Trevor P. Bastow

Distributions of methylated naphthalenes in crude oils: indicators of maturity, biodegradation and mixing

Abstract Variations in the distributions of trimethylnaphthalenes (TMNs), tetramethylnaphthalenes (TeMNs) and pentamethylnaphthalenes (PMNs) were studied in a set of crude oils, which differ mainly in their level of maturity. Comparison with mixtures obtained after heating methylated naphthalenes with clay led to the proposal of three new parameters, one for each class of naphthalenes. Each parameter is based on the same principle, which is the increase with maturity of stable isomers relative to less stable ones. It appears that the relationships between these three parameters are linear for those oils where thermal stress alone has determined the distributions. These relations can be conveniently displayed in a ternary diagram. It is proposed that when an oil plots in the restricted area in the diagram that is defined by these linear relations, the so-called maturity centre, the value of any of the three parameters is an accurate indication of its thermal maturity. When an oil plots outside the maturity centre, factors like in-reservoir mixing, biodegradation or mixing with indigenous organic matter have distorted the distributions of methylated naphthalenes. It is thought that the distributions of methylated naphthalenes reflect the extent to which 1,2-methylshifts and methyl transfer reactions, mediated by clay catalysis, have taken place. It is proposed that the methyl groups involved in these reactions form a ‘methyl pool’ to which most compounds present will have access. Increasing temperature and availability of suitable catalysts can remove methyl groups from this pool through formation of very stable compounds such as methane, leading to a decrease in the degree of methylation of aromatic hydrocarbons.

Geosynthesis of organic compounds: II. Methylation of phenanthrene and alkylphenanthrenes

Abstract A crude oil and several shales which contain anomalously high concentrations of phenanthrene, 1-methylphenanthrene, 1,7-dimethylphenathrene, and retene have been shown to contain relatively high concentrations of their corresponding 9-methyl counterparts. In laboratory experiments carried out under mild conditions, each of the phenanthrenes has been shown to be substituted preferentially at the 9-position when a methyl donor was heated with the substrate in the presence of a clay catalyst. These observations have been interpreted as evidence for a sedimentary methylation process.

2-Methylretene in sedimentary material: a new higher plant biomarker

Abstract 2-Methylretene has been synthesised, characterised and identified in sedimentary material from a range of locations, source types and ages. 2-Methylretene was observed only in samples of Permian to Tertiary age and can be associated with specific higher plant precursors that also yield retene. Laboratory dehydrogenation of simonellite yielded 2-methylretene as the major product. Based on this we suggest that 2-methylretene forms from the aromatisation of diterpenoid type natural products with the abietane and phyllocladane skeletons, similar to those that form simonellite and suggest it can be used as a biomarker for higher plant input.

Biodegradation of aromatic land-plant biomarkers in some Australian crude oils

Abstract Land-plant-derived aromatic hydrocarbons with a range of susceptibilities to reservoir biodegradation have been used to assess the accumulation history of crude oils from two Australian sedimentary basins. The compounds used in this study (retene, 9-methylretene, 6-isopropyl-2-methyl-1-(4-methylpentyl)naphthalene and 6-isopropyl-2,4-dimethyl-1-(4-methylpentyl)naphthalene) are thought to originate from land-plants and are the result of reactions of their natural produce precursors, involving aromatisation, rearrangement and methylation in the sediments. They are therefore suggested as markers for land-plants in severely biodegraded oils in which other biologically derived compounds cannot be recognised. The order of biodegradability of the methylated compounds was assessed relative to their non-methylated counterparts, namely 6-isopropyl-2-methyl-1-(4-methylpentyl)naphthalene and retene. In both cases the methylated homologue is less susceptible to biodegradation. These compounds were used to assess the accumulation history of a crude oil that was previously reported to contain a mixture of a severely biodegraded and a non-biodegraded crude oil.

The identification of crocetane in Australian crude oils

Abstract The C 20 irregular isoprenoid crocetane (2,6,11,15-tetramethylhexadecane) has been unambiguously identified in a series of Western Australian crude oils from the Canning (Ordovician/Devonian) and Perth (Lower Triassic) Basins. PMI (2,6,10,15,19-pentamethylicosane) was found to co-occur with crocetane in the same samples. In addition, a newly observed series of C 22 –C 24 irregular isoprenoids possessing the same carbon skeleton have also been tentatively identified in each of these crude oils on the basis of GC retention times and mass spectral characteristics. This may indicate a source for crocetane, in the Canning and Perth Basin crude oils, from the diagenesis of PMI. The stable carbon isotopic composition for combined crocetane/phytane in these samples suggests crocetane is not unusually depleted in 13 C compared with all previous reports. This is the first reported occurrence of biomarkers attributable to methanogenic archaea in crude oils.

Pentamethylnaphthalenes and related compounds in sedimentary organic matter

Abstract The isomeric pentamethylnaphthalenes (PMNs) 1,2,3,5,6-PMN, 1,2,3,5,7-PMN, 1,2,3,6,7-PMN and 1,2,4,6,7-PMN have been synthesised, characterised and identified in a number of crude oils and sediments, ranging in age from Proterozoic to Tertiary. We suggest that 1,2,3,5,6-PMN forms predominantly from the aromatisation of drimanoid precursors via 1,2,2,5,6-pentamethyltetralin. When 1,2,3,5,6-PMN was heated in laboratory experiments, the proportions of PMNs changed in a manner which reflected the relative stability of the isomers. The other PMNs in sediments are therefore suggested to arise via acid catalysed isomerisation or transalkylation processes. A conveniently measured pentamethylnaphthalene ratio PNR=1,2,4,6,7-PMN/(1,2,3,5,6-PMN+1,2,4,6,7 PMN) has been defined as a maturity parameter. In an Indonesian sedimentary sequence, the values of PNR changed smoothly with increasing depth with values ranging from 0.1 to 0.4. Values in crude oils range from 0.15 to 0.85, suggesting a value of approximately 0.15 for the beginning of the oil window.

Geosynthesis of organic compounds. Part V — methylation of alkylnaphthalenes

Abstract Several crude oils and rock extracts with high concentrations of 1,6-dimethylnaphthalene, 1,2,5-trimethylnaphthalene, 1,2,3,5-tetramethylnaphthalene, 1,2,3,5,6-pentamethylnaphthalene and 6-isopropyl-2-methyl-1-(4-methylpentyl)naphthalene are also shown to contain enhanced distributions of their corresponding methylated counterparts. Of these methylated counterparts, 1,2,3,5,6,7-hexamethylnaphthalene has been synthesised and is identified in sedimentary material for the first time. In laboratory experiments carried out under mild conditions with a methyl donor in the presence of a clay catalyst, each of the parent alkylnaphthalenes was shown to be substituted in preferential positions. The distributions of these products from laboratory methylation experiments are similar to the distributions found in sedimentary material. These observations have been interpreted as evidence for a sedimentary electrophilic aromatic methylation process.

Small-scale and rapid quantitative analysis of phenols and carbazoles in sedimentary matter

Abstract A rapid small-scale method for the quantitative analysis of alkylphenols, alkylcarbazoles and benzocarbazoles from sedimentary matter is described using silica gel liquid chromatography and GC–MS techniques. Alkylphenol, alkylcarbazole and benzocarbazole components of crude oils can be easily, rapidly and economically separated from saturated and aromatic hydrocarbons using silica gel, as a stationary phase, and disposable glassware such as Pasteur pipettes (sample sizes up to 100 mg). Analysis of these components was performed using GC–MS without any further derivatisation. This method affords rapid sample processing with accurate quantification of carbazoles, benzocarbazoles and alkylphenols suitable for routine use in petroleum geochemistry.

Identification of 1,2,2,5-tetramethyltetralin and 1,2,2,5,6-pentamethyltetralin as racemates in petroleum

1,2,2,5-Tetramethyltetralin and 1,2,2,5,6-pentamethyltetralin have been synthesized and identified as recemates in crude oil and are proposed from dehydrogenation experiments as intermediates in the formation of alkylnaphthalenes in crude oil.

The effect of oxidation on the distribution of alkylphenols in crude oils

Alkylphenols in crude oils can be affected by oxidation during storage. A crude oil and an alkylphenol mixture were exposed to light and air for various time periods, resulting in significant alterations of the alkylphenol distributions. Most affected were ortho/para substituted isomers, such as 2,4-/2,6-dimethylphenol and 2,4,6-trimethylphenol, whereas phenol, and meta substituted isomers such as m-cresol and 3,5-dimethylphenol were least affected. The alkylphenol distributions in two crude oils from the same accumulation, one of which was stored for 34 years, were found to be different in a manner consistent with a scenario where the stored oil had been affected by oxidation. These results suggest that the alkylphenol distributions in crude oils can be severely altered by oxidation which is enhanced by exposure to light. Therefore, if precautions are not taken to minimise the effects of oxidation during sampling and storage, alkylphenol distributions may be altered. Significant oxidation can be avoided by storing samples in opaque containers such as metal tins and drums and reducing or flushing air from the containers headspace.