Kerogen Formation Research Articles

Evolution of Organic Matter in a Eutrophic Lake: Transition from Biological Material to Kerogen and Implications for Origin of Lacustrine Type 1 Oils: ABSTRACT

A detailed study of the optically determined amorphous organic matter (type 1) in the water column and the sediments from eutrophic Lake Greifen provide a model for determining the pathway by which lacustrine algae are incorporated into sediments and later transformed into type 1 kerogens. Mass balancing of organic fractions (fulvic acids, humic acids, and stable residues (proto-kerogen)) in combination with infrared spectroscopy and gas chromatographic studies of these fractions suggest that two processes in the formation of kerogens are proceeding simultaneously: (1) selective decomposition of the structurally and functionally unique fulvic acids and (2) polycondensation/polymerization of structurally and functionally related fractions in the lacustrine algae leading to the formation of complex humic acids and finally to the stable residue phase.

Origin of Messel Oil Shale kerogen

Despite many investigations concerning the insoluble organic matter in sediments (kerogen), its chemical nature and origin are only poorly understood. Here we report the results of a combined microscopy and chemical study of the Messel Oil Shale which shed light on the mechanism of kerogen formation. Scanning electron microscopy revealed the overwhelming presence of cell-wall remains of Tetraedron-like microalgae which are virtually indistinguishable from those of the widely occurring extant Tetraedron minimum (Chlorococcales). Flash-pyrolysis gas chromatography/mass spectroscopy indicated the presence of an insoluble, non-hydrolysable highly aliphatic biopolymer in both fossil and extant Tetraedron species. The bulk of the Messel Oil Shale kerogen probably consists of selectively preserved cell-wall material of Tetraedron algae, mainly made up of this newly discovered biopolymer. We therefore suggest that this polymer, and similar types of recently discovered highly aliphatic biopolymers in other algae and plant cuticles, are important precursors of n-alkanes in crude oils.

Cyanobacterial resistant biopolymers. Geochemical implications of the properties of Schizothrix sp. resistant material

Biopolymers highly resistant to drastic chemical treatments occur in the walls of some microalgae. A large level of resistant material, termed PRB, was detected in the green colonial alga Botryococcus braunii. A close relationship between the chemical structure of PRB and of immature Torbanite demonstrated that selective preservation of the resistant material played a major role in the formation of this algal kerogen. Cyanobacterial derived matter probably provided an important contribution to the formation of numerous amorphous kerogens, including stromatolites. On the other hand, five species of cyanobacteria, of six tested, were shown to contain various levels of resistant biopolymer and a quite high content was noted in Schizothrix sp. The resistant materials of Schizothrix sp., Anacystis montana and Oscillatoria rubescens show similar chemical structure but sharply differ from PRB. Accordingly the existence of a new group of resistant biopolymers appears likely in cyanobacteria. Cyanobacterial resistant biopolymers are located in the cell envelope and exhibit an amorphous nature. Such features are in agreement with observations both on Recent mats and stromatolites and may account, firstly, for the formation of cyanobacterial kerogens mainly via selective preservation of resistant envelope constituents and, secondly, for the amorphous nature of such kerogens, well documented in the case of stromatolites.

Characterization of the microbial degradation products of a submerged plant with particular reference to the production of the kerogen-like material

Laboratory microbial degradation (14.2 yr.) of a freshwater submerged plant yielded a polymeric residue, from which humic acid-like and kerogen-like colored materials were isolated. In order to evaluate the submerged plantderived organic matter as one possible contributor to the sedimentary organic matter, chemical analyses (lipids, amino acids and saccharides) of the whole-plant residue and the kerogen-like material were examined. The kerogen-like material was further characterized by alkaline permanganate oxidation and vacuum pyrolysis. The oxidation products as well as the pyrolysate hydrocarbons of the kerogen-like material exhibited a qualitative similarity to the natural kerogens from freshwater lacustrine sediments. However, the long-chain prevalence in fatty acid and alcohol composition and the high cellulose content of the kerogen-like material suggested that the kerogen-like material should have originated from surface material of the plant. The chemical composition of this material, showed an obvious difference from the kerogen of sediment from Lake Haruna, where the submerged plant was collected. The difference was interpreted to indicate a large contribution of the planktonic organic matter to the formation of natural kerogen in lacustrine sediments.

The role of minerals in the thermal alteration of organic matter--IV. Generation of n-alkanes, acyclic isoprenoids, and alkenes in laboratory experiments.

A series of pyrolysis experiments, utilizing two different immature oil-prone kerogens ("type I": Green River Formation kerogen; "Type II": Monterey Formation kerogen) mixed with common sedimentary minerals (calcite, illite, or Na-montmorillonite), was conducted to study the effects of minerals on the generation of n-alkanes, acyclic isoprenoids, and alkenes during laboratory-simulated catagenesis of kerogen. The influence of clay minerals on the aliphatic hydrocarbons is critically dependent on the water concentration during laboratory thermal maturation. Under extremely low contents of water (i.e., dry pyrolysis, where only pyrolysate water is present), C12(+) -range n-alkanes and acyclic isoprenoids are mostly destroyed by montmorillonite but undergo only minor alteration with illite. Both clay minerals significantly reduce alkene formation during dry pyrolysis. Under hydrous conditions (mineral/water = 2:1), the effects of the clay minerals are substantially reduced. In addition, the dry pyrolysis experiments show that illite and montmorillonite preferentially retain large amounts of the polar constituents of bitumen, but not n-alkanes or acyclic isoprenoids. Therefore, bitumen fractionation according to polarity differences occurs in the presence of these clay minerals. By this process, n-alkanes and acyclic isoprenoids are concentrated in the bitumen fraction that is not strongly adsorbed on the clay matrices. The extent of these concentrations effects is greatly diminished during hydrous pyrolysis. In contrast, calcite has no significant influence on the thermal evolution of the hydrocarbons. In addition, calcite is incapable of retaining bitumen. Therefore, the fractionation of n-alkanes or acyclic isoprenoids relative to the polar constituents of bitumen is insignificant in the presence of calcite.

Possible carotenoid-derived structures in fossil kerogens

The unique KMnO 4 degradation products of β-carotene, previously identified as 2,2-dimethyl succinic acid (C 6) and 2,2-dimethyl glutaric acid (C 7) have been found in the oxidation products of Green River shale (Eocene, 52 × 10 6 yr) and Tasmanian Tasmanite (Permian, 220−274 × 10 6 yr) kerogens. These two compounds were also detected in KMnO 4 degradation products of young kerogens from lacustrine and marine sediments. The results indicate that kerogens incorporated carotenoids (possibly β-carotene) at the time of kerogen formation in surface sediments. Both acids are useful markers to obtain information on biological precursors contributing to the formation of fossil kerogens.

ケロージェン生成に対する“メイラード反応”仮説と検証：仮説について

ケロージェン生成に対する“メイラード反応”仮説と検証：仮説について

Steranes and triterpanes generated from kerogen pyrolysis in the absence and presence of minerals

Steranes and triterpanes generated from pyrolysis of immature Monterey Formation kerogen in the presence and absence of calcite, illite and montmorillonite reveal results that are both consistent and divergent with published data that reflect the use of these biological markers as maturation indicators. The extent of isomerization of biomarkers generated from pyrolysis of kerogen at 300°C for 2 hours, at C-20 in 14α(H),17α(H)-steranes, at C-22 in 17α(H),21β(H)-hopanes and of 17β(H),21β(H)-hopanes correspond to early diagenetic stages in rock extracts from sedimentary basins. Isomerization increases with heating time and, after 1000 hours, attains values which correspond to the catagenetic stage in sedimentary basins, or equivalent to that of mature oil. Stepwise pyrolysis of the kerogen indicates faster isomerization rates for steranes and triterpanes in the bitumen than for those retained in the kerogen structure, confirming earlier studies. Presence of a mineral matrix can influence the isomerization of steranes and triterpanes considerably. Comparisons with results from kerogen heated alone, for a given maturation stage, show that calcite inhibits, illite catalyzes slightly and montmorillonite has a pronounced catalytic effect on these reactions. This effect results in early isomerization of steranes and hopanes corresponding to the catagenetic stage in the presence of montmorillonite, while kerogen or kerogen with calcite held at the same temperature (300°C) and time (10 hours) only yield isomerized products which correspond to a diagenetic stage. Further, illite and montmorillonite affect various isomerization reactions differently. The fastest reaction is the isomerization at C-20 in 14α(H),17α(H)-steranes followed by that at C-22 in 17α(H),21β(H)-hopanes and the slowest is the formation of 14β(H),17β(H) steranes. These results show that maturation measurements of rock or oil samples from sedimentary basins which use biological markers have to take into account the mineral matrix effects, which have been largely ignored until present.

Isoprenoid thiophenes: novel products of sediment diagenesis?

Sulphur is a significant component of the organic matter in recent and ancient sediments and in petroleums1,2, yet the precise nature of its association and incorporation is poorly understood. Various sulphur-containing compounds have been recognized in petroleums2–4, but little is known about their origins and mode of generation during sediment burial, and for only a few organo-sulphur compounds with >15 carbon atoms have the structures been determined5,6. Here we identify one of the alkyl thiophenes which occur widely in both recent and ancient deep-sea sediments7–13 as 3-methyl-2-(3,7,l l-trimethyldodecyl)-thiophene, occurring as a limited number of the possible stereoisomers. This compound is presumed to originate from the incorporation of sulphur into chlorophyll-derived phytol, or archaebacterial phytenes or their diagenetic products. Its recognition suggests a novel diagenetic pathway for acyclic isoprenoids involving the introduction of sulphur into specific lipid moieties. Similar, but intermolecular, sulphur incorporation might give rise to sulphur-linked macro molecular materials and thereby contribute significantly to the formation of kerogens.

Role of minerals in the thermal alteration of organic matter—I: Generation of gases and condensates under dry condition

Pyrolysis experiments were carried out on Monterey formation kerogen and bitumen and Green River formation kerogen (Type II and I, respectively), in the presence and absence of montmorillonite, illite and calcite at 200 and 300°C for 2–2000 hours. The pyrolysis products were identified and quantified and the results of the measurements on the gas and condensate range are reported here. A significant catalytic effect was observed for the pyrolysis of kerogen with montmorillonite, whereas small or no effects were observed with illite and calcite, respectively. Catalytic activity was evident by the production of up to five times higher C 1–C 6 hydrocarbons for kerogen with montmorillonite than for kerogen alone, and by the dominance of branched hydrocarbons in the C 4–C 6 range (up to 90% of the total amount at any single carbon number). This latter effect in the presence of montmorillonite is attributed to cracking via a carbonium-ion [carbocation] intermediate which forms on the acidic sites of the clay. No catalytic effect, however, was observed for generation of methane and C 2 hydrocarbons which form by thermal cracking. The catalysis of montmorillonite was significantly greater during pyrolysis of bitumen than for kerogen, which may point to the importance of the early formed bitumen as an intermediate in the production of low molecular weight hydrocarbons. Catalysis by minerals was also observed for the production of carbon dioxide. These results stress the importance of the mineral matrix in determining the type and amount of gases and condensates forming from the associated organic matter under thermal stress. The literature contains examples of gas distributions in the geologic column which can be accounted for by selective mineral catalysis, mainly during early stages of organic matter maturation.

Characteristics of amorphous kerogens fractionated from terrigenous sedimentary rocks

A preliminary attempt to fractionate amorphous kerogens from terrigenous bulk kerogen by a benzene-water two phase partition method under acidic condition was made. Microscopic observation revealed that amorphous kerogens and structured kerogens were fractionated effectively by this method. Characteristics of the amorphous and structured kerogens fractionated by this method were examined by some chemical analyses and compared with those of the bulk kerogen and humic acid isolated from the same rock sample (Haizume Formation, Pleistocene, Japan). The elemental and infrared (IR) analyses showed that the amorphous kerogen fraction had the highest atomic H C ratio and the lowest atomic N C ratio and was the richest in aliphatic structures and carbonyl and carboxyl functional groups. Quantities of fatty acids from the saponification products of each geopolymer were in agreement with the results of elemental and IR analyses. Distribution of the fatty acids was suggestive that more animal lipids participate in the formation of amorphous kerogens because of the abundance of relatively lower molecular weight fatty acids (such as C 16 and C 18 acids) in saponification products of amorphous kerogens. On the other hand, although the amorphous kerogen fraction tends to be rich in aliphatic structures compared with bulk kerogen of the same rock samples, van Krevelen plots of elemental compositions of kerogens from the core samples (Nishiyama Oil Field, Tertiary, Japan) reveal that the amorphous kerogen fraction is not necessarily characterized by markedly high atomic H C ratio. This was attributed to the oxic environment of deposition and the abundance of biodegraded terrestrial amorphous organic matter in the amorphous kerogen fraction used in this work.

Formation of Botryococcus-derived kerogens—Comparative study of immature torbanites and of the extent alga Botryococcus braunii

The outer walls of the extant alga Botryococcus braunii were shown to contain large amounts of a biopolymer, termed PRB (Polymère Résistant de Botryococcus), characterized by a very high resistance towards chemical attacks. Such features suggested that PRB could play a prominent role in the formation of Botryococcus-derived kerogens. In the present work we examined the structure and the origin of PRB using i.r., 13CNMR, pyrolysis-GC and feeding experiments. It appears that PRB synthesized by the living alga comprises chiefly long unbranched, or weakly branched, hydrocarbon chains (containing some cis unsaturations, OH and ester function) probably linked by ether bridges. This biopolymer derives from the dienic hydrocarbons occuring in high level in the alga, or from some long chain close precursors of the latter. A comparative study of PRB with Coorongite and immature Torbanites (i.r., 13CNMR, H/C and O/C atomic ratios, preliminary results from pyrolysis-GC) indicates that PRB is very likely the major constituent of the initial biomass from which Botryococcus-derived kerogens were built. The biosynthesis of PRB by B. braunii accounts for the efficiency of this alga to produce large amounts of kerogens with a high oil potential.

The Contribution of Humic Substances to the Formation of Kerogens in Marine and Terrestrial Environments: An Alternate Approach

Abstract Critical examination of the data on elemental compositions of humic substances and the corresponding kerogens reported by various investigators led to a modified scheme of formation of kerogens. According to the proposed route, only the humic substances rich in carboxyl groups, such as soil fulvic acids, can, in principle, contribute together with lipids to the formation of type II hydrogen-rich kerogen, whereas soil humic acids contribute to the formation of type III kerogen.

Distribution of carbazole derivatives in petroleum

Carbazole, its alkyl derivatives and higher benzologs have been selectively isolated from six different crude oils. In all samples, benzocarbazoles are major constituents of a series which may extend up to dibenzocarbazole derivatives. The distribution includes unsubstituted parent compounds and shows a maximum content of methyl and dimethyl substituted species. Sixteen individual constituents have been identified by means of comparison with reference compounds using gas chromatography, mass spectrometry and high resolution spectrofluorimetry. So far, no generally acceptable explanation has been given for the origin of these compounds, which presumably are the most stable ones in their respective series of aromaticity. Thus, carbazole derivatives may result from a complex sequence of reactions during kerogen formation, not allowing the recognition of precursing molecules. The geochemical significance of this series of nitrogen compounds needs to be clarified, requiring further studies.

Microbiological precursors of coorongite and torbanite and the role of microbial degradation in the formation of kerogen

Permian torbanites and Recent coorongite represent fossil algal accumulations, the affinity of which as well as the extent of their biodegradation is not known. Extensive previous research has been carried out on the geochemical constituents of torbanite and coorongite. However, the study of the microbiological components has been somewhat neglected in the past. In the present study an attempt is made at improving the understanding of the nature and origin of these deposits. The major components of coorongite as observed in the present study are a diatomaceous alga and a biodegraded organism of unknown affinity. Transmission electron microscopy (TEM) also revealed a microbial assemblage most probably responsible for the degradation of the primary organic matter. Torbanites from Glen Davis, N.S.W. (TGD-1) and Carnarvon Creek, Queensland (TC-1) revealed an organism of possible cyanobacterial affinity in various stages of biodegradation. TEM studies showed the ultra-structure of the organism as well as microbial components closely resembling cysts of methanogenic bacteria; the last in the biodegradation chain to survive in the highly anoxic environment. Similar ultra-structures have been observed in the Green River shale. Geochemical evidence for the involvement of bacteria in kerogen formation in general and coorongite (Douglas et al., 1969, Geochim. Cosmochim. Acta, v. 3, p. 56–577) in particular has been reported for sometime. Selected geochemical data are reviewed in the present study and linked to the microbial assemblages observed.

High temperature simulation of petroleum formation—III. Effect of organic starting material structure on hydrocarbon formation

The importance of the structure of organic matter used as a precursor in petroleum formation has been investigated by high temperature simulation reactions. The products of pyrolysis reactions of lignin, cellulose, protein and lipids have been compared on the basis of the yield and composition of the gaseous and liquid products. Each organic matter type was also investigated with a carbonate, water, and ammonia reaction. Lignin and cellulose are poor sources of liquid hydrocarbons, but cellulose in the presence of carbonate gives a high yield of gaseous hydrocarbons. Protein pyrolysis gives a high oil yield with an alkane distribution similar to petroleum. The presence of carbonate increases the yield of all liquid compound classes. The gas yield from protein is also raised by the presence of carbonate. The lipids produced the highest oil yield of the substances tested but the n-alkanes show an odd carbon length predominance unlike the distribution found in petroleum. The results indicate that fatty acids must undergo alteration during kerogen formation sufficient to remove the odd carbon chain length bias and that the alkyl groups of protein may be an important source material for alkane formation.

Catalytic effect of smectite clays in hydrocarbon generation revealed by pyrolysis-gas chromatography

Temperature-programmed pyrolysis-gas chromatography of mixtures of expanding three-layer clays and Eocene Green River Formation kerogen gives insight into the differences in composition of pyrolyzates of specific sediments and of isolated kerogens. Argillaceous sediments containing less than about 2τ total organic carbon yield pyrolyzates containing primarily low molecular weight hydrocarbons in the gas condensate range whereas kerogens isolate from such sediments yield a broad range of volatile products including higher molecular weight hydrocarbons. Pyrolyzates of organic-rich sediments are much more similar in composition to pyrolyzates of isolate kerogens. The ration of expanding three-layer clays to organic matter in sediments apparently controls the degree of catalytic cracking so that argillaceous sediments relatively low in organic matter yield volatiles rich in low molecular weight hydrocarbons due to more carbon—carbon bond cleavage. These observations are related to gas and gas condensates of the northern Gulf Mexico basin and in Indonesia and to oil in the North Sea area.

Depositional environments of the Senonian chert, phosphorite and oil shale sequence in Israel as deduced from their organic matter composition

ABSTRACTIn the Upper Cretaceous sequence of the Negev (southern Israel) the organic matter in phosphorites and cherts differs from that associated with oil shales in its higher content of humic substances and lower kerogen content, and in its more intensive microbial alteration. The n‐alkane distribution pattern of the oil shales, phosphorites and cherts indicates that marine biota, probably algae, are the main organic precursors of their organic matter. In some of the oil shales, however, some contribution of terrestrial organic matter is also evident. Similar high phytane/pristane ratios indicate that the organic matter in the oil shales as well as in the phosphorites and cherts accumulated under reducing conditions. The main differences in the organic matter composition are attributed to early diagenetic processes rather than to different biotic precursors or to late modifications due to temperature‐induced maturation. The depositional model suggested for the sequence involves upwelling conditions at the boundary between the deep Tethys and the shallow shelf, which induced high organic productivity deep into the inner shelf. Bottom water circulation enabled intensive microbial alteration of the organic matter, followed by a winnowing process leading to phosphorite formation. Since humification is considered an oxygen‐consuming reaction, these processes favoured the formation of oxygen‐enriched humic substances and the oxidation of humic substances already present. These humic substances are relatively resistant to further alteration and their conversion into kerogen is thus retarded. Subsequently, syndepositional tectonic activity resulted in the introduction of less saline water, restriction of bottom‐water circulation and the establishment of a density stratification in the water body. Consequently, aeration of the bottom layer and the sediments was inhibited, microbial alteration was reduced and later winnowing processes were prevented. Such conditions favoured the formation of kerogen directly, rather than through humic substances, and also favoured the preservation of most of the organic matter in the form of oil shale deposits instead of phosphorites.

Geochemistry of Artificially Heated Humic and Sapropelic Sediments--I: Protokerogen

Experiments designed to simulate burial maturation have provided information on the geochemistry of classes of organic constituents in Holocene sediments of different origin. Humic acids and kerogens isolated from laboratory-heated Laguna Mormona, Baja California, algal mat (sapropelic facies) and Staten Island, California, peaty deltaic soil (humic facies) were characterized by a variety of techniques including elemental analysis, electron spin resonance, programmed temperature pyrolysis, vitrinite reflectance, and stable isotope ratios. The study shows that the generation of kerogen during burial is the net result of temperature/time dependent synthetic and destructive reactions involving protokerogen and associated solvent-extractable organic matter. For the Staten Island sediment, where the initial content of humic substances is high, conversion of humic acid to gas-prone kerogen is quantitatively the most significant maturation process. For the Laguna Mormona algal mat, where lipid content is high, evidence suggests that some formation of oil-prone kerogen may occur by cross-linkage and binding of free lipids during diagenesis. Kerogen formation from humic acid in this sediment is minor.

Melanoidins—kerogen precursors and geochemical lipid sinks: a study using pyrolysis gas chromatography (PGC)

Melanoidins, acidic polymeric products of amino acid/sugar condensation reactions, are shown to be capable of reacting with and binding functionalized lipid molecules. Pyrolysis gas chromatography is used to monitor the inclusion of lipids (alcohols and acids) into, and to monitor the changes in composition caused by artificial diagenesis of, the melanoidins. Artificial diagenesis of the lipidcontaining melanoidins produces a material resembling kerogen. The possible role of melanoidins in the formation of kerogens is discussed as are internal transformations within kerogens during diagenesis.