Robert I. Kagi

Demethylated hopanes in crude oils and their applications in petroleum geochemistry

Analyses of some Australian crude oils show that many contain varying concentrations of A/ B-ring demethylated hopanes. These range from C26 to C34 and have been identified from their retention times and mass spectral data as 17α(H)-25-norhopanes. Comparison of hopane and demethylated hopane concentrations and distributions in source-related, biodegraded oils suggests that demethylated hopanes are biotransformation products of the hopanes. Further, it appears that the process occurs at a late stage of biodegradation, after partial degradation of steranes has occurred. Demethylated hopanes are proposed as biomarkers for this stage of severe biodegradation. The presence of these compounds in apparently undegraded crude oils is thought to be due to the presence of biodegraded crude oil residues which have been dissolved by the undegraded crude oil during accumulation in the reservoir sands. The timing of hopane demethylation, relative to the degradation of other compounds, has been assessed and the progressive changes in crude oil composition with increasing extent of biodegradation have been identified. The use of demethylated hopanes as maturity parameters for severely biodegraded crude oils, and the applicability of established biomarker maturity parameters to such oils, are also discussed.

Biodegradation of aromatic hydrocarbons in crude oils from the Barrow Sub-basin of Western Australia

Abstract The compositions of aromatic hydrocarbon fractions isolated from four oils from the Barrow Sub-basin of Western Australia have been examined using capillary gas chromatography and gas chromatography-mass spectrometry. Each of the oils is derived from the same source rock formation, and the oils are of comparable maturity, but they contain very different distributions of aromatic hydrocarbons due to the effects of biodegradation and water washing. The susceptibility to biodegradation of the different aromatic hydrocarbon classes has been assessed and, in general, the rate of biodegradation is found to decrease with increasing number of aromatic rings and with increasing number of alkyl substituents. The rate also depends on the positions of the alkyl substituents, and it is very much lower if the aromatic hydrocarbon contains adjacent methyl substituents. Positional isomers of dimethylnaptthalenes are biodegraded at very different rates, with isomers having β-methyl substituents most susceptible to biodegradation. Ethylnaphthalenes are much less rapidly biodegraded than dimethylnaphthalenes. Trace amounts of mono- and triaromatic steroidal hydrocarbons were detected in all of the oils indicating that these compounds are ver resistant to biodegradation and thus can be used as maturity parameters even in severely biodegraded oils. There is evidence to suggest that C 21 and C 22 methyl-substituted triaromatic steroidal hydrocarbons have been removed from the severely biodegraded Mardie oil. A scale is proposed for assessing the extent to which an oil has been biodegraded based on the relative abundance of selected aromatic and saturated hydrocarbons.

The effects of thermal maturity on distributions of dimethylnaphthalenes and trimethylnaphthalenes in some Ancient sediments and petroleums

Abstract The distributions of di- and trimethylnaphthalenes in two sedimentary sequences from Western Australia have been examined by capillary gas chromatography and combined gas chromatography-mass spectrometry. A general decrease was observed in the relative proportions of αα-dimethylnaphthalenes and ααβ-trimethylnaphthalenes with increasing thermal maturity. Similar trends were also observed for six crude oils which have very different ratios of ethylcholestane epimers indicating very different thermal histories. These results suggest that changes in the relative abundances of certain methyl substituted naphthalenes may be useful indicators of thermal maturity of sedimentary organic matter, and the use of a number of isomer ratios is illustrated.

Tetracyclic diterpenoid hydrocarbons in some Australian coals, sediments and crude oils

Abstract Tetracyclic diterpenoid hydrocarbons (diterpanes) based on the ent -beyerane, phyllocladane and ent -kaurane skeletons have been identified in the hydrocarbon extracts of some Australian coals, sediments and crude oils. Structures were assigned to the geological diterpanes by comparison with synthetically prepared reference compounds. Studies of a sample suite consisting of low-rank coals and sediments indicate that the ratios of C-16 epimers of phyllocladane and ent -kaurane are maturity dependent, and that the relative proportion of the thermodynamically preferred 16β (H)-compounds increases with increasing thermal maturity. Thermodynamic equilibrium for the interconversion reactions is attained in sediments before the onset of crude oil generation. The most likely natural product precursors for the tetracyclic diterpanes are considered to be the tetracyclic diterpene hydrocarbons which occur widely in the leaf resins of conifers. Tetracyclic diterpanes have been identified in sediments and coals of Permian age or younger, suggesting that these compounds are markers for both modern and extinct families of conifers. In particular, phyllocladane is proposed as a marker for the Podocarpaceae family of conifers.

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.

Trimethylnaphthalenes in crude oils and sediments: Effects of source and maturity

Abstract Three sedimentary sequences and a number of crude oils have been analysed for trimethylnaphthalenes (TMNs) using gas chromatography. The sediments were found to contain enhanced relative abundances of 1,2,5-TMN and 1,2,7-TMN in samples of Cretaceous age and younger; this was especially pronounced in lower maturity sediments containing type III organic matter. Older samples containing type III organic matter were found to contain enhanced relative abundances of 1,2,5-TMN and lower relative abundances of 1,2,7-TMN. 1,2,7-TMN appears to be derived directly from the structural degradation of oleanane-type triterpenoids present in angiosperms, and as such is proposed as a marker for this class of plant and indirectly, therefore, also for samples deposited since the Early Cretaceous. The crude oils have been classified according to their trimethylnaphthalene compositions. This classification enables crudes of higher plant origin to be identified and to be further classified according to whether the organic matter pre-dated or post-dated the evolution of angiosperms.

Origin of perylene in ancient sediments and its geological significance

The distributions of polycyclic aromatic hydrocarbons (PAHs) in sediments of three Upper Triassic to Middle Jurassic sedimentary sequences from the Northern Carnarvon Basin, Australia have been investigated. Perylene was found to be a major PAH component in the top Lower to base Middle Jurassic sediments that are immature or at low maturity. Its depth/age profiles are not related to the combustion-derived PAHs that have been believed to be produced during ancient vegetation fires before deposition. This suggests a diagenetic origin for perylene. The concentration of perylene in the sediments is proportional to the amount of terrestrial input, decreasing with distance from the source of land sediments. Its carbon isotope composition is slightly heavier than higher-plant derived PAHs, but still in the range of the terrestrially sourced PAHs including higher-plant PAHs and combustion-derived PAHs as suggested previously. Fungi are proposed to be the major precursor carriers for perylene in sediments based on the facts that (1) perylenequinone structures have been previously suggested to be the natural precursors for perylene; (2) perylenequinone pigments exist in many fungal bodies; (3) fungi have played an important role during geological processes.

Diamondoid hydrocarbon ratios as indicators of biodegradation in Australian crude oils

The effect of biodegradation on diamondoid distributions in petroleum has been investigated on a series of crude oils reservoired in two Australian sedimentary basins, the Gippsland Basin and the Carnarvon Basin. The ratio of methyladamantanes to adamantane rises with increasing biodegradation. Significant changes in the ratio occur at extreme levels, indicating that diamondoids can be indicators of petroleum biodegradation especially when most other hydrocarbons have been removed. The methyladamantane/ adamantane ratio can also be used to assess the composition of crude oils that are comprised of a mixture of severely biodegraded and non-biodegraded oil.

A geochemical reconstruction of oil generation in the Barrow Sub-basin of Western Australia

Abstract A suite of crude oils and petroleum source rock extracts from the Barrow Sub-basin of Western Australia have been analysed for biological marker compounds by capillary GC-MS, and for volatile hydrocarbons by whole oil capillary GC. These analyses were used to calculate values for twenty-three biomarker parameters in order to assess aspects of source type, maturity, migration and biodegradation of the hydrocarbons. The crude oils had a source in the Upper Jurassic Dingo Claystone formation. These hydrocarbons accumulated in the reservoir sands and in some cases were biodegraded. Several accumulation and biodegradation episodes have been recognised while the basin continued to subside, which resulted in a suite of oils showing marked differences in composition.

Microbial degradation of aromatic components of crude oils: A comparison of laboratory and field observations

Abstract Small aliquots of crude oil (Brent, North Sea) were incubated aerobically for periods of up to 14 days with an inoculum of recent sediment. The saturated and aromatic hydrocarbon components were then isolated, and the latter were compared, by capillary gas chromatography, with similar fractions isolated from four oils from the Barrow Sub-basin of Western Australia which had experienced different degrees of biodegradation. Biodegradation of the aromatic hydrocarbons of Brent oil produced a series of altered oils whose compositions were comparable to those of the naturally reservoir-degraded oils. Compounds such as naphthalene, phenanthrene and alkylated homologues of these hydrocarbons were oxidised but individual isomers were oxidised to different extents.