Ben G.K van Aarssen

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.

Higher plant biomarkers reflect palaeovegetation changes during Jurassic times

Abstract The relative abundances of three higher-plant-derived biomarkers, retene, cadalene and ip-iHMN, have been measured in marine sedimentary rocks from the northwest margin of Australia. It is thought that each biomarker represents input from a different plant type. The distributions of these three compounds form a fingerprint, representing higher plant input (HPF). Variations in HPF in Oxfordian sediments were nearly identical in all three locations, with retene becoming very abundant relative to the other two compounds with decreasing age of the sediment. This finding strongly suggests that the composition of terrestrial input during deposition largely determines HPF and that the possible effects of diagenesis and catagenesis on the distribution of the three biomarkers are relatively unimportant. The marked increase in the abundance of retene relative to that of cadalene during the Oxfordian is interpreted to reflect an increase in the contribution of plants that produced precursors for retene, i.e., conifers, brought about by a significant change in climate. This was exemplified by measuring the distributions of retene and cadalene, expressed in the higher plant parameter (HPP) for a suite of sediments from the Carnarvon Basin, Western Australia, covering the complete Jurassic period. The HPP profile displays three major cycles, each covering a period of at least 10 million years. This profile not only compared well with published palaeoclimate data, but also showed a remarkable similarity with second order cycles in the global sea level curve, thus strongly supporting the proposal that variations in HPF and HPP are indications of changes in palaeoclimate. The relation with global sea level further suggests that global factors, e.g., the atmospheric carbon dioxide concentration, may play a major role in determining the observed variations in the distributions of these higher-plant-derived biomarkers.

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.

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.

Relative efficiency of free fatty acid butyl esterification: choice of catalyst and derivatisation procedure

The conversion efficiency of alkanoic, alkenoic, branched, alicyclic, aromatic, keto-substituted, and dioic carboxylic acids to their corresponding butyl esters was compared under different reaction conditions (time, temperature, catalyst). We show that boron trifluoride is generally a more efficient catalyst than sulphuric acid. However, optimum derivatisation conditions vary strongly for different acids and no single derivatisation protocol can be employed without certain losses. Therefore, care must be taken when the simultaneous quantitative analysis of different types of carboxylic acids in one sample is envisaged. Addition of water-scavenging reagents to the reaction mixture caused the formation of artefacts and selectively decreased reaction yields.

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.

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.