Christopher J. Boreham

Petroleum geology and geochemistry of the Middle Proterozoic McArthur Basin, Northern Australia: III. Composition of extractable hydrocarbons☆

Abstract Middle Proterozoic organic-rich sediments from the McArthur Basin in northern Australia contain abundant hydrocarbons which are derived from syngenetic kerogen and hence, are representative of the organic remains of microorganisms living at that time. The major classes of hydrocarbons identified were n-alkanes, monomethyl branched alkanes, cyclohexyl alkanes and acyclic isoprenoids. There were also low abundances of pentacyclic triterpanes comprising hopanes and methyl hopanes. Low concentrations of steranes were present in most samples, but like the triterpanes, they were only easily detected in the least thermally altered sediments, and hence were of limited use in detailed assessments of thermal maturity. The presence of steranes in sediments of this age is strong evidence for the existence of eukaryotic organisms as far back as 1690 Ma, although the relatively high abundances of branched alkanes indicates that most of this primitive organic matter was probably derived from prokaryotes.

Dinosterane and other steroidal hydrocarbons of dinoflagellate origin in sediments and petroleum

Abstract The steroidal alkanes of a selection of sediments and oils have been examined by GC-MS with multiple metastable reaction monitoring. Specific 4-methyl sterane isomers have been identified by comparison with isomers synthesized from sterols isolated from dinoflagellates. An immature marine oil shale and two mature marine oils of Triassic to early Cretaceous age contained high concentrations of C30 steranes comprising desmethyl, 24-ethyl-4α-methylcholestane and 4α,23,24-trimethylcholestane (dinosterane) isomers. An immature non-marine oil shale and two non-marine oils of Cretaceous to Eocene age contained stereoisomers of 24-ethyl-4α-methylcholestane as the dominant C30 steranes. Reaction monitoring analyses in GC-MS are particularly suited to unravelling complex distributions of homologous and stereoisomeric steroids encountered in oils and their source rocks.

Biomarker and n-alkane isotope profiles for Tertiary oils: relationship to source rock depositional setting

Abstract Biomarker and n -alkane isotope profiles have been measured for 29 Late Cretaceous/Tertiary oils from SE Asia, China, Papua New Guinea, New Zealand and the U.S.A. The results are interpreted with respect to six kinds of source rock depositional setting: fluvio-deltaic (FD), freshwater transitional (TR), lacustrine (LAC), saline lacustrine (SAL), marine deltaic (MD) and marine carbonate (MC). A comparison of biomarker and n -alkane isotope results suggests that parameters such as the oleanane/hopane ratio may overestimate the higher plant contribution to marine oils. The abundance of bicadinanes is much more variable than that of oleanane, probably because of a specific association with the highly resinous, dipterocarp hardwoods of SE Asia. However, traces do occur in an oil from outside the paleogeographic range of the dipterocarp family, confirming that there is also a more general angiosperm source. The bicadinane isomeric distribution is shown to depend on maturity, leading to new maturity parameters which are resistant to even very heavy biodegradation. Retention indices based on the regular hopane series are provided for the main bicadinane and methyl bicadinane isomers and for a group of oleanane-related triterpanes of unknown structure. The presence of the latter compounds may indicate locally deposited as opposed to transported higher plant matter in the source. Source rock depositional setting is the primary control on the shape of the n -alkane isotope profile, with negatively sloping curves being characteristic of FD and TR oils and flat or positively sloping curves typical of marine oils. The difference is probably related to the bacterial reworking of higher plant matter in the FD environment. A slight isotopic anomaly at n -C 17 correlates with the abundance of algal-derived steranes and may indicate a minor marine contribution to the source of an FD oil.

Secular and environmental constraints on the occurrence of dinosterane in sediments

Abstract The distribution patterns of sedimentary A-ring methylated steranes have changed markedly over geological time. Although dinosterane and its isomer 24-ethyl-4α-methylcholestane have been tentatively identified in three Proterozoic rock units, they are either not detectable or occur in low abundance relative to 3-methyl steranes throughout most of the Palaeozoic. Between Permian and middle Triassic times (260-220 Ma ago), 4-methyl sterane abundances in marine sediments increased markedly. The presence of dinosterane in some middle Triassic marine sediments is contemporaneous with the appearance of fossil cysts of uncontested dinoflagellate affinity. 4-Methyl steranes, including dinosterane or their precursor sterenes and sterols, then show a continuous presence, often in high abundance, in marine sediments from the late Triassic through to the present day. Assemblages of 4-methyl steranes and their precursors, but with dinosterane absent or in low relative abundance, are often the predominant steroids in lacustrine sediments in the Cainozoic. Dinosterane appears to arise predominantly from marine dinoflagellates and, as a consequence, is a useful biological marker for Mesozoic and Cainozoic marine organic matter. The isomer 24-ethyl-4α-methylcholestane is likely to have multiple origins although its very high abundance in Tertiary lacustrine sediments and oils, compared to older materials, suggests that dinoflagellates could also be the source in these cases.

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.

Hydrocarbon biomarkers, thermal maturity, and depositional setting of tasmanite oil shales from Tasmania, Australia

This study represents the first geological and organic geochemical investigation of samples of tasmanite oil shale representing different thermal maturities from three separate locations in Tasmania, Australia. The most abundant aliphatic hydrocarbon in the immature oil shale from Latrobe is a C19 tricyclic alkane, whereas in the more mature samples from Oonah and Douglas River low molecular weight n-alkanes dominate the extractable hydrocarbon distribution. The aromatic hydrocarbons are predominantly derivatives of tricyclic compounds, with 1,2,8-trimethylphenanthrene increasing in relative abundance with increasing maturity. Geological and geochemical evidence suggests that the sediments were deposited in a marine environment of high latitude with associated cold waters and seasonal seaice. It is proposed that the organism contributing the bulk of the kerogen, Tasmanites, occupied an environmental niche similar to that of modern sea-ice diatoms and that bloom conditions coupled with physical isolation from atmospheric CO2 led to the distinctive “isotopically heavy” δ13C values (−13.5‰ to −11.7‰) for the kerogen. δ13C data from modern sea-ice diatoms (−7‰) supports this hypothesis. Isotopic analysis of n-alkanes in the bitumen (−13.5 to −31‰) suggest a multiple source from bacteria and algae. On the other hand, the n-alkanes generated from closed-system pyrolysis of the kerogen (−15‰) are mainly derived from the preserved Tasmanites biopolymer algaenan. The tricyclic compounds (mean −8‰) both in the bitumen and pyrolysate, have a common precursor. They are consistently enriched in 13C compared with the kerogen and probably have a different source from the n-alkanes. The identification of a location where the maturity of the tasmanite oil shale approaches the “oil window” raises the possibility that it may be a viable petroleum source rock.

The effect of aromatization on the isotopic compositions of hydrocarbons during early diagenesis

Polycyclic aromatic hydrocarbons with varying degrees of aromatization were isolated from the Eocene Messel Shale (Rheingraben, Germany). The high abundances of these compounds and their structural resemblances to cyclic triterpenoid lipids are consistent with derivation from microbial rather than thermal processes. Compounds structurally related to oleanane contain from five to nine double bonds; those within a series of aromatized hopanoids contain from three to nine. All are products of diagenetic reactions that remove hydrogen or methyl groups, and, in several cases, break carbon-carbon bonds to open rings. Aromatized products are on average depleted in 13C relative to possible precursors by l.2% (range: l.5% enrichment to 4% depletion, n = 9). The dependence of 13C content on the number of double bonds is not, however, statistically significant and it must be concluded that there is no strong evidence for isotopic fractionation accompanying diagenetic aromatization. Isotopic differences between series (structures related to ursane, des-A-ursane, des-A-lupane, des-A-arborane, and possibly, des-A-gammacerane are present) are much greater, indicating that 13C contents are controlled primarily by source effects. Fractionations due to chromatographic isotope effects during HPLC ranged from 0.1 to 2.8%.

Distributions and stable carbon isotopic compositions of biomarkers in torbanites from different palaeogeographical locations

Abstract The relative distributions and stable carbon isotopic compositions of certain aliphatic and aromatic biomarkers from 11 Botryococcus braunii rich torbanites from Scotland, South Africa and Australia covering the Late Carboniferous to Late Permian were investigated. The data was scrutinised for any evidence of molecular features which may be characteristic of palaeogeography. The torbanites studied were selected to cover a range of age and palaeoclimatic conditions (e.g. Permian and Late Carboniferous when Gondwana was covered by an extensive ice sheet, the Late Permian when the climate warmed from glacial to cool to temperate and the Carboniferous when Laurasia was located within the equatorial zone). All torbanites are composed of abundant n-alkanes and novel macrocyclic alkanes which, based on similar δ13C values, are ascribed a common origin. All samples are also characterised by a high cyanobacterial input as indicated by abundant hopanoids. Other biomarker distributions and stable carbon isotopic compositional differences separated the torbanites into three groups, which also correlate to three different palaeogeographical/palaeoclimatic precincts: (i) Group A—southern Africa and eastern Australia (Temi) torbanites which are characterised by a high relative abundance of cyanobacterial and methylotrophic hopanoids, abundant branched hydrocarbons and 13C enriched homologous series of monomethylakanes; (ii) Group B—eastern Australia (Newnes and Glen Davis) torbanites contain relatively high amounts of drimanes and abundant 13C enriched homologous series of monomethylakanes; and (iii) Group C—Scottish torbanites (Torbane Hill and Westfield) contain high relative amounts of cyanobacterial hopanoids, methylotrophic hopanoids and abundant branched aliphatics (but no 13C enriched homologous series of monomethylakanes).

Chemical, molecular and isotopic differentiation of organic facies in the Tertiary lacustrine Duaringa oil shale deposit, Queensland, Australia

Abstract Total organic matter, extractable organic matter (bitumen), and pyrolysates released on low-temperature sealed-tube pyrolysis of kerogen were analysed chemically and isotopically. Based on bulk bitumen and biomarker parameters, the sediments are immature to a depth of approx. 1300 m. Within the terrestrial and lacustrine depositional environments of the Tertiary Duaringa Basin, various organic facies have been classified according to inorganic input and maceral abundances of organic matter derived from land-plant and algal sources; the latter derived mainly from Pediastrum and with a varying Botryococcus content. The lacustrine upper lamosite is chemically and isotopically distinct from the lower lamosite due to a changing algal community, relative contributions from allochthonous land-plants and, for the lower lamosite, a slightly more saline and restricted lake environment. The lower lamosite is associated with the highest concentrations of C 28 -C 30 4-methylsteranes and 13 C depletion in TOC. In the upper lamosite, there is a strong positive correlation between Botryococcus content and 13 C enrichment. The organically lean claystones have sources nearly identical to the lamosites, but the organic matter is diluted due to rapid influx of siliciclastics. The terrestrial facies, the cannel coal and carbonaceous shales, are mainly distinguished from the lacustrine facies by higher relative proportions of C 29 steranes, C 30 4-methylsteranes and δ 13 C values for the pyrolysed hydrocarbons intermediate between the two lamosite facies. The carbonaceous oil shale has a composite geochemical signature intermediate between the two end-members, lamosite and cannel coal. Oleanane and ring-A-degraded triterpanes derived from lupane, arborane and oleanane skeletons are distributed throughout all organic facies, representing both autochthonous and allochthonous higher plant inputs. Higher concentrations of oleanane and des-A-lupane are associated with the terrestrial facies. Hopanes dominate over steranes in all organic facies and their concentration ranges are independent of facies. For the lamosites, the aliphatic biopolymer algaenan is implicated as the dominant source for n -alkanes from analysis of their yields, homologue abundances, and individual δ 13 C values. In the terrestrial facies, resinites are paraffinic and are a major source for the n -alkanes released on pyrolysis. Compound Specific Analysis of extractable and pyrolysate hydrocarbons allowed deconvolution of different source inputs to free lipids and kerogen. For the lamosites, low-molecular-weight n -alkanes in the bitumen are derived mainly from a Pediastrum source while Botryococcus is associated with waxy n -alkanes. However, the situation is reversed in the kerogen; Pediastrum algeanan has a longer average alkyl chain-length than Botryococcus algaenan. The magnitude of the isotopic differences between single n -alkanes released at different temperatures is useful in determining multiplicity of sources.

Carbon isotope biogeochemistry of plant resins and derived hydrocarbons

Abstract Hydrocarbons derived from plant resins are major components of some terrigenous oils and bitumens. These compounds are structurally distinct and this makes them useful biomarkers applicable in petroleum exploration as well as sources of biogeochemical information about palaeoenvironment and palaeobotany. Although recent studies have elucidated the molecular structure of resinites, very little information has been available for the carbon isotope composition of resinites and no studies of resin-derived compounds in oils had been performed prior to the present study. Hence, carbon stable isotope analyses were carried out on a suite of modern and fossil resins of diverse origins, including compound specific isotope analysis of individual hydrocarbons produced during resin pyrolysis. Oils derived from resinitic source organic matter were also analysed. The results showed that “Class I” resinites derived from gymnosperms were enriched in the heavy carbon isotope compared with those from angiosperms (“Class II” resinites). Furthermore, both fossil resinites themselves and individual hydrocarbons derived from them were isotopically heavy compared with modern plant resins. The isotopic signatures of diterpanes and triterpanes in various early Tertiary oils from Australasia and Southeast Asia reflect their origins from gymnosperms and angiosperms, respectively.