Trevor D. Gilbert

Biomarker distributions in Australian oils predominantly derived from terrigenous source material

Oils found in many Australian sedimentary basins contain a relatively high input of terrigenous organic source material. A study of a large number of these oils has permitted a number of biomarker distributions to be determined that appear to be characteristic for oils derived from terrigenous source material. These features include the presence of sesquiterpanes, absence of tricyclic terpanes, relatively high concentrations of a C24-tetracyclic terpane, predominance of C29-steranes, presence of two unidentified C27- and C30-terpanes and virtual absence of 18α(H)-oleanane. The latter observation is significant since it appears that 18α(H)-oleanane appears only in oils of Tertiary age derived from terrigenous source material. An attempt has also been made to compare the results for the Australian oils with published results for oils derived from terrigenous source material in other parts of the world. The number of reports in the literature for these oils is fairly limited but it appears that some of the observations for the Australian oils can be extended to them. This study is an initial attempt to devise a classification scheme that can be used to determine the origin of an oil unambiguously from biomarker parameters. In order for this approach to be successful a relatively large number of biomarker parameters need to be assigned as characteristic of a particular type of source material.

Geochemical application of sterane and triterpane biomarkers to a description of oils from the Taranaki Basin in New Zealand

Abstract The distributions of sterane and terpane biomarkers in a suite of 25 oils from onshore and offshore regions of the Taranaki Basin of New Zealand have been determined by gc-ms. Features of these distributions include the presence of relatively high concentrations of a number of C 24 tetracyclic terpanes, the virtual absence of tricyclic terpanes, a predominance of C 29 normal-, iso- and dia-steranes and the presence of a number of non-hopanoid triterpanes. In one member of the suite (Urenui) 18α(H)-oleanane is the most abundant component ( m/z 191 ion chromatogram) of the terpane class of biomarkers. Based upon the distributions of 18α(H)oleanane, the McKee and Moturoa families of oils can be delineated. These terpane distributions are qualitatively similar to those encountered in crude oils from a number of Far-eastern Basins and from the Handil field of the Mahakam delta (Indonesia). The Taranaki oils are of approximately the same thermal maturity. Maturity parameters calculated from the distributions of C 29 steranes and of 17s(H),21α(H)- and 17α(H),21s(H)- hopanes confirm recent findings of incomplete nuclear isomerisation in oils derived from Tertiary sediments. An attempt has been made to correlate the McKee-1 and Maui-4 crude oils with potential source rocks from depths of 3708 m and 3825 m respectively. The rocks studied have attained insufficient maturity to have been the sources of these oils.

Bicyclic sesquiterpenoids and diterpenoids in Australian crude oils

Abstract Bicyclanes previously reported only in heavily biodegraded Texas Gulf Coast crudes have been found to be ubiquitous in Australian crude oils of non-marine origin from four different basins. The compounds are present in oils, thought to be derived from the same or similar sources, that have undergone varying degrees of biodegradation. They are also found to be present in oils of different geological age. In addition a series of tricyclic diterpenoid hydrocarbons was common to four oils from the Gippsland Basin. Four of these compounds had the molecular formula C20H34 and mass spectral fragmentation patterns suggested they were mono-unsaturated diterpenoids. The presence of unsaturated diterpenoids in crude oils appears to be a unique observation. It is proposed that the diterpenoids may be the source of the bicyclanes also observed in these oils.

Comparison of the structures of humic substances from aquatic and terrestrial sources by pyrolysis gas chromatography-mass spectrometry

Abstract Four humic extracts isolated from terrestrial, mahne, plankton and freshwater sources have been investigated by pyrolysis-gas chromatography-mass spectrometry. The results show that substantial quantities of phenols (some not derived from lignin), pyrroles and nitrites (derived from proteins, nucleic acids or porphyrins) are decomposition products. Small amounts of furans (derived from carbohydrates) and unsaturated ketones (derived from polycarboxylic aliphatic acids) are also present. The results illustrate how parent material, transport factors and biological activity affect the composition of humic extracts.

Characterization of victorian soft brown coal wood by microscopic techniques and Curie-Point pyrolysis combined with gas chromatography-mass spectrometry

Abstract Pyrolysis combined with gas chromatography-mass spectrometry is used in an attempt to distinguish lignins derived from an angiosperm and a gymnosperm on the basis of their pyrolysis products. The study is extended to examine the pyrolysis products of Australian soft brown coal woods that have undergone varying degrees of gelification during the soft brown coal stage of coalification. Comparison of the lignin and brown coal pyrograms shows that the brown coals may be derived from gymnosperms, which is in agreement with the results of previous microscopic studies. Poorly gelified brown coals are also shown to contain significant amounts of furan-type products which are not present in the gelified soft brown coal woods. No systematic variation was observed in the distribution of methoxy compounds and the macroscopically and microscopically defined degree of gelification. Finally pyrolysis of low rank vitrinites shows a relative lack of substituted phenols in their pyrograms, but a relative increase in concentration of substituted polycyclic aromatics, compared with highly gelified brown coals.

Source rock and asphaltene biomarker characterization by pyrolysisgas chromatography-mass spectrometry-multiple ion detection

Abstract Biomarkers produced by microscale pyrolysis of extracted source rocks, kerogens and asphaltenes have been analysed directly by gas chromatography-mass spectrometry in the MID mode. A series of experiments have been undertaken. These include an investigation into the production of the hopanes at different pyrolysis temperatures and a comparison of results obtained from Curie point pyrolysis and the Chemical Data Systems pyroprobe. Steranes and triterpanes produced by pyrolysis of a series of vitrinites were investigated and correlated with maturity variations. Finally, the biomarkers produced from asphaltenes were compared with those produced from the extract and extracted rock in order to further investigate the theory that asphaltenes are a liquid kerogen. The results of these experiments showed that the biomarkers released by pyrolysis of source rocks have a potential use for source and maturity determinations. The method is advantageous in that it can be said with some certainty that the biomarkers are indigenous to the rock and not present in the rock as a possible result of migration.

Hydrous pyrolysis of model compounds

Abstract In pursuit of a method for the determination of the total n -alkyl content of petroleum source rocks, the reactions of model organic compounds containing a long-chain n -alkyl constituent with water have been studied in sealed tubes. At 330°C, the n -alkyl component of n -alcohols and n -ethers and the n -alcohololic components of esters are largely converted to, and release as, the corresponding n -alkanes. Some fragmentation of carbon chains accompanies this reaction. The conversion of n -alkanoic acids to n -alkanes is only partial, even at 350°C, and n -alkyl aromatics and n -alkyl hydroaromatics remain virtually unaffected under these conditions. Where significant n -alkane generation from model compounds is observed, additions of brown coal generally promote this process. In view of the stability of n -alkanoic acids and n -alkylated hydrocarbons observed under the conditions of hydrous pyrolysis used in this study, this technique is not entirely suitable for determining the total n -alkyl content of all source rocks nor, presumably, their full petroleum potential.

Characterization of petroleum source rocks and shales by pyrolysis-gas chromatography-mass spectrometry-multiple ion detection

Abstract A number of organic rich shales and potential source rocks from Australia and Alaska have been examined by pyrolysis-gaschromatography-mass spectrometry-multiple ion detection. Attempts have been made to observe variations in the distributions of biomarkers released from the samples by pyrolysis. Particular emphasis has been placed on the release of pentacyclic triterpanes to evaluate whether or not a one-step pyrolysis method can be used to determine relative maturity of the samples. Immature shales are observed to produce C27 and C29 hopenes, not present at higher levels of maturity, as well as number of hopanes and moretanes. Variations in the distributions of alkylbenzenes, alkanes, alkenes, sesquiterpenoids and diterpenoids can also be monitored during the pyrolysis reaction and provide information on the nature of the source material in the samples.

A review of biomarkers in kerogens as determined by pyrolysis-gas chromatography and pyrolysis-gas chromatography-mass spectrometry

Abstract Biomarkers are compounds, generally hydrocarbons, present in crude oils and source rocks whose carbon skeletons can be readily related to a precursor molecule occurring in the original source material. The use of biomarkers in source rock extracts and crude oils for the purpose of determining source materials, thermal maturity, extent of biodegradation and migration and for making oil/oil and oil/source rock correlation studies has been well documented. One of the major problems with this approach is the possibility of contamination either naturally, through crude oil migration, or artificially during sample collection or work-up procedures. A solution to this potential problem can be found in the use of biomarkers produced by kerogen pyrolysis. The actual isolation of the kerogen eliminates any potential contaminants and pyrolysis of the kerogen produces indigenous biomarkers. In this paper it is intended to review progress that has been made in the study of biomarkers produced by kerogen pyrolysis. The various techniques that have been used in this approach will be discussed and examples given to show, in particular, the distributions of triterpanes and steranes produced by kerogen pyrolysis. In addition, results obtained from a combination of hydrous pyrolysis to artificially mature kerogens, and anhydrous pyrolysis to characterize the artificially matured kerogens, will be discussed. Certain biomarkers such as the C 28 -bisnorhopane and gammacerane are not produced during anhydrous pyrolysis and possible reasons for these observations will be discussed. Finally, the production of sulphur-containing compounds during pyrolysis reactions will be briefly discussed with reference to their role as possible indicators of maturity and depositional environments.

Characterization by pyrolysis-gas chromatography-mass spectrometry of the insoluble organic residues derived from the hydrogenation of Tasmanites sp. oil shale

Abstract The insoluble organic residues from the hydrogenation of Tasmanites sp. oil shale have been characterized by Curie point pyrolysis combined with gas chromatography and computerized gas chromatography-mass spectrometry. The resulting pyrograms show that very little, if any, hydrogenation of the residues occurs. Rather, the initial step in the sequence is pyrolysis of the residue followed by hydrogenation of the liquid products. Major compounds identified from the pyrograms of the residues include alkene/alkane doublets, mixtures of diterpenoids and mono- di- and triaromatic compounds. Changes in the distribution of these compounds with changes in reaction conditions are discussed.