Andrew P. Murray

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.

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.

Oleananes in oils and sediments: Evidence of marine influence during early diagenesis?

Abstract The oleananes, as markers for the angiosperms, provide valuable source and age information when present in an oil. Nevertheless, they are not quantitatively related to the land plant input and indeed their presence reflects only a small leak in diagenetic processes leading primarily to aromatic oleanoids. Because they are minor products, the abundance of oleananes in terrigenous oils and sediments may be highly sensitive to changes in early diagenetic conditions. Here we present evidence that contact of plant matter with seawater during early diagenesis enhances the expression of oleananes in a mature sediment or oil. Oleananes are absent or present at very low concentrations in samples from the base of an Eocene coal seam affected by postdepositional seawater intrusion. However, their abundance increases toward the top of the seam in correlation with % organic sulphur, dibenzothiophene/phenanthrene, and the homohopane index. Similarly, in deltaic sediments from the South Sumatra Basin, oleanane/hopane is strongly correlated with indicators of marine influence such as C 27 C 29 steranes and the homohopane index. In each case, increasing oleanane abundance is accompanied by a reduction in the extent of aromatisation and, for the South Sumatra Basin, the proportion of A-ring contracted oleananes. An angiosperm-derived Miocene coal from the Philippines, deposited under freshwater conditions, shows abundant aromatic oleanoids but no oleananes. These results show that oleananes need to be used with caution as age and source markers in fluvio-deltaic and lacustrine petroleum systems. On the other hand, their sensitivity to early diagenetic conditions may make them useful in locating effective source rocks in such systems.

Polycyclic aromatic hydrocarbons in ancient sediments and their relationships to palaeoclimate

Combustion-derived and land-plant-derived polycyclic aromatic hydrocarbons (PAHs) have been investigated for an Upper Triassic to Middle Jurassic sedimentary sequence from the Northern Carnarvon Basin, Western Australia. The former included fluoranthene, pyrene, benzofluoranthenes, benzo[e]pyrene and benzo[a]pyrene, and the latter included retene, cadalene and simonellite. Combustion-derived PAHs are most abundant in the Upper Triassic and late Lower Jurassic sediments. The abundances of land-plant-derived markers all maximise in the upper part of the Lower Jurassic section, but vary independently of one another through time. The changes in the relative abundances of these markers have been related to the variation of regional climatic conditions, especially the changes in humidity and seasonality during the Late Triassic to Middle Jurassic times.

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.

Carbon isotopic composition of hydrocarbons in ocean-transported bitumens from the coastline of Australia

Abstract Bitumens stranding along the coastlines of the Northern Territory, Western Australia, South Australia, Victoria and Tasmania often have biomarker signatures which closely match those of oils of S.E. Asian origin. A suite of these bitumens was studied to determine the isotopic signatures of their alkanes and to compare them to similar oils from the Central Sumatra Basin. Saturated hydrocarbons were separated into a fraction containing n -alkanes and isoalkanes and one containing the multibranched/cyclic components using adduction into silicalite. Gas chromatography-isotope ratio mass spectrometry (GC-IRMS) of the n -alkanes revealed δ 13 C signatures covering a wide range of values from −25.5‰ to −29‰ PDB but generally falling between those of the botryococcane-containing Minas and Duri oils from the Central Sumatra Basin. n -Alkanes of the Minas oil are 3–4‰ lighter than their Duri counterparts. The isotopic compositions of pristane and phytane cover a similar range, δ 13 C = −24‰ to −27‰. Botryococcane is consistently heavy with δ 13 C values in the range −11 to −14‰. One bitumen sample with no botryococcane but abundant bicadinanes showed n -alkane δ 13 C values in the range −28 to −29‰, at the light extreme for waxy bitumens in our sample suite. Sample to sample differences in carbon isotopic signatures of n -alkanes are attributed mainly to variations in primary source, with weathering and biodegradation as minor causes. Instances of low precision for replicate analyses (± 2‰) are caused by the relative abundance of co-eluting isoalkanes. Overall, the isotopic patterns of waxy bitumens from the Australian coastline provide independent confirmation of their similarity to Central Sumatran lacustrine oils. On the other hand, asphaltic bitumens from the southern Australian coastline are isotopically light with n -alkane δ 13 C values in the range −31.5 to −33‰. This distinctive feature may assist identification of their source.

Oceanic transport of fossil dammar resin: The chemistry of coastal resinites from South Australia

Abstract Fragments of resin and fossilised resin (resinites) occasionally wash ashore along the southern Australian coast. Chemical and isotopic analyses were carried out on a suite of coastal and reference resinites to determine the likelihood of a local, as opposed to a distant, origin. All of the coastal resinites were found to contain a polymer based on the sesquiterpenoid cadinene skeleton and were markedly different to the diterpenoid resinites found in local Victorian coals. The coastal resinites closely resemble both fossil and recent dammar resin—a material associated primarily with the tropical angiosperm hardwoods of Southeast Asia, and one which has no known Australian source. Minor variations in the composition of our resinite samples are attributed to differences in their thermal history. These findings confirm the viability of long-distance oceanic transport, not only for the resinites but also for the waxy bitumens that strand along the same coastline. Analytical data on the coastal resinites also help to clarify the role of dammar resin in generating bicadinanes—a class of source and age-specific triterpanes found commonly in the Cainozoic oils and sediments of Southeast Asia.

Bicadinanes and related compounds as maturity indicators for oils and sediments

Abstract Biomarker maturity indices have proven useful in petroleum exploration to assess the thermal maturity of oils and sediments. However, their application to deltaic petroleum systems is often hindered by source and diagenetic interferences. Recently, we have described several new maturity indices based on the catagenetic products of the plant biopolymer polycadinene. These indices should be largely immune to biochemical effects during diagenesis, since little microbial activity will remain at temperatures sufficient to dissociate the polymer. Here we report changes in five polycadinene-related maturity indices through a marine-fluvio-deltaic depositional sequence in the South Sumatra Basin, Indonesia. The results are compared with the maturity required for oil generation as estimated by kinetic modelling and with conventional maturity indicators such as sterane epimerisation and the methyl phenanthrene index. The data confirm that the polycadinene indices are less subject to source and diagenetic interferences than those based on the sterane and aromatic hydrocarbon distributions. Values for several of the indices have also been measured for 17 Tertiary oils from Southeast Asia, Papua New Guinea, New Zealand and Australia. Contrary to an earlier report, the bicadinane maturity indicator (BMI-1) continues to change into the oil window and hence may be useful in ranking the relative maturity of oils as well as sediments. The BMI-1 values for two oils having unusually low sterane maturities are normal, leading us to question whether the low sterane maturity of many Tertiary-age oils is due to their source rocks having been heated more rapidly than those of older oils.

Integrated petroleum systems analysis to understand the source of fluids in the Browse Basin, Australia

The Browse Basin is located offshore on Australia’s North West Shelf and is a proven hydrocarbon province, hosting gas with associated condensate in an area where oil reserves are typically small. The assessment of a basin’s oil potential traditionally focuses on the presence or absence of oil-prone source rocks. However, light oil can be found in basins where source rocks are gas-prone and the primary hydrocarbon type is gas-condensate. Oil rims form whenever such fluids migrate into reservoirs at pressures less than their dew point (saturation) pressure. By combining petroleum systems analysis with geochemical studies of source rocks and fluids (gases and liquids), four Mesozoic petroleum systems have been identified in the basin. This study applies petroleum systems analysis to understand the source of fluids and their phase behaviour in the Browse Basin. Source rock richness, thickness and quality are mapped from well control. Petroleum systems modelling that integrates source rock property maps, basin-specific kinetics, 1D burial history models and regional 3D surfaces, provides new insights into source rock maturity, generation and expelled fluid composition. The principal source rocks are Early–Middle Jurassic fluvio-deltaic coaly shales and shales within the J10–J20 supersequences (Plover Formation), Middle–Late Jurassic to Early Cretaceous sub-oxic marine shales within the J30–K10 supersequences (Vulcan and Montara formations) and K20–K30 supersequences (Echuca Shoals Formation). These source rocks contain significant contributions of terrestrial organic matter, and within the Caswell Sub-basin, have reached sufficient maturities to have transformed most of the kerogen into hydrocarbons, with the majority of expulsion occurring from the Late Cretaceous until present.