Simon C. George

Coal as a source rock for oil: a review

The geological debate about whether, and to what extent, humic coals have sourced oil is likely to continue for some time, despite some important advances in our knowledge of the processes involved. It is clear that not only liptinites, but also perhydrous vitrinites have the potential to generate hydrocarbon liquids in the course of natural coalification. Some liptinites, especially alginite, cutinite, and suberinite, contain a higher proportion of aliphatic moieties in their structure than other liptinites such as sporinite and resinite and are, therefore, more oil-prone. It is of potential value to be able to predict the several environments of deposition in which coals with high liptinite contents or containing perhydrous vitrinites may have been formed. Review of the distribution of oil-prone coals in time and space reveals that most are Jurassic–Tertiary with key examples from Australia, New Zealand, and Indonesia. Methods based both on experimental simulations and the examination of naturally matured samples have been used to determine the order of generation of hydrocarbons from different macerals. Results are not entirely consistent among the different approaches, and there is much overlap in the ranges of degradation, but it seems probable that in the natural environment vitrinites begin to generate early, followed by labile liptinites such as suberinite, then cutinite, sporinite, and, finally, alginite. Petroleum potential may be determined by experimental simulation of natural coalification or inferred through various micro-techniques, especially fluorescence and infrared (IR) spectroscopy, or bulk techniques such as elemental analysis and 13C NMR spectroscopy. The latter three techniques enable a measure of the polymethylene component of the coal, which now appears to be one of the best available approaches for determining petroleum potential. No method of experimental simulation of petroleum generation from coals is without criticism, and comparative results are highly variable. However, hydrous pyrolysis, confined pyrolysis, and forms of open-system hydrous pyrolysis approach acceptable simulations. Whether, and to what degree generated liquid hydrocarbons are expelled, has long been the central problem in ‘oil from coal’ studies. The structure of vitrinite was believed until recently to contain an interconnected microporous network in which generated oil would be contained until an expulsion threshold was attained. Recent studies show the pores are not interconnected. Combined with a dynamic model of pore generation, it now seems that expulsion of hydrocarbons is best explained by activated diffusion of molecules to maceral boundaries and ultimately by cleats and fractures to coal seam boundaries. The main reason for poor expulsion is the adsorption of oil on the organic macromolecule, which may be overcome (1) if coals are thin and interbedded with clastic sediments, or (2) if the coals are very hydrogen-rich and generate large quantities of oil. The existence of oil in vitrinite is attested to by solvent extractions, fluorescence properties, and by microscopic observations of oil and bitumen. Experimental simulation of expulsion of oil from coals has only recently been attempted. The relative timing of release of generated CO2 and CH4 could have considerable importance in promoting the expulsion of liquid hydrocarbons but the mechanism is unclear. As it is universally agreed that dispersed organic matter (DOM) in some shales readily generates and expels petroleum, it is curious that few consistent geochemical differences have been found between coal macerals and DOM in interbedded shales. Unambiguous evidence of expulsion from coals is limited, and in particular only a few commercial oil discoveries can be confidently correlated to coals. These include Upper Cretaceous Fruitland Formation coals in the USA, from which oil is produced; New Zealand Tertiary coals; and Middle Jurassic coals from the Danish North Sea. It is likely that coals have at least contributed to significant oil discoveries in the Gippsland Basin, Australia; in the Turpan Basin, China; and in the Kutei and Ardjuna basins in Indonesia, but this remains unproven. Early reports that early Jurassic coals in mid-Norway were a major source of the reservoired oils have been shown to be inaccurate. None of the proposed ‘rules of thumb’ for generation or expulsion of petroleum from coals seem particularly robust. Decisions on whether a particular coal is likely to have been an active source for oil should consider all available geological and geochemical information. The assumptions made in computational models should be well understood as it is likely with new understandings of processes involved that some of these assumptions will be difficult to sustain.

The effect of minor to moderate biodegradation on C5 to C9 hydrocarbons in crude oils

Abstract A suite of 18 oils from the Barrow Island oilfield, Australia, and a non-biodegraded reference oil have been analysed compositionally in order to detail the effect of minor to moderate biodegradation on C5 to C9 hydrocarbons. Carbon isotopic data for individual low molecular weight hydrocarbons were also obtained for six of the oils. The Barrow Island oils came from different production wells, reservoir horizons, and compartments, but have a common source (the Upper Jurassic Dingo Claystone Formation), with some organo-facies differences. Hydrocarbon ratios based on hopanes, steranes, alkylnaphthalenes and alkylphenanthrenes indicate thermal maturities of about 0.8% Rc for most of the oils. The co-occurrence in all the oils of relatively high amounts of 25-norhopanes with C5 to C9 hydrocarbons, aromatic hydrocarbons and cyclic alkanes implies that the oils are the result of multiple charging, with a heavily biodegraded charge being overprinted by fresher and more pristine oil. The later oil charge was itself variably biodegraded, leading to significant compositional variations across the oilfield, which help delineate compartmentalisation. Biodegradation resulted in strong depletion of n-alkanes (>95%) from most of the oils. Benzene and toluene were partially or completely removed from the Barrow Island oils by water washing. However, hydrocarbons with lower water solubility were either not affected by water washing, or water washing had only a minor effect. There are three main controls on the susceptibility to biodegradation of cyclic, branched and aromatic low molecular weight hydrocarbons: carbon skeleton, degree of alkylation, and position of alkylation. Firstly, ring preference ratios at C6 and C7 show that isoalkanes are retained preferentially relative to alkylcyclohexanes, and to some extent alkylcyclopentanes. Dimethylpentanes are substantially more resistant to biodegradation than most dimethylcyclopentanes, but methylhexanes are depleted faster than methylpentanes and dimethylcyclopentanes. For C8 and C9 hydrocarbons, alkylcyclohexanes are more resistant to biodegradation than linear alkanes. Secondly, there is a trend of lower susceptibility to biodegradation with greater alkyl substitution for isoalkanes, alkylcyclohexanes, alkylcyclopentanes and alkylbenzenes. Thirdly, the position of alkylation has a strong control, with adjacent methyl groups reducing the susceptibility of an isomer to biodegradation. 1,2,3-Trimethylbenzene is the most resistant of the C3 alkylbenzene isomers during moderate biodegradation. 2-Methylalkanes are the most susceptible branched alkanes to biodegradation, 3-methylalkanes are the most resistant and 4-methylalkanes have intermediate resistance. Therefore, terminal methyl groups are more prone to bacterial attack compared to mid-chain isomers, and C3 carbon chains are more readily utilised than C2 carbon chains. 1,1-Dimethylcyclopentane and 1,1-dimethylcyclohexane are the most resistant of the alkylcyclohexanes and alkylcyclopentanes to biodegradation. The straight-chained and branched C5–C9 alkanes are isotopically light (depleted in 13C) relative to cycloalkanes and aromatic hydrocarbons. The effects of biodegradation consistently lead to enrichment in 13C for each remaining hydrocarbon, due to preferential removal of 12C. Differences in the rates of biodegradation of low molecular weight hydrocarbons shown by compositional data are also reflected in the level of enrichment in 13C. The carbon isotopic effects of biodegradation show a decreasing level of isotopic enrichments in 13C with increasing molecular weight. This suggests that the kinetic isotope effect associated with biodegradation is site-specific and often related to a terminal carbon, where its impact on the isotopic composition becomes progressively ‘diluted’ with increasing carbon number.

Assessing the maturity of oil trapped in fluid inclusions using molecular geochemistry data and visually-determined fluorescence colours

Abstract The thermal maturity of oils extracted from inclusions and the fluorescence colours of oil-bearing fluid inclusions have been measured in 36 sandstone samples from Australasian oil fields. The inclusion oils were analysed using an off-line crushing technique followed by GC–MS. A maturity assessment was made for each inclusion oil using 25 molecular maturity ratios, including a newly defined dimethyldibenzothiophene ratio (DMDR). Each inclusion oil was placed in one of 4 maturity brackets, approximately equivalent to early, mid, peak and post oil generation windows. The fluorescence colours of oil inclusions were visually-discriminated into “blue”, “white” and “yellow plus orange” and their proportions estimated using point counting techniques. Sixteen samples have >85% of oil inclusions with blue fluorescence, whilst other samples have more variable fluorescence colours. One sample has 100% of oil inclusions with yellow plus orange fluorescence. The results show that samples containing mainly blue-fluorescing oil inclusions have thermal maturities anywhere within the oil window. In particular, the molecular geochemical data strongly suggests that oil inclusions with blue fluorescence can have relatively low maturities (calculated reflectance

Reappraisal of hydrocarbon biomarkers in Archean rocks

Significance The advent of oxygenic photosynthesis set the stage for the evolution of complex life on an oxygenated planet, but it is unknown when this transformative biochemistry emerged. The existing hydrocarbon biomarker record requires that oxygenic photosynthesis and eukaryotes emerged more than 300 million years before the Great Oxidation Event [∼2.4 billion years ago (Ga)]. We report that hopane and sterane concentrations measured in new ultraclean Archean drill cores from Australia are comparable to blank concentrations, yet their concentrations in the exteriors of conventionally collected cores of stratigraphic equivalence exceed blank concentrations by more than an order of magnitude due to surficial contamination. Consequently, previous hydrocarbon biomarker reports no longer provide valid evidence for the advent of oxygenic photosynthesis and eukaryotes by ∼2.7 Ga. Hopanes and steranes found in Archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes, but the syngeneity of these hydrocarbon biomarkers is controversial. To resolve this debate, we performed a multilaboratory study of new cores from the Pilbara Craton, Australia, that were drilled and sampled using unprecedented hydrocarbon-clean protocols. Hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit, but when they were detectable, they had total hopane (<37.9 pg per gram of rock) and total sterane (<32.9 pg per gram of rock) concentrations comparable to those measured in blanks and negative control samples. In contrast, hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389.5 pg per gram of rock and 1,039 pg per gram of rock, respectively. Polycyclic aromatic hydrocarbons and diamondoids, which exceed blank concentrations, exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1,000 ng per gram of rock in hydropyrolysates from the ultraclean cores. These results demonstrate that previously studied Archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons. Therefore, existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by ∼2.7 billion years ago. Although suitable Proterozoic rocks exist, no currently known Archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation, so future exploration for Archean biomarkers should screen for rocks with milder thermal histories.

Effect of igneous intrusion on the organic geochemistry of a siltstone and an oil shale horizon in the Midland Valley of Scotland

Abstract The effect of rapid thermal stress on the organic geochemistry of a siltstone intruded by a quartz-dolerite dyke and an oil shale intruded by an alkali-dolerite sill has been studied using optical petrography, Rock-Eval pyrolysis, gas chromatrography and gas chromatography-mass spectrometry. Maturity-dependent ratios derived from both the aliphatic and aromatic hydrocarbon fractions corroborate the observation of Raymond and Murchison (1988b) that the alkali-dolerite sills, which were intruded soon after sedimentation, had a lesser effect than the later quartz-dolerite intrusions. The maturity parameters based on the alkylnaphthalene and alkylphenanthrene isomer ratios are more suitable for studying maturity variations in heat-affected samples than those based on the commonly-used aliphatic biological markers. These aromatic compounds survive to higher ranks and the isomer ratios continue changing.

Constraining the oil charge history of the South Pepper oilfield from the analysis of oil-bearing fluid inclusions

Abstract The South Pepper oilfield, located in the Barrow Sub-basin on the NW margin of the Australian continent, has experienced a multi-phase charge history. Abundant oil-bearing fluid inclusions are present in samples from within the current gas cap, suggesting that an oil column existed prior to gas. This palaeo-oil (gas-leg FI oil) has Ts/Tm and C29/C30 αβ hopane ratios of ∼1 and the C35 homohopanes are a significant proportion of the extended homohopanes. It has lower Pr/Ph and diasterane/sterane ratios than the currently reservoired live oil and contains gammacerane, a series of peaks tentatively identified as C30 to C34 17α(H)-30-norhopanes and a large amount of 2α-methylhopanes. Collectively, geochemical analysis of the gas-leg FI oil suggests that it was generated from a less mature, more calcareous source rock, deposited under more reducing conditions than the Upper Jurassic Dingo Claystone, the main source of the live oil. In addition the presence of C30 dinosteranes in the gas-leg FI oil provides a Triassic or younger age constraint. This makes Palaeozoic carbonates an unlikely source. Possible source intervals for the gas-leg FI oil are thin, Lower Jurassic limestones and marls which occur at the base of the Lower Dingo Claystone, or a thin limestone unit (the Cunaloo Member) at the base of the Triassic Locker Shale. Samples from within the present oil-leg also contain abundant oil inclusions, consistent with high oil saturations at the present day. However, these oil inclusions exhibit different fluorescence colours, suggesting they represent a second oil charge. Geochemically the oil-leg FI oil has an intermediate composition between the live oil and the gas-leg FI oil, suggesting that gas charge displaced the first oil charge, samples of which were preserved as fluid inclusions in the oil-leg sample. Biodegradation of the first oil charge, indicated by the presence of 17α(H)-25-norhopanes in the currently reservoired live oils, can be attributed to the ingress of meteoric waters, probably during sub-aerial exposure of the basin margin during Miocene wrenching. Changing environmental conditions curtailed bacterial activity and allowed unaltered oil sourced from the Dingo Claystone to accumulate below the gas cap and mix with the biodegraded residues of the first oil charge to achieve the live oil composition. The biodegradation event must have preceded the second charge as the live oil contains compounds such as n-alkanes which would have been removed had alteration occurred after the second charge. Complex charge histories are common and the analysis of palaeo-oils trapped within fluid inclusions provides the opportunity to achieve a more comprehensive assessment of hydrocarbon charge

Applications of laser micropyrolysis-gas chromatography-mass spectrometry

Abstract An apparatus which couples a laser microprobe with a GC–MS has recently been constructed in our laboratories to facilitate analytical micropyrolysis studies. The utility and versatility of the technique is demonstrated through the analysis of micro-sized quantities of various organic fossils. Results are reported from a torbanite coal, Green River oil shale, Tasmanite oil shale, an isolated Tasmanites microfossil and oil-bearing fluid inclusions. A diverse range of aliphatic products including distributions of n-alkene/n-alkane pairs, isoprenoids, polycyclic biomarkers and various alkylaromatics have been detected from these materials. The analytical credibility of the laser micropyrolysis GC–MS technique is confirmed by the favourable comparison of the laser derived molecular data to corresponding data obtained from more traditional methods.

Tricyclic terpenoid composition of Tasmanites kerogen as determined by pyrolysis GC-MS

The high abundance with which tricyclic terpenoids have previously been detected in Tasmanite oil shales has led to the strong suspicion that the source of these compounds is the Tasmanites microfossil prevalent in these oil shales. In this study, the hydrocarbon composition of a Tasmanite oil shale and isolated Tasmanites were separately investigated by laser micropyrolysis gas chromatography–mass spectrometry, a recently developed technique that facilitates the analysis of small samples such as microfossils. Major products comprised C19–C28 tricyclic terpanes, including the ubiquitous 13-methyl, 14-alkylpodocarpanes, as well as a number of additional tricyclic terpane isomers, a C19 monoaromatic hydrocarbon, and several C19–C21 tricyclic terpenes (one and two orders of unsaturation). There have been few previous reports on the tricyclic terpenes and their production is likely attributable to the pyrolytic cleavage of analogous (probably saturated) tricyclic precursors within the macromolecular biopolymer. The only major difference between the tricyclic terpenoid compositions observed from these samples was the absence of the less concentrated oil shale products in the Tasmanites analyses, probably due to the lower organic content of the preextracted fossil. The very similar tricyclic content of both samples strongly supports the proposal of an inherent relationship between the Tasmanites and tricyclic terpenoid production. The integrity of the laser data was confirmed by comparison to a conventional data set obtained by the pyroprobe pyrolysis of the Tasmanite oil shale.

Biomarkers, brines, and oil in the Mesoproterozoic, Roper Superbasin, Australia

Gas chromatography–mass spectrometry of oil inclusions from the ca. 1430 Ma marine Roper Group in the Roper Superbasin, Australia, provides a new source of information about the early biosphere and Proterozoic petroleum systems. Oil most likely derived from an overlying shale was trapped at ∼60 °C as abundant oil inclusions within transgranular microfractures in detrital quartz during Mesoproterozoic basin inversion. The oil is very mature and has a wide range of biomarkers, derived mainly from cyanobacteria, but lacks eukaryote biomarkers. Unlike associated solid bitumens, the inclusion oil is nonbiodegraded. Evidently, the inclusions remained closed systems, sheltered from postentrapment alteration and contamination. Because fluid inclusions have preserved biomarkers for >1000 m.y., they constrain the diversity of primordial ecosystems, whereas other forms of early Precambrian organic matter are usually absent or metamorphosed.

A new apparatus for laser micropyrolysis—gas chromatography/mass spectrometry

Abstract A new instrumental configuration for laser micropyrolysis-gas chromatography/mass spectrometric analysis is reported. The principal devices are a high powered continuous wave Nd:YAG laser; a commercial reflected light/fluorescence microscope (slightly modified to accommodate laser radiation), a pyrolysis chamber, a gas inlet system, a gas Chromatograph (modified to accommodate the inlet system) and a hybrid (double focusing-quad) mass spectrometer. The mass spectrometer offers very high sensitivity ( max m = 5 × 10 − 7 C μg − 1 ) and mass resolution (maxm = 80000). A Sydney Basin torbanite sample was analysed to test the efficiency and potency of the instrument. High concentrations of pyrolysis products were obtained from the sample with mild laser conditions. The dominant products are homologous series of straight chain aliphatic hydrocarbons ranging from C6 to greater than C30. Secondary contaminant products from sample charring were minimal with laser energies in the region of 0.5–3 W, even for continuous exposure of up to 300 s duration. To make comparison with more conventional techniques used for the pyrolysis of organic sediments, the Sydney Basin torbanite sample was also subjected to gas chromatography/mass spectrometric analysis with the following pyrolysis methods: 1. (i) a pyroprobe; 2. (ii) a pyrojector; and 3. (iii) a quantum microscale sealed vessel (MSSV-1) pyrolysis unit. A comparison reveals similar aliphatic dominated product distributions from each pyrolysis method. The consistency of these results proves the capability of the new laser apparatus to perform analytical pyrolysis at a microscopic level, rather than on bulk samples as required by more traditional techniques.