Modeling of catagenetic transformation of organic matter from a Riphean mudstone in hydrous pyrolysis experiments: Biomarker data

The catagenesis of organic matter (OM) was modeled by the hydrous pyrolysis of a Riphean mudstone. Microscopic observations of the processes operating during kerogen heating to 600°C were conducted in a diamond anvil cell. The results of pyrolysis in an aqueous environment were used to calculate the activation energies of kerogen cracking and derive chemical kinetic models for OM catagenesis. Isothermal experiments were carried out for 3 days at temperatures of 300, 310, …, 360, and 370°C. The maximum bitumen yield was obtained at 330°C followed by thermal cracking at higher temperatures. The aromatic and saturated hydrocarbons from rock bitumen, hydrous pyrolyzates, and kerogen flash pyrolyzates were analyzed by chromatography-mass spectrometry. We also discuss the problem of extrapolation of high-temperature pyrolysis results to geologic observations under the conditions of regional catagenesis.     

泥岩有水热解产生低分子量有机酸实验研究

直接采用泥岩岩屑进行有水热解,测定实验后水溶液中几种常见低分子量有机酸.结果表明,在实验的水溶液中检测到了丰富的低分子量有机酸,组成上主要为一元羧酸,其中又以乙酸占优势.讨论了不同热解条件对有机酸产率以及组成的影响.实验结果表明,有机酸总量随加热温度和时间增加而增加,并且溶液中不同有机酸相对组成也发生变化.当温度高于1...     

Hydrothermal petroleum from lacustrine sedimentary organic matter in the East African Rift.

Cape Kalamba oil seeps occur at the south end of the Ubwari Peninsula, at the intersection of faults controlling the morphology of the northern basin of the Tanganyika Rift, East Africa. Oil samples collected at the surface of the lake 3-4 km offshore from Cape Kalamba have been studied. The aliphatic hydrocarbon and biomarker compositions, with the absence of the typical suite of polynuclear aromatic hydrocarbons, indicate an origin from hydrothermal alteration of immature microbial biomass in the sediments. These data show a similarity between a tar sample from the beach and the petroleum from the oil seeps, and confirm that the source of these oils is from organic matter consisting mainly of bacterial and degraded algal biomass, altered by hydrothermal activity. The compositions also demonstrate a < 200 degrees C temperature for formation/generation of this hydrothermal petroleum, similar to the fluid temperature identified for the Pemba hydrothermal site located 150 km north of Cape Kalamba. The 14C age of 25.6 ka B.P. obtained for the tar ball suggests that Pleistocene lake sediments could be the source rock. Hydrothermal generation may have occurred slightly before 25 ka B.P., during a dry climatic environment, when the lake level was lower than today. These results also suggest that the Cape Kalamba hydrothermal activity did not occur in connection with an increased flux of meteoric water, higher water tables and lake levels as demonstrated in the Kenya Rift and for the Pemba site. Hydrothermal petroleum formation is a facile process also in continental rift systems and should be considered in exploration for energy resources in such locales.     

Limitations of ‘Rock-Eval’ pyrolysis for typing organic matter

Laboratory experimentation on whole-rock ‘Rock-Eval’ pyrolysis has shown that the characterization of organic matter through the use of a modified van Krevelen diagram, in which the hydrogen and oxygen indices are substituted for the atomic H/C and O/C ratios, produces questionable results. The hydrogen and oxygen indices have been found to be strongly affected by both matrix mineralogy and level of organic enrichment. It appears, therefore, that although the modified van Krevelen diagram maybe useful for tracing evolutionary pathways as organic matter matures, it can be very misleading when used to assess kerogen type.     

Investigation of catalytic effects of indigenous minerals in the pyrolysis of Aleksinac oil shale organic matter

The catalytic effect of indigenous minerals in the pyrolysis of Aleksinac (Yugoslavia) oil shale was studied in this paper. The substrates were prepared by gradual removal of the mineral constituents (carbonates, silicates, pyrite) and the free and bound bitumens. The substrates were analyzed by chemical methods, X-ray diffraction, porosimetry, thermal analysis, ¹³C NMR, and standard ASTM Micro Activity Test (MAT) designed for the investigation of cracking catalysts. The liquid pyrolysis products were analyzed by organic geochemical techniques as well. Based on the yields of gaseous and liquid products and the coke, conversion degrees, GC analyses (MAT parameters) and weight losses (TG parameter), the catalytic effect of indigenous mineral components in the pyrolysis of Aleksinac oil shale organic matter was found to be very low. The results suggested that principal organic matter changes should be attributed to thermal rather than to catalytic cracking.     

Influence of HCl/HF treatment on organic matter in aquifer sediments: A Rock-Eval pyrolysis study

Rock–Eval pyrolysis is increasingly used for the routine characterization of natural organic matter in soils and sediments. In this work the bulk composition of sedimentary organic matter (SOM) in sandy aquifer sediments is studied, as well as purified samples (isolation of SOM) by HCl/HF treatment. This treatment is necessary to avoid detection limit problems for samples with low SOM contents, but the results presented here indicate that this treatment influences the organic geochemistry of the aquifer sediment samples. The FID and CO₂/CO pyrograms show a shift of 10–40 °C of the major peak to a lower temperature. Organic matter alteration or removal of components containing O-bearing groups may explain this. It is also suggested that destruction of the mineral matrix may lead to the reduced retention of the material. For the change of the CO₂/CO pyrograms of the RC fraction only organic matter alteration seems to be likely. Concentrated organic matter samples may also accelerate the release of exothermic energy and influence the pyrograms. Results indicate that the organic matter concentration in the sample influences the measured total organic matter (TOM) content and the T_max of the FID pyrogram, while the sample loading (absolute organic matter amount) up to 80 mg in the Rock–Eval apparatus does not. The FID pyrograms can be deconvoluted into four subpeaks, which allows comparison of samples at various depths. Rock–Eval pyrolysis may only be routinely applied to characterize SOM in aquifer sediments when such systematic and analytical phenomena are taken into account.     

Generation characteristics of organic matter and distribution of biomarkers in bitumens of the Riphean,Vendian, and Cambrian source rocks of the Siberian Platform

The distribution of biomarkers in bitumens of source rocks of different ages in the southern Siberian Platform is studied to elucidate the correlation between oils and their sources. It is shown that the bitumens of Riphean, Vendian, and Lower-Middle Cambrian source rocks are different in some parameters reflecting the type of primary biogenic material, first of all, in the distribution of n-alkanes and acyclic isoprenoids and in the portions of steranes and triterpanes among polycyclic naphthenes. The revealed difference in the composition and distribution of biomarkers in bitumens of source rocks of different ages can be used to identify the sources of hydrocarbons in discovered pools.     

Evolution of organic matter in lignite-containing sediments revealed by analytical pyrolysis (Py–GC–MS)

Newly vegetated sites provide opportunities to enlighten organic matter (OM) transformation mechanisms in soils and sediments at very early stages of development which, in turn, is relevant to better understand general ecosystem functioning. Mine acid soils and sediments in the Lusatian open cast lignite mining district (Germany) contains a high concentration of fossil carbon (lignite) in ad mixture with recent OM from the local vegetation, both contributing to the humified OM pool. In this study, analytical pyrolysis (Py–GC–MS) was used to monitor the different C sources (lignite or plant derived) in developing mine tailing soils and sediments and their degree of degradation in contrasting environments. Representative vegetation and the organic carbon (OC) rich soil/sediment fraction (humus fraction) were sampled at two depths (0–5 and 5–10 cm) in three plots along a transect covering an upland forest soil, a partially submerged sediment at the land–water interface and a constantly submerged sediment. The analysis of plant (lipds, isoprenoids, methoxyphenols and carbohydrates) and possible lignite (alkyl napththalenes, alkyl benzenes and PAHs) biomarkers released after pyrolysis supports previous findings in the area using other proxies. It was possible to discern OM sources in soil/sediment humus fractions, both from the substrate (lignite) as well as from the prevailing vegetation of the area. Environmental conditions in the submerged sediment seem to favour OM protection and the accumulation of decomposing plant material, whereas more intense OM degradation seems to prevail in the land–water interface areas characterized by fluctuating water level. In addition, a well resolved series of organic sulfur compounds (OSCS) found in the submerged sediments of rehabilitated acid lakes, indicates the possible occurrence of particular mechanisms of C preservation in this extreme anoxic S rich environment, i.e. via sulphur “quenching” with plant derived lipids during early diagenesis.     

A Novel Type of Oil—generating Organic Matter —Crystal—enclosed Organic Matter

The comparative study of organic matter in carbonate rocks and argillaceous rocks from the same horizon indicates that the organic thermal maturities of carbonate rocks are much lower than those of argillaceous rocks .Ana extensive analysis of extracted and inclused organic matter from the same sample shows that inclused organic matter is different from extracted organic matter,and the thermal maturity of the former is usually lower than that of the latter in terms of biomarker structural parameters.It seems that carbonate mineras could preserve organic matter and retard organic maturation.The inclused organic matter,abundant in most carbonate rocks,will be released from minerals and transformed into oil and gas during the high-thermal maturity stage.     

Interstellar organic matter in meteorites

Hydrogen which is highly enriched in deuterium is present in organic matter in a variety of meteorites including non-carbonaceous chondrites. The concentrations of this hydrogen are quite large. For example Renazzo contains 140 μmoles/g of the 10,000‰ δD hydrogen. The $\text{D}\text{H}$ ratios of hydrogen in the organic matter vary from 8 × 10^?5 to 170 × 10^?5 (δD ranges from ? 500‰ to 10,000‰) as compared to 16 × 10^?5 for terrestrial hydrogen and 2 × 10^?5 for cosmic hydrogen. The majority of the unequilibrated primitive meteorites contain hydrogen whose $\text{D}\text{H}$ ratios are greater than 30 × 10^?5. If the $\text{D}\text{H}$ ratios in these compounds were due to enrichment relative to cosmic hydrogen by isotope exchange reactions, it would require that these reactions take place below 150 K. In addition the organic compounds having $\text{D}\text{H}$ ratios above 50 × 10^?5 would require temperatures of formation of < 120 K. These types of deuterium enrichments must take place by ion-molecule reactions in interstellar clouds where both ionization and low temperatures exist. Astronomically observed $\text{D}\text{H}$ ratios in organic compounds in interstellar clouds are typically 180 × 10^?5 and range between about 40 × 10^?5 and 5000 × 10^?5. The $\text{D}\text{H}$ values we have determined are the lower limits for the organic compounds derived from interstellar molecules because all processes subsequent to their formation, including terrestrial contamination, decrease their $\text{D}\text{H}$ ratios.In contrast, the $\text{D}\text{H}$ ratios of hydrogen associated with hydrated silicates are relatively uniform for the meteorites we have analyzed with an average value of 14 × 10^?5; very similar to the terrestrial value. These phyllosilicates values suggest equilibration of H₂O with H₂ in the solar nebula at temperatures of about 200 K and higher.The ${}^{13}\text{C}{}^{12}\text{C}$ ratios of organic matter, irrespective its $\text{D}\text{H}$ ratio, lie well within those observed for the earth. If organic matter originated in the interstellar medium, our data would indicate that the ${}^{13}\text{C}{}^{12}\text{C}$ ratio of interstellar carbon five billion years ago was similar to the present terrestrial value.Our findings suggest that other interstellar material, representing various inputs from various stars, in addition to the organic matter is preserved and is present in the meteorites which contain the high $\text{D}\text{H}$ ratios. We feel that some elements existing in trace quantities which possess isotopic anomalies in the meteorites may very well be such materials.     

Geochemical characterisation of the Xiagou Formation carbonate-bearing mudstone in southwestern Jiuquan Basin,China: implications for paleo-environment reconstruction and the origin of organic matter

Since the Xiagou Formation was confirmed to be the primary source rock of the crude oil discovered in the Yumen oilfield, geologists have performed many studies on the sedimentology of the formation, source rock evaluation and migration, as well as petroleum accumulation. In this study, the carbonate-bearing mudstones were investigated using synthetic organic and inorganic geochemical analyses to reconstruct the paleo-environment and study the origin of organic matter during deposition in correlation with organic matter enrichment. The analysed samples have total organic carbon (TOC) and S₁ + S₂ values in the range of 0.20–3.39 wt% and 0.07–19.50%, respectively, which indicate fair to good hydrocarbon potential. Biomarker studies on the pyrolysis data of the samples suggest that the predominant origin of the organic matter is terrigenous high plants with partial aquatic algae and micro-organisms, primarily oil- and gas-prone. The trace elements P and Ba are considered to be the main proxies for paleoproductivity, and values for the two elements hosted in samples range from 48 to 1724 ppm (a mean of 658 ppm) and from 322 to 1701 ppm (a mean of 550.34 ppm), respectively, which indicates that the sediments in the Xiagou Formation have high paleoproductivity. Additionally, organic geochemical studies reveal that the sediments were deposited in a lacustrine environment with brackish water (low C₃₁-22R-homohopane/C₃₀-hopane ratios; moderate gammacerane and β-carotane), which is consistent with the medium degree of the Sr/Ba ratio, which ranges from 0.22 to 1.42 (a mean of 0.64). Furthermore, the wide range of Cu/Zn and Rb contents are important indicators of a suboxic to relatively anoxic paleoredox condition. This outcome is consistent with the biomarker parameter Pr/Ph, which ranges from 0.05 to 1.37. Moreover, the major oxides and trace elements [high Fe/Mn, low Mg/Ca, low CIA and the plot of SiO₂ vs (Al₂O₃ + K₂O + Na₂O)] also indicate that the sedimentary paleoclimate was semiarid to arid. Lack of correlation between TOC content and paleo-environment (suboxic, brackish and semiarid) or productivity indicators indicates that the accumulation of organic matter was controlled by combined action, rather than a single factor, such as redox condition or productivity.     

Hydrothermal alteration of organic matter in sediments of the Northeastern Pacific Ocean: Part 2. Escanaba Trough,Gorda Ridge

Samples from the Ocean Drilling Program Leg 169 in Escanaba Trough, Gorda Ridge, NE Pacific Ocean, were analyzed to study maturation by accelerated diagenesis and/or by catagenesis of the sedimentary organic matter to hydrothermal petroleum. At Site 1038 the hydrothermal petroleums have migrated after generation to shallower horizons. The n-alkane maturation was indicated by the strong even C number preference (i.e., CPI <1.0) as in the case of Middle Valley. All samples contained admixed organic matter of terrigenous and marine components as indicated by the distributions of the biomarkers. The biological precursors were catagenetically altered to their equivalent mature compounds. The presence of high molecular weight PAHs in some sediment sections at Site 1038 reflected the high temperature alteration and reworking of organic matter into mature hydrothermal petroleum. At Site 1037, the reference hole in the Escanaba Trough, the n-alkanes with a strong predominance of odd C number homologs reflected immature non-marine lipid components essentially throughout the hole. Maturation of organic matter was only observed below 450 mbsf with n-alkanes showing a CPI <1.0. The strong even C number predominance in those intervals was attributed to initial maturation by high heat flow during the early rifting process.     

Monocyclic alkanes in Ordovician organic matter

The major compounds in the C₁₅₊ branched/cyclic alkane fractions of two Ordovician oils (Pine Unit and Midland Farms oils) and an immature Ordovician rock (Guttenberg Oil Rock) are monocyclics. One series of these compounds was identified as n-alkylcyclohexanes and another tentatively identified as methyl-n-alkylcyclohexanes. The carbon number distribution of these compounds resembled those of the n-alkanes found in these samples with an odd carbon number predominance. It is suggested that the monocyclic alkanes, in these Ordovician samples, may be principally derived from the cyclisation of straight-chain algal fatty acids, by mechanisms that involve decarboxylation. However, there is evidence from the m/z 97 fragmentograms of these samples, to indicate that methyl-n-alkylcyclohexanes can also be derived from fatty acids by a less preferred mechanism that does not involve decarboxylation or from other precursors.In addition, a sample of kerogen from the Guttenberg Oil Rock was hydrously pyrolysed. The saturated hydrocarbon products of this experiment showed very similar distributions (including monocyclics) to those observed in the Ordovician oils. This suggests that although the oils and the Guttenberg Oil Rock come from widely differing geographical locations, their precursor (algal derived) organic matter was very similar.     

Composition of dissolved organic matter in groundwater

Groundwater constitutes a globally important source of freshwater for drinking water and other agricultural and industrial purposes, and is a prominent source of freshwater flowing into the coastal ocean. Therefore, understanding the chemical components of groundwater is relevant to both coastal and inland communities. We used electrospray ionization coupled with Fourier-transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) to examine dissolved organic compounds in groundwater prior to and after passage through a sediment-filled column containing microorganisms. The data revealed that an unexpectedly high proportion of organic compounds contained nitrogen and sulfur, possibly due to transport of surface waters from septic systems and rain events. We matched 292 chemical features, based on measured mass:charge (m/z) values, to compounds stored in the Kyoto Encyclopedia of Genes and Genomes (KEGG). A subset of these compounds (88) had only one structural isomer in KEGG, thus supporting tentative identification. Most identified elemental formulas were linked with metabolic pathways that produce polyketides or with secondary metabolites produced by plants. The presence of polyketides in groundwater is notable because of their anti-bacterial and anti-cancer properties. However, their relative abundance must be quantified with appropriate analyses to assess any implications for public health.     

Raman spectroscopy of irradiated organic matter

Raman spectroscopy of a range of irradiated and nonirradiated natural terrestrial bitumens has revealed that radiolytic alteration is generally associated with an increase in structural disorganisation. An interpretational methodology designed to overcome the considerable difficulties in obtaining reproducible, meaningful parameters of structural disorganisation is also presented, and should prove useful for future Raman applications. Raman investigation of a set of bitumens reported to have formed by the radiolytic polymerisation of light hydrocarbons, such as methane, has revealed excessive structural disorganisation, relative to biogenic complex-hydrocarbon-derived bitumens of similar radioelement concentrations, which may indicate the importance of precursor materials on the organic products of irradiation. Variations in the R1 ratio (D1/G band intensity) are found to be the best guide to variations in structural organisation. Comparisons of Raman spectra of the same sample, but produced by different exciting wavelengths, reveal the importance of the selection of a suitable laser wavelength. The results are discussed in terms of analyses of irradiated organic matter in the solar system, especially cometary nuclei and carbonaceous chondrites.     

Subaerial weathering of sedimentary organic matter

Small diameter core samples were taken from outcrops of the Permian Phosphoria Formation and the Cretaceous Pierre Shale of the Western United States to determine the effects of weathering on organic matter in shale outcrops. While the Pierre Shale core showed no evidence of weathering, the Phosphoria Formation showed significant reduction of overall organic content and pronounced changes in organic composition over the near-surface interval of the core. Total organic carbon is lower by as much as 60% over the upper 2 ft of the core. Chloroform-soluble organic matter and total hydrocarbon (C₁₅₊) concentrations are 50% lower over this same interval. The ratio of saturated to aromatic hydrocarbons decreases steadily with core depth over the upper 2.6 ft of the core. Aromatic hydrocarbons are enriched in the stable carbon-13 isotope by an average of 1.7%. over this same interval. Shallow core samples also show a loss of n-paraffins relative to branched/cyclic compounds in the saturated C¹⁵⁺ fraction.Although the extent of weathering is variable, certain characteristic effects are recognizable and can be applied to the interpretation of outcrop data in organic geochemical studies.     

Preservation of organic matter in mound-forming coral skeletons

This study demonstrates that intracrystalline organic matter in coral skeletons is well preserved over century timescales. The extent of preservation of organic matter in coral skeletons was investigated by measuring total organic carbon (TOC), total hydrolyzable amino acid (THAA), chloropigment, and lipid concentrations in 0-300 year old annual growth bands from Montastraea annularis (Florida Keys) and Porites lutea (Red Sea). Organic matter intrinsic to the calcium carbonate mineral (intracrystalline) was analyzed separately from total skeletal organic matter. The Red Sea coral had less TOC (0.02-0.04 wt%) than the Florida Keys coral (0.04-0.11 wt%), but a higher percent of intracrystalline organic matter in all annual bands measured. Carbon in the form of THAA, most likely from mineral-precipitating proteins, contributed 30-45% of the TOC in both corals. Carbon in lipids represented about 3% of the TOC in the coral skeletons. Chlorophyll-a and b were present in annual bands where endolithic algae were present, but these compounds were minor contributors to TOC. The distribution of specific organic compounds showed that organic matter was well preserved throughout the time period sampled in both the total and intracrystalline pools. Variations in THAA were not correlated with TOC over time, suggesting that organic matter that is involved in biomineralization, like amino acids, may be deposited in response to different environmental factors than are other components of skeletal organic matter. Differences in the quantity and composition of organic matter between the two corals investigated here were assessed using principal components analysis and suggest that location, species and skeletal structure may all influence organic matter content and possibly the degree of physical protection of organic matter by the coral skeleton. Further, our study suggests that intracrystalline organic matter may be better protected from diagenesis than non-intracrystalline organic matter and may therefore be a more reliable source of organic matter for paleoceanographic studies than total skeletal organic matter.     

Effects of weathering on organic matter in shales

The most drastic changes of the organic geochemical parameters examined (organic carbon, soluble organic matter content and composition, δC¹³ insoluble organic matter) occur over the near-surface 3 m intervals of two shallow core holes drilled into outcrops of the Upper Cretaceous Mancos Shale in Utah. This observation is believed to reflect the effects of weathering under the semi-arid climate of the study area. Therefore, organic geochemical data obtained from analysis of surface samples should be interpreted with caution.     

Distribution of organic matter in a shale clast

The mechanism of petroleum migration is not fully understood. One approach has been to study the distribution of organic matter near contacts between rocks with source character and those with reservoir character. Published studies have been restricted to profiles across the bedding, but the ease of movement of hydrocarbons along and across the bedding may be very different. Gradients for volatile bitumens and kerogen both along and across shale bedding have been established by analyzing a shale clast encased in a sand matrix. In this case, the distribution of volatile bitumens is not controlled by the distribution of kerogen but shows evidence of bitumen movement through the clast. The amount and composition of the volatile bitumens and their distribution suggest that in part they have been introduced into the clast from the surrounding medium, and that the movement of hydrocarbons occurs preferentially along the shale bedding. The composition of the material moving into the clast is not the same as the bulk composition of the external supply but shows a preferential enrichment in normal alkanes, particularly those of shorter chain length.