indicator for heavy oil biodegradation at Peters and Moldowan biodegradation level (PM level) 4–5, when hopanes and steranes are still intact. Changes in the distribution patterns of some NSO compound species also suggest PM level 4–5.

Introduction

Bicyclic sesquiterpanes (bicyclanes) form one of the terpenoid classes with a decahydronaphthalene skeleton substituted by alkyl side chains (C₁, C₂ and C₃) (Stout et al., 2005). The C₁₄–C₁₆ homologues with either a drimane type or a eudesmane type skeleton are ubiquitous in crude oil and source rocks regardless of age or organic sources (e.g., Stout et al., 2005). C₁₅ and C₁₆ drimanes with the 8α(H) biological configuration are unstable, and they are transformed to the more stable 8β(H) configuration before the oil window is reached (Noble et al., 1987). Two types of rearranged drimanes were identified and their stereochemical structures deciphered by Bendoraitis, 1974, Alexander et al., 1983, Alexander et al., 1984). They were widely acknowledged to be maturity sensitive (Luo et al., 1991, Gordadze et al., 2011, Ji et al., 2016) and also related to clay-rich environments (e.g., Zhang et al., 2004, Cesar and Grice, 2018). Two additional types of rearranged sesquiterpanes were identified and proposed by Nytoft et al. (2009) as terrigenous organic input indicators. Bicyclanes can be generated from a wide range of biological precursors from higher plants to algae and bacteria (Alexander et al., 1983, Wang et al., 1990, Wang et al., 2005, Fan et al., 1991). In addition, they can be the biodegradation products of tricyclic terpanes and hopanes (Alexander et al., 1983, Alexander et al., 1984). The significance of this compound class as an organic source indicator widely justifies the applications of relative proportions of the C₁₄–C₁₆ bicyclanes in oil-source correlation and maturity estimation (Al-Arouri et al., 1998, Wang et al., 2005).

Biodegradation of sesquiterpanes has been mostly investigated in oil environmental weathering studies (Stout et al., 2005), but rarely in studies about oil biodegradation under reservoir conditions. Williams et al. (1986) reported higher biodegradation resistance of C₁₅ 8β(H)-drimane than C₁₆ 8β(H)-homodrimane in Texas oils, while Yang et al. (2011) observed the opposite trend in Alberta oil sands, where C₁₅ 8β(H)-drimane was degraded faster than C₁₆ 8β(H)-homodrimane but both were more resistant to biodegradation than their rearranged counterparts. Similarly, Weston et al. (1989) determined higher biodegradation susceptibilities of the rearranged sesquiterpanes identified by Bendoraitis (1974) (Fig. 2, peaks 2 and 3) than those of the isomers identified by Nytoft et al (2009). Generally, C₁₄–C₁₆ sesquiterpanes can survive even when n-alkanes and isoprenoids in the saturated hydrocarbon fraction or C₀–C₂ naphthalenes in the aromatic hydrocarbon fraction are greatly biodegraded (Volkman et al., 1984, Williams et al., 1986, Stout et al., 2005). However, our study found the most variable behavior in a rarely investigated C₁₅ sesquiterpane isomer under the influence of biodegradation. This isomer has an ethyl group similar to the C₁₅ desmethylhomodrimane first reported by Weston et al. (1989).

Since no practical biodegradation parameter based on the bicyclanes has been derived, our study investigated bicyclane distributions in a sequentially biodegraded sample suite and correlated the X/C₁₅ bicyclanes concentration ratio with other robust biodegradation indicators derived from aromatic hydrocarbons and NSO compounds. The purpose of our study is to elucidate biodegradation behavior of bicyclanes and deliver a practical tool for assessing moderately high biodegradation levels.

Section snippets

Geological background and sample information

Thirty-nine heavy oil samples were collected from an onshore oil field in southeastern Mexico from comingled producing reservoirs ranging in depth from 655 m to 1585 m (Table 1). The samples are typical heavy–extra heavy oils with an API gravity range of 7.4–18.7° and a broad dead oil viscosity range of 141–3,362,040 centipoise at 20 °C. Sulfur contents of these oils are 2.6–4.1 wt%. (Table 1). The oils are sourced from the same organic-rich Tithonian marly-argillaceous mudstones dominated by

Methods

The heavy oil samples were first passed through activated sodium sulfate columns to remove water and then an aliquot of each sample was filtrated through a pre-cleaned Florisil solid phase extraction (SPE) column to remove the asphaltene fraction. The saturated and aromatic hydrocarbon fractions were subsequently separated via a column chromatography method developed by Bastow et al. (2007). Geochemical fingerprinting of both fractions was obtained using an Agilent 6890 N gas chromatograph

Saturated hydrocarbons

n-Alkanes and isoprenoids exhibit changes from complete destruction (sample S9, Fig. 1A) to seemingly intact distributions (sample S23, Fig. 1A) with the summed n-alkane concentration ranging from 0 to 21.4  mg/g oil (Table 1). This reflects distinct degrees of biodegradation.

Terpanes and steranes show no apparent distinction in distribution pattern, but notable changes in total concentration with different biodegradation levels (Fig. 1B–C, Table 1). Higher contents of hopanes than tricyclic

Source and maturity influences on the hydrocarbon compounds

The elevated C₂₄TeT abundance relative to C₂₆TT might be related to carbonate–evaporite environments or abundant terrigenous organic matter (Wang et al., 2010). This coincides with a higher relative amount of C₂₉ αβ-hopane than C₃₀ αβ-hopane (Table 2), which is characteristic of organic matter derived from a carbonate–evaporite marine environment (Connan et al., 1986). Generally low St/H ratio values of 0.27–0.36 and the dominance of the C₂₉ homologues in the C₂₇–C₂₉ regular steranes

Conclusions

A previously unknown C₁₅ demethylated bicyclic sesquiterpane isomer (compound X) has been detected in SE Mexico onshore heavy oil samples and confirmed by a molecular mass of m/z 208 and a characteristic fragment mass of m/z 179, similar to the speculated C₁₅ desmethylhomodrimane in the literature. This compound shows a higher biodegradation resistance than the other C₁₅ isomers investigated. Relieved 1,3-diaxial tension resulted from removal of a methyl group from a quaternary carbon of the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors extend their gratitude to Petróleos Mexicanos (PEMEX), Pemex Exploración y Producción PEP and Instituto Mexicano del Petróleo (IMP) for offering the heavy oil samples and the bulk data of these oils, to the PRG lab and the Tesla Petroleomics Centre of University of Calgary for their contributions of the GC–MS and the FTICR–MS data. Dr. Lloyd Snowdon is acknowledged for helpful discussions about this work. Mike Finn is thanked for improving the readability of this manuscript.