Sterane Distributions Research Articles

Bitumen Accumulation in Grosmont Platform Complex, Upper Devonian, Alberta, Canada

The Upper Devonian Grosmont Formation, a broad carbonate platform complex in Alberta, Canada, contains an estimated 300 billion bbl of bitumen. It has been suggested that these vast reserves are related to Lower Cretaceous Athabasca oil sands. Detailed gas chromatographic-mass spectrometric studies of a wide range of biologic marker compounds confirm this suggestion. The Grosmont Formation contains bitumen of similar maturity and source to the Athabasca deposit, but it has been subjected to a greater degree of biodegradation and water washing, possibly as a result of its reservoir rock characteristics. The difference in the degree of biodegradation is manifested by the absence of bicyclic terpanes and by the reduced concentrations of the C30 and the 22R epimers of the extended hopanes in the Grosmont bitumen. Also, the greater degree of water washing of the Grosmont bitumen is inferred from the observed distribution of the bicyclic, tricyclic, and tetracyclic terpenoid sulfides, which shows a characteristic loss of the lower molecular weight members in the carbonate bitumen. The correlation established here between the two deposits suggests that if the precursor oil has indeed undergone long-distance migration, the Paleozoic carbonates could have acted as a path for migration. Finally, the observed distribution of steranes in the Grosmont bitumen corresponds to the suggestion that the Mannville Group shales were not the major source rocks of the oil-sand and carbonate bitumen accumulations of northern Alberta.

Unusual biological marker geochemistry of oils and possible source rocks, offshore Beaufort-Mackenzie Delta, Canada

Tertiary oils and source rocks from the offshore Beaufort-Mackenzie Delta have been examined for their biological marker content. The triterpane and sterane distributions suggest a predominant land plant contribution to the oils. The oils range in maturity from extremely immature to moderately mature 18(α)H-Oleanne is a major triterpane of the oils together with probable bisnorlupanes, and an excellent correlation is observed in the concentration of these components in individual oils. An excellent correlation is also observed between the concentration of bisnorlupanes and oil maturity, based on sterna and triterpane maturation parameters, suggesting that the decrease in relative concentration of these components with increasing maturity is a result of progressive dilution by hydrocarbons generated by the cracking of kerogen. Only one formation in cores taken from the delta has been found to contain bisnorlupanes, and were present they are the dominant hydrocarbons of the branched/cyclic fraction.

The occurence of unusual C 27 and C 29 sterane predominances in two types of Oman crude oil

Two types of Oman crude oils reveal unusual sterane distributions. Type “A”, which is the more common (74 examples), is characterised by a predominance of C 29 iso- and normal-steranes and generally none or only very low relative concentrations of rearranged-steranes. The triterpanes are characterised by the predominance of the C 29 17αH, 21βH norhopane over the C 30 17αH, 21βH hopane and non-predominant C 20–C 30 tricyclic terpanes. The C 29 steranes of this type of crude were not derived from the C 29 sterols of land-plant origin (frequently proposed as the source of C 29 steranes) since there is good geological evidence that these crudes were generated from a pre-Cambrian source rock, a geological period when land-plants did not exist. The type “B” crude oil (11 known examples) is characterised by a strong predominance of C 27 iso-, normal- and rearranged-steranes, relatively lower concentrations of 17αH, 21βH hopanes and relatively high concentrations of C 20–C 30 tricyclic terpanes. The remarkably different biomarker characteristics of these crude oils imply that the organisms active in the depositional environment of the respective source rocks were significantly different.

Characterisation of the non-asphaltene products of mild chemical degradation of asphaltenes

The major steranes of the non-asphaltene fraction of Nigerian tar sand bitumen (maltene) are the c 27-c 29 diasteranes [13β(H),17α(H); 20R + S] and C 28-C 29 regular steranes [14β (H),17β (H); 20S]. The reducing metal reaction products of the corresponding asphaltenes (maltene-I) contain mainly C 27-C 29 regular steranes with the 14β(H),17β(H); 20R + S and 14α(H),17α(H); 20R + S configurations as well as the corresponding diasteranes having the 13β(H),17α(H); 20R + S configuration. These sterane distributions suggest that maltene-I corresponds to an unaltered oil whilst the maltene is equivalent to the product of severe biodegradation of maltene-I. This is consistent with maltene-I being the remnant of “original oil” trapped within the asphaltene matrix and protected from the effect of in-reservior biodégradation. Degradation of Nigerian asphaltenes by refluxing with ferric chloride-acetic anhydride or methanolic potassium hydroxide also releases soluble reaction products having the characteristics of unaltered oil such as the presence of n-alkanes having an unbiased distribution. These methods appear to be milder and more suitable than reducing metal reactions for releasing hydrocarbons occluded by asphaltenes.

Relationship Between Petroleum Composition and Depositional Environment of Petroleum Source Rocks

Crude oils of nonmarine source can be distinguished from those of marine shale source and from oils originating in marine carbonate sequences by using a battery of geochemical parameters, as demonstrated with a sample suite of nearly 40 oils. A novel parameter based on the presence of C30 steranes in the oil was found to be a definitive indication of a contribution to the source from marine-derived organic matter. A second novel parameter based on monoaromatized steroid distributions was effective in helping to distinguish nonmarine from marine crudes and can be used to gauge relative amounts of higher plant input to oils within a given basin. Sterane distributions were similarly useful for detecting higher plant input but were less effective than monoaromatize steroid distributions for making marine versus nonmarine distinctions. Concentrations of high molecular-weight paraffin can also be effective nonmarine indicators but are influenced by maturation and biodegradation processes. Certain algal-derived nonmarine oils may show little high molecular-weight paraffin response. Oils from carbonate sources (with a few exceptions) can be distinguished by having low pristane-phytane ratios, low carbon preference indexes, and high sulfur contents. Gammacerane indexes and carbon isotope ratios of the whole crude are not effective in distinguishing these types of environmental differences on a global basis.

Characterization of hydrocarbons in a subsurface oil-rich layer in the Sargasso Sea

The hydrocarbon composition of seawater samples collected within a subsurface oil-rich layer in the southwest North Atlantic Ocean (Sargasso Sea) during February–March, 1978, has been examined in detail. Chromatograms generated through gas chromatographs/mass spectrometry (GC/MS) analyses of saturated and aromatic hydrocarbons are characterized by a narrow unresolved envelope (UCM or unresolved complex mixture) containing isoprenoids as major resolved peaks, suggesting a petroleum source which has undergone moderate biodegradative weathering. Distributions of terpanes, steranes and polycyclic aromatic hydrocarbons corroborate the inferred fossil origin. Stable carbon isotopic compositions ( δ 13 C) suggest that the source of these hydrocarbons is enriched in 13C in comparison with most petroleums. The results support the previous suggestions (Harvey et al., 1979) that this layer is part of a massive submarine oil seep originating somewhere on the Venezuelan shelf, although its origin cannot be attributed to seepage of a specific oil formation.

C 30-steranes, novel markers for marine petroleums and sedimentary rocks

A series of C 30-steranes has been identified in petroleum through the application of metastable scanning gas chromatography-mass spectrometry. Their Chromatographic pattern in crude oil resembles an epimerized distribution of steranes. It is proposed that the biological precursors for the C 30-steranes are marine sterols which contain 30 carbon atoms, hence C 30-steranes are markers for input to sedimentary lipids from marine organisms.

Reworked triterpenoid and steroid hydrocarbons in a recent sediment

The distributions of steranes, pentacyclic triterpanes and aromatic steroid hydrocarbons in core sections of a recent intertidal sediment (Bridgwater Bay, Severn Estuary, U.K.) have been examined by gas chromatography-mass spectrometry. Depending on sample depth, the distributions are explicable in terms of 1. (i) a reworked origin for these compounds from a geological source, probably as a result of erosion of nearby organic-rich sediments of Lower Liassic age, or 2. (ii) a dual origin from both (i) and a more mature source, probably an anthropogenic input of petroleum origin. These results show that not all of these steroid and triterpenoid hydrocarbons in surface sediments necessarily result from fossil fuel pollution. They indicate also that attention should be paid to the possibility of contributions from reworked components when using the distributions of these compounds in geochemical studies of sediments that are low in extractable organic matter.

Hydrocarbon Source Potential of Organic Facies of Lacustrine Elko Formation (Eocene-Oligocene), Northeastern Nevada: ABSTRACT

The Elko Formation, an Eocene-Oligocene lacustrine deposit cropping out in northeastern Nevada, is composed of two distinctly different organic-rich facies: a lignitic, gas-prone siltstone and an oil shale. A third organic-lean facies is represented by mudstones containing small amounts of fine-grained reworked kerogen. The deposit is thermally immature with respect to oil generation. Geochemical indicators of the two major organic facies are distribution of steranes and diterpanes, presence or absence of a specific triterpenoid biomarker (gammacerane), kerogen form, and Rock-Eval pyrolysis parameters, S2/S3 and (S1 + S2)/TOC. The siltstones are characterized by: (1) vitrinitic kerogen, (2) pristane/phytane ratios slightly greater than 1.0, (3) relatively less negative ^dgr13C values for kerogen and C15+ hydrocarbons, (4) a predominance of C29 steranes and small amounts of rearranged steranes, (5) primarily C19 and C20 tricyclic diterpanes, and (6) a predominance of 17s(H)-22, 29, 30-trisnorhopane with respect to 17^agr(H) hopanes and 17s(H) mor tanes. The oil shales are characterized by: (1) algal kerogen, (2) pristane/phytane ratios less than 0.5, (3) relatively more negative ^dgr13C values for kerogen and C15+ hydrocarbons, (4) a mixture of C27, C28, and C29 steranes and 4-methyl steranes, (5) a mixture of C19 to C26 diterpanes, and (6) a predominance of hopane and moretane and the presence of gammacerane. Hydrous pyrolysis of solvent-extracted oil shale produced a waxy oil-like bitumen whose more mature biomarkers and stable carbon isotopic composition resembled the unreacted oil shale. End_of_Article - Last_Page 945------------

Effects of the mineral matrix on the distribution of geochemical markers in thermally affected sedimentary sequences

The distribution of geochemical markers in bitumen fractions, both free and associated with minerals, was studied in bituminous chalk and black shale samples (Upper Cretaceous, Israel; Upper Cretaceous, West Greenland) affected by combustion metamorphism and the latter also by dyke intrusion. The bitumen fractions were obtained by stepwise extraction in conjunction with stepwise dissolution of the carbonate and subsequently silicate minerals. They show differences in their bulk composition and in the distribution of n-alkanes, alkylcyclohexanes, isoalkanes, isoprenoids, steranes and triterpanes. At low maturity, in the country rocks, the distribution of the biomarkers in the various bitumen fractions reflects mainly the source or organic matter and minerals (detrival vs autochthonous) associated during transport and sedimentation. With increasing maturation, processes take place like: (i) the formation of hydrogen-bonded associations of clay minerals with organic matter, (ii) the isomerisation of steranes which reaches different degrees; free bitumen>carbonate associated>silicate associated in the bituminous chalk, while bound > free in the black shales, (iii) the formation of isoalkanes and alkylcyclohexanes which occur only in the mineral-associated fractions. The particular effects of mineral groups on the geochemical markers during sedimentation, generation and primary migration necessitates the consideration of the composition of the mineral matrix for the interpretation of the geochemical markers. The generation of hydrocarbons during maturation probably involves the mobilization of bitumen from minerals and kerogen.

Effect of Water Washing on C15+ Hydrocarbon Fraction of Crude Oils from Northwest Palawan, Philippines

Recognition and subsequent correlation of biodegraded and/or water-washed oils with their unaltered counterparts has been a long-recognized problem in petroleum geochemistry. Because water washing and biodegradation can occur concurrently, it is difficult to delineate the individual effects of these two oil-altering processes, especially on the C15+ hydrocarbon fraction. Both processes can cause an oil to become enriched in sulfur and to have a lower API gravity owing to loss of the more volatile components. Thus, the altered oil may appear to be a high-sulfur crude of low maturity. Altered and unaltered oil fractions extracted from cores of a water-washed carbonate reservoir offshore northwest Palawan were studied to observe the effect of alteration on correla ion parameters. Water washing resulted in the following changes: (1) depletion of 13C in the C15+ branched-cyclic alkane fraction, whereas virtually no change occurred in isotope composition of the aromatic fraction; (2) selective loss of dibenzothiophenes followed by loss of aromatic hydrocarbons; and (3) virtually no change in steranes or tricyclic and pentacyclic terpanes, with the exception of enhancement of 17^agr(H) norhopane and decrease of 18^agr(H) trisnorhopane. In situations of severe water washing, mild biodegradation also occurred, as indicated by the loss of n-alkanes (and possible selective loss of n-alkanes having less than 25 carbon atoms) relative to branched alkanes, cycloalkanes, and aromatic components. Because cycloalkanes and aromatic hydrocarbons are not generally preferentially utilized by bacteria as long as n-alkanes are present, the changes observed in these two fractions most likely result from water washing. Because of the potential for extreme compositional changes in the n-alkane and aromatic fractions, the most useful correlation parameters between the altered and unaltered oils are sterane and terpane distributions. Stable carbon isotope ratios are also useful; however, more studies of oils in which water washing is known to have occurred need to be made to explain the general response (or various responses) of the isotopic composition of oils to water washing.

Biological markers in bitumens and pyrolyzates of Upper Cretaceous bituminous chalks from the Ghareb Formation (Israel)

The sterane and triterpane distributions of three bituminous chalks from the Upper Cretaceous Ghareb Formation (Israel) were investigated both in the original extractable bitumens and in extracts obtained after pyrolysis of whole rock and isolated kerogen samples at 450°C. Pyrolysis was performed in a closed system under hydrous (whole rock) and anhydrous conditions (isolated kerogens). The carbon number distributions of steranes and triterpanes differ significantly between original bitumen and pyrolyzates. Unlike the bitumens in which diasteranes were not detected, the anhydrous pyrolyzates contain small amounts of diasteranes. The presence of water during pyrolysis leads to an increase of sterane isomerization, the abundant formation of diasteranes and an increase of the 18α( H)- trisnorneohopane 17α( H)- trisnorhopane ratio. Sterane isomerization maturation parameters show a closer match between original bitumen and pyrolyzates after pyrolysis in a closed system when compared with an open system.

Sterane distribution of solid bitumen pyrolyzates. Changes with biodegradation of crude oil in the Ouachita Mountains, Oklahoma

Solid bitumens (grahamite and impsonite) of southeastern Oklahoma have been shown to originate from near-surface alteration of crude oil ( Curiale, 1981; Curiale and Harrison, 1981). Pyrolysis of these solids has been employed to compare the sterane distribution of geographically proximate oils to that of the bitumens. The ratio of rearranged to regular steranes is higher in the pyrolyzates than in the oils, a finding consistent with a bitumen origin due to biodegradation of oil. The remaining presence of steranes, particularly regular steranes, in the bitumens suggests that sterane occlusion may have occurred prior to or during the alteration process, thus removing tetracyclic compounds from the influence of microbial attack. These data suggest that pyrolysis- GC MS offers a viable approach to correlation problems involving solid bitumens.

Use of Porphyrins as a Maturity Parameter for Oils and Sediments

Summary There is considerable interest at present in the use of biological markers, both as maturity parameters for oils and sediments, and oil-oil and oil-source rock correlations. Earlier work on biomarkers concentrated on the distribution of steranes and triterpanes; but with new analytical techniques becoming available other biomarker classes are under investigation, such as aromatics, sulphur compounds and porphyrins. Porphyrins in oils and sediments are probably derived from chlorophylls in algae or photosynthetic bacteria. Until recently, little was known about the structure of geological porphyrins, but now several compounds have been isolated, purified and characterized. Two main types of porphyrins are present, DPEP and ETIO, and it has been shown that with increasing thermal maturity the relative concentration of ETIO-porphyrins increases with respect to DPEP-porphyrins. Thus a measure of ratio (1) in sediments and oils should reveal the thermal maturity of these samples. A suite of oils and related source rocks have been examined, and their porphyrin distributions recorded. Porphyrins show large changes in ratio (1), ranging from 0.8, in the least mature oils, to zero in the most mature oils. Changes in molecular distribution were similar in the sediment extracts, and porphyrins seem to be useful maturation parameters in the range %Ro 0.3 to 0.8; after which they are thermally destroyed.

Determination of petroleum sterane distributions by mass spectrometry with selective metastable ion monitoring

Determination of petroleum sterane distributions by mass spectrometry with selective metastable ion monitoring

Asphalts, Oils, and Bituminous Rocks from the Dead Sea Area--A Geochemical Correlation Study

Oils and asphalts (gilsonite) from the Dead Sea area, Israel, were characterized by bulk composition, specific C15+ hydrocarbons (n-alkanes, isoprenoids, steranes, triterpanes), and aromatics including sulfur compounds and light hydrocarbons (C2 to C8 molecular range). These indicators, complemented by literature data (sulfur content, sulfur isotope composition and distribution of petroporphyrins) define the oils and asphalts as belonging to a single geochemical province. The geochemical characteristics suggest a source material of bacterially reworked algae and a source rock poor in clay minerals, probably a calcareous rock. Calcareous bituminous rocks of the Campanian-Maestrichtian Ghareb Formation are exposed on the margin of the Dead Sea graben, on downwarped blocks, and they probably extend under the thick graben fill. The agreement of the geochemical indicators suggests that these bituminous rocks are a potential source of the asphalts and oils in the Dead Sea area. Differences between the various types of hydrocarbon occurrences are due to a combination of several effects such as variations in source material, the thermal history of the different downwarped blocks, water washing, and biodegradation. The distribution characteristics of hydrocarbons (C6 to C35 and C2 to C8) is sensitive to biodegradation and/or water washing. The aromatic fractions (except very soluble compounds like benzene, toluene, etc) are less affected by these processes and record the variability due to source and possibly thermal history. The interpretation of the triterpane and sterane distribution corroborates that of other hydrocarbons with respect to maturation and type of source rock. Differences in maturation are pronounced: the bituminous rocks and asphalts are immature, and the oils are derived from source rocks in an initial stage of petroleum generation.

Investigation of the sources of aliphatic hydrocarbons in the musselMytilus edulisfrom north sea oil production platforms by Capillary glc and CGCMS†

Aliphatic hydrocarbons isolated from mussels collected over a 20 month period from three North Sea Forties field oil production platforms have been examined by capillary gas chromatography and gas chromatography‐mass spectrometry. The biological hydrocarbons consist predominantly (300–700 μh g‐1 lipid) of C31 and C33 n‐alkenes with 2, 3 and 4 double bonds probably derived from a dietary intake of Emiliania huxleyi, a microscopic coccolithophorid alga. In some of the mussels C18, C20 and C22 n‐alkanes are present in unusually high abundance compared to their odd carbon number homologues. Possible reasons for this distribution are discussed. The concentrations of fossil fuel hydrocarbons (mainly unresolved complex mixtures) in the mussels ranged from 330 to 5,298 μg g‐1 of lipid. No significant increase in values was detected two months after the start of discharge of treated Forties production water from the Forties D platform. To determine the pollutant hydrocarbon sources, the sterane and terpane distributions of hydrocarbon fractions isolated from the mussels and from a number of fossil fuels, including Forties crude oil, were examined by mass fragmentography. The steranes in the mussels from the Forties C and D platforms contained higher proportions of regular 14α(H), 17α(H) components than Forties oil in which the steranes were mainly 13ß(H), 17α(H) diasteranes. In addition, the terpane distributions of the mussel fractions differed markedly from that of Forties crude oil, particularly in the relative abundance of diterpanes to triterpanes, which was higher in the mussel fractions than in the oil, and in the absence of 17α(H), 18α(H), 21ß(H)‐18, 30‐bisnorhopane, a known component of Forties crude. Furthermore, the ratio of C29 to C30 triterpanes was greater than unity in the mussels but much less than unity in the Forties oil. These data preclude Forties or other similar North Sea crudes as the major source of pollutant aliphatic hydrocarbons in the mussels. The relative concentrations and distributions of steranes and triterpanes suggest that the most likely source of pollutants is a Middle Eastern based oil derived either from rig activities, or from background pollution in the North Sea. The low concentration of ≥ C27 steranes in the more polluted mussels (e.g. Forties B; UCM > 200 ppm dry wt.) suggests that gas oils used on the platforms may be the major source of petrogenic hydrocarbons in these samples.