Normal Alkane Distributions Research Articles

Relationship between petrographic and geochemical characterisation of selected South African coals

Fifteen samples of South African coals from the Waterberg, Soutpansberg (Northern Province) and Main Karoo (Mpulanga and KwaZulu-Natal) provinces were investigated by petrographic and geochemical methods. The aim of this work was to establish the relationship between these two methods for evaluation of coal maturity in the case of South African coals. The petrographic analyses included maceral and microlithotype evaluation by routine quantitative methods as well as random reflectance measurements. Maceral analysis included identification of vitrinite, liptinite and inertinite groups. The same samples were used for microlithotype analysis. Vitrite, liptite, inertite, clarite, durite,vitroinertite, duroclarite, clarodurite and carbominerite were identified. To assess the facies trends, liptinite-poor and liptinite-rich durite and clarodurite were counted separately. Sporoclarite and cutinodetrite were also treated independently. Both maceral and microlithotype analyses were performed to confirm the geochemical and petrographic compatibility in interpretation of the primary organic material which was deposited in the biochemical stage of coal formation. The reflectance measurements were carried out in incident light using a Zeiss Universal microscope. The 546 nm interference filter was applied. Distributions of aliphatic and aromatic hydrocarbons present in coal extracts were analysed by gas chromatography-mass spectrometry (GC-MS) for assessment of both source/environment of deposition and coal thermal maturity. Normal alkane distribution shows odd-over-even carbon number predominance in the case of coals in the range of 0.63–0.80% vitrinite reflectance. Several groups of compounds related to conifer resin were identified indicating predominant source of organic matter of the coals, for example tetracyclic diterpanes, retene and simonellite. The coal rank was assessed by 13 thermal maturity parameters based on isomerisation reactions of sterane and triterpane diastereomers together with ratios and indices of aromatic hydrocarbons and methyldibenzothiophenes based on different thermal resistance of their α- and β-alkyl isomers. The correlation of reflectance ( R rv%) with two sterane diasteromer parameters: C 29αββ/(ααα+αββ), C 29ααα20S/(20S+20R) and aromatic hydrocarbon parameters of thermal maturity such as: MDR (Methydibenzothiophene ratio), MPI-1 (MethylPhenanthrene Index-1) revealed good agreement. Lower correlation coefficients were obtained for TMR-2 (Trimethylnaphthalene ratio), TeNR (Tetramethylnaphthalene ratio) and DMPR (Dimethylphenenthrene ratio) with respect to R rv%. Other correlations of biomarker and aromatic hydrocarbon parameters with R rv% were poor.

CENTRAL SAUDI ARABIAN CRUDE OILS: A GEOCHEMICAL INVESTIGATION

ABSTRACT A suite of Paleozoic oils from Central Arabia was studied in order to evaluate conventional hopane and sterane biomarkers, as well as non-conventional markers such as phenanthrene and its methylated isomers. Because it was uncertain whether conventional biomarkers would be found in these light oils, additional analyses were performed on the oils for correlation purposes. These include light-hydrocarbon and mid-range micro-scale correlation techniques. Paleozoic oils and condensates from several fields from Central Arabia as well as Paleozoic condensate and two Jurassic oils from the Eastern Province (for comparison) were studied. Bulk properties, such as API gravity, percent sulfur, and nitrogen content were determined. The oils and condensates were then fractionated into their major compound classes: saturates, aromatics, polars, and asphaltenes. The full range gas chromatographic analysis of the saturate and aromatic fractions did not permit meaningful distinctions for bimodal distribution of normal alkanes or identification of aromatic compounds. Micro-scale correlation technique was used to correlate mid-range hydrocarbons and the mid-range star diagrams (MRSD) are drawn. The MRSD showed that all Paleozoic fluids (Central Arabian oils) follow patterns that are different from those followed by the Eastern Province. Bulk properties, pristane/phytane ratios, methyl penanthrene ratio (MPR), and calculated reflectance (% R c) values showed that Paleozoic oils from Central Arabia differ significantly from Jurassic oils of the Eastern Province. Central Arabian Paleozoic oils and condensates are high-API gravity, exceedingly low sulfur, low nitrogen and low metals fluids. They have pristane/phytane ratios that are greater than one indicating dysoxic conditions at the time of deposition of their source rocks. Their MPR and % R c values are indicative of differences in timing of generation from the source rock as well as in maturity and, perhaps most significantly, indicative of post-generative alteration such as water washing. Eastern Province Jurassic oils are: medium-gravity, high sulfur, high nitrogen crude oils. They have pristane/phytane ratios that are less than one indicating highly reducing conditions at the time of source rock deposition. Their MPR and % R c values are indicative of generation from a source rock that was at peak oil maturity (VRe ≈ 0.80–1.0%).

Influence of addition of pure components on the structural state of multi-n-alkane mixtures

Commercial multi-alkane product whose normal alkane distribution is of the ‘normal logarithmic type’, has been modified by adding pure components (n-C20H42, n-C32H66, n-C50H102) in different proportions. The molecular alloys resulting have been studied by means of X-ray diffraction, at room temperature. It appears that addition of pure components to the initial product has no influence on the structural state of the wax when the proportion is lower than that of the component of higher concentration contained in the initial multi-alkane mixture. On the contrary, when the added proportion is higher than the concentration of the average alkane representative of the distribution, a new series of 00l reflections characteristic of the pure component structure added, is observed. Moreover, this work has shown the possible existence of a solubility of wax in pure n-alkane.

00/02676 Emissions from biomass energy use in some selected Asian countries

Thirty oils from the Pauls Valley area of the Anadarko Basin, Oklahoma, have been analyzed for n-alkane, terpane and sterane distributions in an effort to classify genetically related oils. Ten rock samples from the area were also analyzed in an attempt to relate oil types to their possible source rocks. The Pauls Valley area, located near the southeast corner of the Anadarko Basin, has producing formations ranging in age from Middle Ordovician to Late Pennsylvanian. Trapping mechanisms responsible for the accumulation of hydrocarbons vary from purely structural traps, to fractured reservoirs, to up-dip truncations of reservoirs and up-dip pinchouts of sandstones.Results show that 85% of oils in the area probably have a common source, the Woodford Shale. A group of oils reservoired in the Viola Group exhibit distinct geochemical characteristics and are thought to have a different source. The differences between the oils from these two sources include: enhanced C24 tetracyclic terpane concentrations relative to C26 tricyclic terpanes; diminished C28–30 tricyclic terpanes relative to Ts and Tm; diminished C30-steranes relative to their C29 counterparts; predominance of C35 over C34 extended hopanes and an even/odd predominance of the normal alkane distribution. Extracts from the Viola Group generally exhibit the same qualities as the associated oils and the limestone is proposed as their probable source rock. The predominance of C29-steranes for oil samples which appear to have a marine source (inferred by their n-alkane distribution, the presence of C30-steranes and high levels of tricyclic terpanes) suggests that caution be used in the interpretation of terrigenous sources based on the high abundance of C29-steranes.The Deese Group, the Springer Formation, the Sylvan Shale, and the Arbuckle Group are not likely sources for the oils analyzed from the Pauls Valley area. Differences in API gravity and color, previously used to classify oils in this region, are not source-related but are related to maturation and migration effects.

98/00916 Methods for studies of coal structure: Jasienko, S. et al. Chemical Fiz. Wegla, 1995, 90–145. (in Polish). Edited by Jasienko, S., Oficyna Wydawnicza Politechniki Wroclawskiej, Wroclaw, Poland

Fifteen samples of South African coals from the Waterberg, Soutpansberg (Northern Province) and Main Karoo (Mpulanga and KwaZulu-Natal) provinces were investigated by petrographic and geochemical methods. The aim of this work was to establish the relationship between these two methods for evaluation of coal maturity in the case of South African coals. The petrographic analyses included maceral and microlithotype evaluation by routine quantitative methods as well as random reflectance measurements. Maceral analysis included identification of vitrinite, liptinite and inertinite groups. The same samples were used for microlithotype analysis. Vitrite, liptite, inertite, clarite, durite,vitroinertite, duroclarite, clarodurite and carbominerite were identified. To assess the facies trends, liptinite-poor and liptinite-rich durite and clarodurite were counted separately. Sporoclarite and cutinodetrite were also treated independently. Both maceral and microlithotype analyses were performed to confirm the geochemical and petrographic compatibility in interpretation of the primary organic material which was deposited in the biochemical stage of coal formation. The reflectance measurements were carried out in incident light using a Zeiss Universal microscope. The 546 nm interference filter was applied. Distributions of aliphatic and aromatic hydrocarbons present in coal extracts were analysed by gas chromatography-mass spectrometry (GC-MS) for assessment of both source/environment of deposition and coal thermal maturity. Normal alkane distribution shows odd-over-even carbon number predominance in the case of coals in the range of 0.63–0.80% vitrinite reflectance. Several groups of compounds related to conifer resin were identified indicating predominant source of organic matter of the coals, for example tetracyclic diterpanes, retene and simonellite.The coal rank was assessed by 13 thermal maturity parameters based on isomerisation reactions of sterane and triterpane diastereomers together with ratios and indices of aromatic hydrocarbons and methyldibenzothiophenes based on different thermal resistance of their α- and β-alkyl isomers. The correlation of reflectance (Rrv%) with two sterane diasteromer parameters: C29αββ/(ααα+αββ), C29ααα20S/(20S+20R) and aromatic hydrocarbon parameters of thermal maturity such as: MDR (Methydibenzothiophene ratio), MPI-1 (MethylPhenanthrene Index-1) revealed good agreement. Lower correlation coefficients were obtained for TMR-2 (Trimethylnaphthalene ratio), TeNR (Tetramethylnaphthalene ratio) and DMPR (Dimethylphenenthrene ratio) with respect to Rrv%. Other correlations of biomarker and aromatic hydrocarbon parameters with Rrv% were poor.

Pristane/phytane ratios in the mineralized Kupferschiefer of the Fore-Sudetic Monocline, southwest Poland

The interaction of organic matter with fluids is important in geochemical processes in surface and diagenetic environments. In the Kupferschiefer copper deposits of southwestern Poland organic-fluid interactions are inferred to have been important in the genesis of the ore deposits. Previous organic geochemical studies of the Kupferschiefer report evidence of a late diagenetic oxidation event that altered the organic geochemistry. This study uses the saturated hydrocarbons as indicators of the biological input and the earliest diagenetic processes to affect the organic geochemistry of the Kupferschiefer. The normal alkane distributions are lithofacies controlled indicating that they reflect biological input. The ratio of the saturated isoprenoids pristane/phytane are potential indicators of the redox conditions during sedimentation and diagenesis. Very low Pr Ph ratios are associated with lithofacies that were deposited in hypersaline environments. Increases in the Pr Ph ratio within individual traverses are interpreted as indicating increasing oxidation of organic matter during deposition and early diagenesis. Trends of increasing pristane/phytane ratios are observed at the base of the Kupferschiefer and at the transition, within the Kupferschiefer, from the clay-rich to the carbonate-rich Kupferschiefer which corresponds approximately to the transition from Cu-zone to the Pb-zone. Early diagenetic, copper-depleted Cu- and CuFe-sulfides have also been reported form the top of the Weissliegendes and the Cu-zone/Pb-zone transition, and it is conceivable that both could be related to the same processes.

Sedimentology, Organic Geochemistry, and Petroleum Potential of Jurassic Coal Measures: Tarim, Junggar, and Turpan Basins, Northwest China

Lower and Middle Jurassic coal-bearing strata occur widely throughout central Asia and are well developed in northwestern China, where their thicknesses in the southern Junggar, northern Tarim, and Turpan basins exceed 2500, 2300, and 1500 m, respectively. Examination of these strata along 13 transects across basin margin outcrop belts indicates that they are entirely nonmarine meandering fluvial deposits with local development of braided fluvial and lacustrine deltaic facies. Chinese subsurface data suggest that regional Jurassic lacustrine facies are present down depositional dip, consistent with predictions from global circulation modeling of Early and Middle Jurassic monsoonal precipitation. Laboratory analyses of coals and organic-rich shales show a dominance of terrestrial, higher plant components. Visual kerogen analysis indicates that vitrinite, inertinite, and exinite are the dominant macerals, and elemental analysis characterizes most kerogens as type III. Rock-Eval analyses yield moderate hydrogen index values (50-300) and very low oxygen index values (<20). Jurassic source rock extracts are characterized by odd-over-even normal alkane distributions, high pristane/phytane and high hopane/sterane ratios, dominance of C29 sterane homologs, local abundance of diterpenoid compounds, and low abundance of tricyclic terpanes. Geochemical correlation with four petroleums from the Junggar, Tarim, and Turpan basins strongly suggests that the Jurassic coaly deposits and their lacustrine equivalents downdip are petroleum source rocks. Sterane and hopane distributions of petroleums and extracts of their putative Jurassic source rock are similar and can be easily distinguished from published distributions of these compounds in other source rock layers. Additional correlation parameters include high pristane/phytane; low abundance or lack of tricyclic terpanes, but similar distributions where present; and lack of gammacerane (with one exception) and carotanes, compounds that characterize Permian and Ordovician source rocks and their respective petroleums. Pyrolysis-gas chromatography of selected Jurassic samples s ggests that they possess potential for liquid hydrocarbon generation. Expulsion of C15+ hydrocarbons from Jurassic source rocks appears likely, despite the traditional view that bituminous coals are incapable of expelling long-chain hydrocarbons. End_Page 929------------------------------

Geochemistry of selected crude oils from Sabah and Sarawak

A total of 35 crude oils from 15 fields from offshore Sabah and Sarawak have been analysed and characterised using liquid and gas chromatography, and gas chromatography-mass spectrometry. The normal alkane distribution shows that the oils are of three different types, namely, i) normal, non-waxy crude oils; ii) waxy oils (high proportion of C20+ n-alkanes); iii) biodegraded oils (less abundance ofn-alkanes relative to isoprenoids). Biological marker distributions (i.e. steranes and triterpanes) show that the oils were derived from terrigenous source rocks containing mixtures of different types of landplant derived organic matter, including resins. Features of these distributions include the presence of relatively high concentrations of 18n (H)-oleanane, a number of resin-derived compounds, a predominance of C29 steranes compared to the relatively low concentrations of C28 and C27 steranes, and the absence of CSD steranes in all the samples. Source rock matter for these oils were probably deposited under peat swamp environments, as indicated by the high pristanelphytane (>3.0) and pristanelnC17 (>1.0) ratios. Biological marker distributions indicate that the oils were generated from source rocks having maturities of 0.5% to at least 0.8% VRe (Vitrinite Reflectance Equivalent).

Source and Thermal Maturity of Bitumen in Precambrian Nonesuch Formation Inferred from Isotopic Compositions of Individual Normal Alkanes

As part of a continuing study of organic geochemistry of the 1.05 Ga Nonesuch Formation in the North American Mid-Continents rift, isotopic compositions of individual normal alkanes were measured using compound-specific isotopic analysis. This is a relatively new technique that can be used in conjunction with traditional biomarker analysis in interpretation of thermal maturity, oil-source rock correlation, and paleoecologic reconstruction. Carbon-isotopic compositions of individual normal alkanes from C{sub 15} to C{sub 25} in five bitumens and one oil from Nonesuch strata vary from {minus}28.8 to {minus}34.5{per thousand} (versus PDB). Within a given sample, isotopic compositions of n-alkanes span a range of 2 to 4 {per thousand}%. Homologues from C{sub 15} to C{sub 19} are isotopically light (depleted) by approximately 0.5{per thousand} relative to higher homologues from C{sub 20} to C{sub 25}. Two sources of normal alkanes are interpreted from the distribution of isotopic compositions. Based on chain length, the series from C{sub 15} to C{sub 19} may have been derived from algal components. Isotopically heavy, higher homologues are suspected to have been derived from bacterial sources. The pattern of isotopic compositions of bitumens is similar to that of the oil, demonstrating an oil-source rock relationship. Superimposed upon this primary, bimodalmore » distribution of normal alkanes is the influence of thermal maturity on isotopic composition. Normal alkanes in samples of higher thermal maturity are enriched isotopically by 0.25 to 2{per thousand} relative to those of lower thermal maturity. Isotopic enrichment with increasing thermal maturity is interpreted to be the result of either addition of isotopically heavy components derived from kerogen and/or preferential loss through cracking of isotopically light components in bitumen.« less

Evidence of primary migration of condensate by molecular solution in aqueous phase in Yacheng Field, offshore South China

The composition of Yacheng condensate provides possible evidences of primary migration of petroleum by molecular solution in the aqueous phase. Aromatic hydrocarbons (HCs), especially benzene and toluene, make up a high percentage of the condensate. The abundance of each hydrocarbon (HC) in the condensate is mainly controlled by compound type and carbon number. The distribution of normal alkanes is discontinuous at C 16. Aromatic hydrocarbons are 3.8‰ isotopically heavier than the saturated hydrocarbons. The isoprenoid hydrocarbons are very abundant, especially pristane. The specific gravity of the Yacheng condensate is 0.85530 g cm −3, which is much higher than the average for condensates. These features are in good agreement with the HC solubilities in aqueous solution.

Oils and source rocks from Pauls Valley, Anadarko Basin, Oklahoma, U.S.A.

Thirty oils from the Pauls Valley area of the Anadarko Basin, Oklahoma, have been analyzed for n-alkane, terpane and sterane distributions in an effort to classify genetically related oils. Ten rock samples from the area were also analyzed in an attempt to relate oil types to their possible source rocks. The Pauls Valley area, located near the southeast corner of the Anadarko Basin, has producing formations ranging in age from Middle Ordovician to Late Pennsylvanian. Trapping mechanisms responsible for the accumulation of hydrocarbons vary from purely structural traps, to fractured reservoirs, to up-dip truncations of reservoirs and up-dip pinchouts of sandstones. Results show that 85% of oils in the area probably have a common source, the Woodford Shale. A group of oils reservoired in the Viola Group exhibit distinct geochemical characteristics and are thought to have a different source. The differences between the oils from these two sources include: enhanced C 24 tetracyclic terpane concentrations relative to C 26 tricyclic terpanes; diminished C 28–30 tricyclic terpanes relative to T s and T m ; diminished C 30-steranes relative to their C 29 counterparts; predominance of C 35 over C 34 extended hopanes and an even/odd predominance of the normal alkane distribution. Extracts from the Viola Group generally exhibit the same qualities as the associated oils and the limestone is proposed as their probable source rock. The predominance of C 29-steranes for oil samples which appear to have a marine source (inferred by their n-alkane distribution, the presence of C 30-steranes and high levels of tricyclic terpanes) suggests that caution be used in the interpretation of terrigenous sources based on the high abundance of C 29-steranes. The Deese Group, the Springer Formation, the Sylvan Shale, and the Arbuckle Group are not likely sources for the oils analyzed from the Pauls Valley area. Differences in API gravity and color, previously used to classify oils in this region, are not source-related but are related to maturation and migration effects.

Unusual normal alkane distributions in offshore New Zealand sediments

The occurrence of unique even-carbon dominated normal alkanes in sediment extracts was confirmed in sediments from the eastern continental shelf offshore of North Island, New Zealand in Hawke Bay. The range in carbon chain-lengths was from n-C 16 to least n-C 36, extending distributions well beyond the previously reported 24 carbons. Stable isotope analysis of individual alkanes suggest an independent origin for the odd and even n-alkanes. It is suggested that sedimentary even number n-alkane predominances are more widespread than previously thought but go unrecognized due to substantial overprinting by terrestrial derived odd carbon number n-alkanes.

Simple gas chromatographic determination of the distribution of normal alkanes in the kerosene fraction of petroleum

An internal standard technique has been applied to the determination of the distribution of normal alkanes in the kerosene fraction of petroleum using a capillary column. The applicability of packed columns for such a determination has also been studied and compared with the existing Universal Oil Products (UOP) method.

Organic geochemistry of the British Kimmeridge Clay: 1. Composition of shale oils produced from Kimmeridge sediments

A series of shale oils produced by Fischer assay of Upper Jurassic Kimmeridgian oil shales, claystones and cementstones from on-shore Britain has been analysed. Oil yields are related to lithology via the quantity of sedimentary organic matter present in each lithologic unit, and also to stratigraphic horizon for certain rich, laterally uniform and persistent units. Shale oils are dark and sulphurous, containing < 50% hydrocarbons. NSO compounds are relatively important in the chromatographically separable fractions, while aliphatic hydrocarbons are usually subordinate. Unimodal normal alkane distributions maximize around nC16, while bimodal distributions have a secondary maximum around nC29. Many alkane gas chromatograms are characterized by a prominent peak due to the nC23 alkane. Certain compositional features are of pyrolytic origin (aromatics, heterocompounds, alkenes), while others are determined by the organic sedimentary input (alkane distributions, prominent nC23 alkane). Broad similarity in geochemical parameters suggests a consistency in the composition of sedimented organic matter. Smaller-scale compositional variations are thought to result from minor fluctuations in the composition of the organic sedimentary input, for example changes in the amount of terrestrial organic detritus reaching the depocentre. Results suggest a dominantly algal or bacterial organic matter source continuously operative throughout much of the Upper Jurassic over the entire region, with a smaller, more variable magnitude contribution from terrestrial sources, probably deposited in a near-shore marine shelf-sea environment under anoxic conditions.

THE ORGANIC GEOCHEMISTRY OF OIL AND GAS GENERATION AND ITS APPLICATION TO BASS STRAIT AND THE NORTHWEST SHELF

A study of the chemical reactions occurring within the geological environment can throw new light on the occurrence of petroleum and natural gas deposits. The principles of chemical kinetics enable extrapolations to be made so that the depths and temperatures of typical hydrocarbon-forming reactions can be deduced when intervals of geological time are available. In general, temperature is the crucial parameter with corrections being possible for shorter or longer heating times.Living organic matter available for incorporation in sediments is of four basic types: carbohydrates, proteins, lignin and lipids. Only the last two are significant precursors of organic matter (kerogen) in consolidated sediments. Lipid-derived material is the principal source of oil, while lignin-related kerogen gives methane as its main gaseous hydrocarbon product. Generation occurs in zones where the rate of cracking of the source material into volatile hydrocarbons due to increasing temperature becomes significant on the geological time-scale. During and after migration of oil from its source to reservoirs (usually at shallower depths and lower temperatures), other processes can alter the initial light oil. In particular, biodegradation by microorganisms can result in a heavier naphthenic or aromatic crude by the loss of normal alkanes.Application of organic geochemistry to specific basins involves the measurement on uncontaminated core samples of certain parameters (organic carbon, extractable hydrocarbons, normal alkane distributions, reflectance of vitrinite, H/C ratio of kerogen and spore coloration). These data, together with activation energies for specific chemical reactions, geothermal gradients, rates of subsidence and knowledge of the geological history of the basin enable a general view of generation, migration and preservation of oil and gas to be obtained. Such an overview is given for the Gippsland Basin, where generation of both oil and gas in the lower Latrobe Group is indicated, and for the Rankin Platform of the Northwest Shelf where a deep source at greater than 4000 m is suggested for the gas and condensate.

Organic compounds from the Rhaetic-Liassic coals of Scoresby Sund, East Greenland

A large number of aromatic and hydroaromatic hydrocarbons were detected by gas chromatography and mass spectrometry from the bituminous coals. A minor part consists of saturated aliphatic hydrocarbons in which the distribution of normal alkanes in the range C24 to C32 shows a similarity to that of recent plant material with an odd/even distribution rate higher than one. A relatively high amount of pristane is present which is not known in higher plants. A heterocyclic compound, dibenzofuran, was isolated and characterized, and a methyl dibenzofuran seems to be present. Monoterpenoid compounds and minor amounts of acids were detected; myristic and palmitic acids were characterized as methyl esters. The coal is derived from the Kap Stewart Formation which bears a well-known fossil flora dominated by gymnospermous plants and without angiosperms. Some of the extracted organic compounds are present in recent gymnosperms. In recent material dibenzofuran and derivatives are only known in lichens and one angiospermous species.

High resolution mass spectrometric investigations of the organic constituents of the Murray and Holbrook chondrites

Two methods intended for the rapid preliminary analysis of organic geochemical samples are described. In illustration of the first, samples from a cross sectional core of a fragment of the Murray C2 chondrite were heated in the ion source of a high resolution mass spectrometer and the mass spectra of the vaporising organic material were recorded. The spectra obtained indicate that the organic material held in the cored sample was uniformly distributed and that it is a complex mixture of hydrocarbons ranging from alkanes to aromatic systems more highly condensed than phenanthrene or anthracene. Oxygen, nitrogen, sulfur and chlorine containing compounds are all found, the last being by far the least abundant. The second method takes advantage of the capabilities of the gas chromatograph-high resolution mass spectrometer combination. Studies of another Murray sample lead to the specific indication of a large number of compounds and give a good indication of the normal alkane distribution, in which pentadecane is found to be the most abundant compound. Mass spectra obtained from cross-sectional core samples taken from the Holbrook L6 chondrite indicate that this stone is also homogeneous and that its organic contents, present at a level of about 50 ppm, must be largely polymeric, the most abundant compounds being evolved at 300–500°. The pyrolysis products are predominantly alkylbenzenes, but various aromatic C H O, C H N, and C H S compounds are also observed along with a few more highly condensed aromatic hydrocarbons.

Normal Alkane Distributions in Marine Organisms; Possible Significance to Petroleum Origin: GEOLOGICAL NOTES

The ratio of odd to even-numbered n-alkanes (which comprise 10 to 15 percent by weight of most petroleums) is about 1, with maximum of 1.3. In most land plants (alkanes in waxy parts) it is 10 or more. The marine sponges and corals investigated consisted of odd-even numbered carbon ratios in alkanes of 1.0 to 1.4, averaging 1.15, suggesting a relation of marine organisms to origin of petroleum.