C.A. Lewis

A biological marker study of coals, shales and oils from the Mahakam Delta, Kalimantan, Indonesia

Abstract The distributions of steroidal and triterpenoidal alkanes and aromatic hydrocarbons and, where appropriate, triterpenoid alkenes and ketones, in nine oils, nine coals and three shales from the Mahakam Delta have been examined by gas chromatography-mass spectrometry. In all of the fractions there are high relative abundances of components of higher-plant origin, in keeping with the previous assignment of the organic matter in Mahakam sediments as type III. Comparison of molecular maturity measurements for the sediments and the oils extends earlier ideas that the source for the Handil oils must have a present depth of at least 3000 m. No major difference was observed here in the maturity (based on the molecular measurements) measured for coal-shale pairs with similar burial depths. In the oils, variations in the relative abundances of the monoaromatic steroid hydrocarbons relative to the triaromatic steroid hydrocarbons, taken with the results of earlier studies of the oils, have been ascribed tentatively to a migration effect.

Occurrence of 2,6,10-trimethyl-7-(3-methylbutyl)-dodecane and related hydrocarbons in the green alga Enteromorpha prolifera and sediments

The isoprenoid alkane 2,6,10-trimethyl-7(3-methylbutyl)-dodecane, a related monoene and a pseudohomologous C25 diene have been identified in field specimens of the green alga Enteromorpha prolifera. The identifications were based on gas chromatographic retention data, microscale hydrogeneration, mass spectral evidence and, for the C20 alkane, comparison with the synthesised hydrocarbon. The same compounds, and related hydrocarbons with the same two carbon skeletons, appear to occur widely in freshwater and marine sediments and sedimenting particles from different parts of the globe. Some of these highly branched sedimentary compounds may be algal in origin.

A quantitative study of microbial decomposition of biopolymers in Recent sediments from the Peru Margin

The relationship between depth changes in total organic matter (TOC), biopolymers (“carbohydrates”, “proteins” and lipids) and bacterial populations and their activity, were investigated on two cruises to an upwelling site on the Peru Margin. Samples were obtained from the sediment surface to 8020 cm below sea floor (cbsf). Bacterial concentrations increased from near surface to a maximum at 12.5 cbsf, then rapidly decreased to 20 cbsf and a slower decrease to 8020 cbsf. The bacterial population at 8020 cbsf was 9.5% of the near surface value, but at 3.3 × 108 cells/cm3 it was still very significant. The presence of dividing bacterial cells indicated that a portion of the population was active and this was confirmed by radiotracer measurements. Biopolymers were overestimated in the surface 4 cm (138% of the TOC). By 5.5 cbsf, however, organic matter uncharacterized as biopolymers was present. This increased rapidly to 17% of the TOC at 10 cbsf and then more slowly to 79% at 2229 cbsf. Bacterial populations and activity were significantly correlated with depth changes in both TOC and biopolymers (p < 0.002). Calculation of rates of anaerobic carbon metabolism from the measured rates of sulphate reduction and methanogenesis demonstrated that these processes could account for all (101%) of the decrease in “carbohydrate” and “protein” in the 2.5 to 22.5 cbsf interval, where bacterial activity was intense. These data confirm the importance of anaerobic bacterial processes in these high organic matter sediments. At deeper intervals, 22.5–480 and 480–8020 cbsf, the decrease in “carbohydrate” and “protein” only accounted for 40% and 15% respectively of the anaerobic bacterial metabolism and hence the significant bacterial population present must have been utilizing a portion of the increasing uncharacterized organic matter. There was a negative relationship between the total bacterial population and the percentage of uncharacterized organic matter, suggesting that a portion of the uncharacterized organic matter may have come from dead bacterial cells. Estimates of bacterial necromass production from rates of thymidine incorporation were significantly correlated with increases in uncharacterized organic matter (p < 0.05, 2.5–197.5 cbsf) and could account for 16% of the increase within the top 25 cm. However, as the thymidine incorporation technique underestimates bacterial productivity in anoxic sediments it is likely that a much greater proportion of the increase in uncharacterized organic matter should be attributed to “bacterial necromass” production. It is suggested that bacterial necromass production may be an important mechanism for the production of recalcitrant, and hence preserved, organic matter in sediments in high productivity regions.

Identification of a C25 highly branched isoprenoid (HBI) diene in Antarctic sediments, Antarctic sea-ice diatoms and cultured diatoms

Abstract A highly branched isoprenoid (HBI) diene found in Antarctic sea-ice diatoms and surface layer (0–2 cm) sediments from Antarctica is identified as 2,10,14-trimethyl-6-methylene-7-(3′-methylpent-1-enyl)pentadecane. The identification was made by comparison of GC–MS mass spectra and retention indices on both polar and apolar GC stationary phases with those of the diene authenticated by NMR after isolation from a laboratory culture of the diatom, Haslea ostrearia .

Molecular parameters of maturation in the Toarcian shales, Paris Basin, France—IV. Laboratory thermal alteration studies☆

Abstract An immature Toarcian shale from the Paris Basin (Semecourt, reported maximum burial depth 700m) was heated in the laboratory under reduced pressure at different temperatures (201, 257, 285°C) for different times (6–91 days). We measured in the resulting extracts by gas chromatography or combined gas chromatography-mass spectrometry: 1. (a) extent of configuational isomerisation at (i) C-6 and C-10 in pristane, (ii) C-20, and C-14 and C-17 in 5α(H),14α(H),17α(H)-steranes, (iii) C-17 and C-22 in hopanes; 2. (b) extent of aromatisation in aromatic steroid hydrocarbons. Both configurational isomerisation and aromatisation occur during heating, but the latter process is accelerated relative to the former, in comparison with the maturation effect experienced by the Toarcian shales as a result of burial depth and the associated temperature rise. The results suggest that it may be possible to distinguish the thermal histories of sedimentary sequences by comparison of the relative extents of isomerisation at C-20 in steranes with the extent of aromatisation in aromatic steroid hydrocarbons.

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.

The use of high temperature gas chromatography to study the biodegradation of high molecular weight hydrocarbons

Abstract A consequence of the biodegradation of petroleum is that lower molecular weight compounds are removed preferentially to higher molecular weight (HMW) compounds greater than triacontane ( n -C 30 ). The extent to which the latter compounds are biodegraded has rarely been studied. Reasons for this include the technical difficulties associated with carrying out biodegradability tests with solid, water-insoluble substances and the limits of the analytical techniques, such as gas chromatography (GC). A quantitative high temperature GC (HTGC) method was developed to monitor the biodegradation of the aliphatic fraction of a waxy Indonesian oil by Pseudomonas fluorescens . Recoveries of over 90% were obtained for n -alkanes up to hexacontane (C 60 ) using liquid-liquid continuous extraction. After only 14 days, 80% of the aliphatic hydrocarbons had been degraded. At the end of the 136-day study, 14% of the original fraction remained. This comprised mainly C 40+ compounds. No decrease in the concentrations of compounds above C 45 was observed. However, the use of a rapid screening biodegradation method provided tentative proof that Pseudomonas fluorescens was capable of utilising n -alkanes up to C 60 once the bacteria had acclimated to HMW alkanes.

Geochemical characteristics of oils from the Chaidamu, Shanganning and Jianghan Basins, China

Abstract Thirty oil samples from the Shanganning, Jianghan and Chaidamu Basins in China have been examined by a number of geochemical techniques. The techniques included gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS) and tandem mass spectrometry using a triple stage quadrupole mass spectrometer (MS/MS), stable isotope mass spectrometry. There were several reasons for undertaking this study. Firstly was the attempted oil-source rock correlation studies within the individual basins. Secondly was the continuing quest for novel, or sets of, biomarkers that could be assigned to saline and hypersaline environments and subsequently used to characterize other similar depositional environments. Thirdly was the desire to compare and contrast results obtained from these three basins with those from a similar study being undertaken on the South Florida Basin and the Anadarko Basin in the U.S.A. Whereas the Chinese basins are lacustrine, those in the U.S.A. are marine. For the purposes of this paper, only the results from the three Chinese basins will be discussed. In addition to examining the results from the biomarker distributions as determined by GC-MS and GC-MS/MS, the results are correlated with those obtained from the δ 13 C isotropic determinations. For example, three oils in the Shanganning Basin showed anomalous isotopic data, which immediately suggested that they should be examined in greater detail than the remaining oils, all of which correlated quite closely with each other. A combination of results used in this way is far more valuable than the biomarker data alone. In summary, the results demonstrate that various families of oils in the three basins can be distinguished on the basis of geochemical data and in many cases on the basis of the carbon isotopic composition alone. The oils from the Shanganning Basin had the lightest values, around −32%, whereas the Chaidamu were the heaviest in the −26% region. The Jianghan oils had values intermediate to the other two basins. Variations in the isotopic composition appear to correlate with variations in salinity of the depositional environment as do a number of other biomarker parameters. In particular the relative concentrations of gammacerane, β-carotanes and tricyclic terpanes all increase as the relative salinity increases. Oils from within particular basins that had anomalous isotopic compositions could also be distinguished on the basis of their biomarker composition.