R.P. Philp

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.

Long-chain alkenones in Qinghai Lake sediments

Abstract Long-chain methyl and ethyl alkenones (C 37 -C 40 ) containing two to four double bonds have been detected in sediments from Qinghai Lake, China. This is the first report of the alkenones in lake sediments, deposited under a variety of salinity conditions, where the lake has not encountered any marine incursions. The presence of the alkenones suggests the presence of Prymnesiophyceae in the lake which may have reached the lake via eolian transportation or seabirds. Sediment samples from the more saline areas of the lake contained higher concentrations of the more highly unsaturated compounds. Widespread use of these compounds as paleotemperature indicators in marine sediments introduces the possibility of extrapolating this approach to lacustrine environments if appropriate calibrations can be undertaken.

Waxes and asphaltenes in crude oils

Abstract High molecular weight (HMW) hydrocarbons (>C40) and asphaltenes are important constituents of petroleum and can cause problems related to crystallization and deposition of paraffin waxes during production and transportation as well as in the formation of tar mats. However, traditional methods to isolate asphaltene fractions, by adding 40 volumes in excess of low boiling point solvents such as pentane, hexane or heptane, can produce asphaltene fractions which are contaminated with a significant amount of microcrystalline waxes (>C40). The presence of these microcrystalline waxes in the asphaltene fractions has the potential to provide misleading and ambiguous results in modeling and treatment programs. The sub-surface phase behaviour of an asphaltene fraction will be quite different from that of a wax-contaminated asphaltene fraction. Similarly accurate modelling of wax drop-out requires information on pure wax fractions and not asphaltene-dominated fractions. Hence the goal of this paper is to describe a novel method for the preparation of wax-free asphaltene fractions. In addition, this method provides a quantitative subdivision of the wax fraction into pentane soluble and insoluble waxes which, when correlated with physical properties of crude oil such as viscosity, pour point, cloud point, etc., may help explain causes of wax deposition during production, transportation and storage of petroleum.

Effects of effluents and water pressure on oil generation during confined pyrolysis and high-pressure hydrous pyrolysis

Abstract In order to test experimentally the effects of hydrostatic pressure on oil generation from source rocks, immature Woodford shale was pyrolyzed in confined (Au cells + external pressure = effluents pressure) and hydrous (water pressure) conditions (260–365°C during 72 h) at pressures ranging between 220 and 1300 bars. Comparisons of the results obtained in both systems also allowed the effect of the presence of water to be investigated. Increasing the external hydrostatic pressure in confined pyrolysis retards the oil potential consumption in the kerogen. However, the effect of pressure on the yields of extractable organic matter is limited. In hydrous pyrolysis, increasing water pressure significantly lowers the total yields of bitumen + expelled fraction and the consumption of the oil potential in the extracted solid residue is correspondingly reduced. The quantitative composition of the pyrolyzates is significantly affected, although, the C15 + n -alkane distributions do not show significant variations. The total yields of pyrolyzates are similar in confined pyrolysis and hydrous pyrolysis performed between 300 and 700 bars. However, during equivalent time-temperature-pressure experiments, the degradation rates of kerogen and asphaltenes is lower in hydrous pyrolysis than in confined pyrolysis. The very different effects of increasing effluent pressure (confined pyrolysis) and water pressure on oil generation indicate that the nature of the pressurizing medium is an important factor. Tentative explanations are given to elucidate these differences, as well as to estimate the importance of the effects of pressure in the natural environment.

Triterpanes and ring-A degraded triterpanes as biomarkers characteristic of Tertiary oils derived from predominantly higher plant sources

Abstract A number of oils, previously proposed to be derived from terrigenous source materials, have been obtained from the Taranaki Basin, New Zealand; Niger Delta; Beaufort-Mackenzie region; Taiwan and Indonesia and their C24 tetracyclic and C30 pentacyclic terpenoid distributions examined by GC-MS and GC-MS/MS. The C24-tetracyclic terpanes are thought to be derived from their corresponding C30 hopane, oleanane, ursane and lupane counterparts as previously reported. A mixture of at least nine C30 pentacyclic terpanes are found to be present with remarkably uniform distributions in oils from these widely separated provinces. Whilst definitive structures have not been proposed for these compounds, information has been obtained as to whether these pentacyclic terpanes are all six-membered rings, or contain one or two five-membered rings. The presence of this mixture of tetracyclic and pentacyclic terpanes provides a good indicator for oils derived from terrigenous source materials and, in the case of these basins, of Tertiary age.

Geochemical investigation of oils and source rocks from Qianjiang depression of Jianghan basin, a terrigenous saline basin, China

Abstract Six oils and seven suspected source rocks from the Jianghan saline basin, in the central eastern part of China, have been examined using a variety of geochemical techniques. The results from this study show that oils from the third and fourth sections of the Qianjiang Formation (Eocene) are derived from similar source materials but have a different source from the oils of Xingouzui Formation (Eocene) in the same basin. The oils of the Xingouzui Formation (Eocene) are more mature, and probably have a greater input of terrigenous source material and in all probability have migrated greater distances than the Qianjiang oils. Oils from these sections are derived from an environment characterized as being extremely anoxic, strongly reducing and highly saline. The suspected source rocks examined in the study were from the Qianjiang Formation and were generally mature, except for the shallowest sample. The biomarker distributions in the suspected source rocks examined from the Qianjiang Formation were very different from those of the oils present in the same sections. It is proposed that oils from the Qianjiang Formation have sources in addition to, or exclusive of, the suspected source rocks examined in this study.

Flash pyrolysis of artificially matured kerogens from the Kimmeridge Clay, U.K.

Abstract Laboratory heating of an immature Kimmeridge kerogen (type II) at different temperatures under conditions reported to approximate to natural maturation, produced a series of kerogen residues which were characterised by flash pyrolysis gas chromatography (PY-GC) and PY-GC-mass spectrometry. The residues showed trends which were similar to those reported in natural burial sequences—for example an increase in the aliphatic nature of the pyrolysates with increased temperature or time. However, quantitative PY-GC proved that the proportion of kerogen represented by the pyrolysate decreased with increasing maturation. An examination of the sulphur-containing components of the pyrolysates revealed a sharp decrease in some compounds with increasing maturation temperature. This was illustrated by the rapid disappearance of 2-methylthiophene relative to toluene. At the highest maturation temperatures an increase in the proportion of benzothiophene relative to alkylated benzothiphenes was observed. The significance of these trends is discussed.

An organic geochemical investigation of crude oils from Shanganning, Jianghan, Chaidamu and Zhungeer Basins, People's Republic of China

Abstract Eighteen oils from four different oil producing basins in China have been characterized using a variety of organic geochemical techniques. The source materials for these oils were deposited under a variety of environmental conditions and this study investigates the use of geochemical parameters in differentiating the resultant oils. The parameters discussed are based on relative amounts and distributions of both individual and groups of biomarkers, including organosulphur compounds, and the carbon isotopic composition of the aliphatic and aromatic fractions. Examination of the oils showed that those from the Zhungeer basin, thought to be derived from source rocks formed in a brackish lacustrine environment, possessed high concentrations of tricyclic terpanes, relative to pentacyclic hopanes, and a predominance of 24-methyl- and 24-ethylcholestanes (C 28 and C 29 , respectively) over cholestanes (C 27 ), which were virtually absent. Oils believed to be derived from source rocks deposited in the saline depositional environment of the Jianghan basin were characterized by a large amount of gammacerane, relative to 17α, 21β-30-homohopanes (C 31 ; 22 R and 22 S ). In general, those oils considered to be derived from source rocks deposited in fresh water lacustrine environments (Shanganning and Chaidamu basins) had small amounts of tricyclic terpanes, relative to pentacyclic hopanes, the presence of an unknown C 30 pentacyclic terpane, small amounts of extended hopanes (C 31 –C 35 ), relative to 17α,21β-hopane and relatively large amounts of 24-ethylcholestanes (20 R and 20 S ), relative to total stearanes. It appears, therefore, that the distribution of biomarkers may be used to differentiate oils derived from different lacustrine environments.

Polycyclic alkanes in a biodegraded oil from the Kelamayi oilfield, northwestern China

Abstract A comparison is made of the distributions of certain classes of saturated hydrocarbon biomarkers in an unaltered oil and a severely biodegraded oil from the Kelamayi oilfield in the Zhungeer Basin of northwestern China. Both oils are thought to have the same source rock. The n-alkanes, acyclic isoprenoids, carotenoid-derived alkanes and regular C27C29 steranes which are present in the nondegraded oil were not detected in the biodegraded sample. Additionally, the biodegraded oil shows an increase in the abundance oil, and the presence of 17α(H)-25-norhopanes, 17s(H)-25-norhopanes, C-10 demethylated tricyclic terpanes and a C-10 demethylated C23 tetracyclic terpane which were not detected in the nondegraded oil. C26 and C25 regular steranes appear to show a much greater resistance to microbial attack than the C27C29 regular steranes, as indicated by an isomer distribution for these low molecular weight steranes that is expected for nondegraded samples in the biodegraded oil. Unknown tricyclic compounds showing a base fragment ion of m/z 219 and a similar carbon number abundance to the steranes in the nonbiodegraded sample are present in relatively high concentrations in the biodegraded oil. These compounds are also present in very low abundance in some nondegraded oils from the Kelamayi Basin, indicating that their much higher concentration in the microbialy altered oil is possibly due to their resistance to biodegradation. However, at present their derivation from the microbial alteration of steranes cannot be ruled out. Several series of bicyclic compounds are also present in higher concentrations in the biodegraded oil.

Biodegradation of tar-sand bitumens from the Ardmore and Anadarko Basins, Carter County, Oklahoma

Abstract A series of tar-sand bitumens collected from a single shallow well in the Ardmore Basin, Carter County, Oklahoma has been characterized by various geochemical techniques. Particular emphasis has been placed on determination of variations in biomarker distributions, both hydrocarbons and porphyrins, as a function of biodegradation. The tar-sand bitumens have been severely biodegraded and most of the n-alkanes, low molecular weight cycloalkanes, isoprenoid alkanes, C27C29 steranes, light aromatics and sulfur compounds have been removed. In addition, the hopane distributions have been altered to differing degress with the pseudohomolugoues above C30decreasing in concentration prior to the C20–21 The triaromatic steroid hydrocarbons were also altered with preferential removal of 20R epimers (C20–21 and C27–28). Diasterane and C30-sterane distributions appear to be relatively unaffected by biodegradation. The high resistance of tricyclic terpanes, The C24-tetracyclic terpane and monoaromatic steroid hydrocarbons to biodegradation indicate that the distributions of these compounds are well suited to serve as correlation parameters in heavily altered samples. It was also observed in this study that the main ring structure of the porphyrin molecule was not affected by these very high levels of biodegradation. Previously hypothesized changes in porphyrin structure, due to biodegradation, have included n-alkyl side chain cleavage and possible conversion from DPEP- to ETIO-type porphyrins. An investigation has also been made with an active seep collected from Sulphur in the Anadarko Basin, Oklahoma and the results show that the relative rate of biodegradation of various classes of biomarkers differs between these two locations, although it was proposed that the original oil responsible for these two seeps is derived from the same source rock formation.