Raymond Michels

Alternative origin of aliphatic polymer in kerogen

The origin of sedimentary organic matter (kerogen) has been attributed to random recombination reactions of biological components in sediments or to selective preservation of decay-resistant macromolecules. Neither hypothesis explains the aliphatic composition of the cuticle of fossil arthropods. Thermal maturation experiments on modern arthropods, involving confined pyrolysis at 250–360 °C, degrade the chitin-protein complex of the cuticle and transform free aliphatic components into a polymeric structure. The results of the application of electron microscopy and spectroscopic methods to modern, thermally matured, and fossil arthropod cuticles indicate that in situ polymerization of free and ester-bound cuticular lipids can lead to kerogen formation. Thus, fossil arthropod fragments can contribute to sedimentary organic matter.

Effects of effluents and water pressure on oil generation during confined pyrolysis and high-pressure hydrous pyrolysis

Abstract In order to test experimentally the effects of hydrostatic pressure on oil generation from source rocks, immature Woodford shale was pyrolyzed in confined (Au cells + external pressure = effluents pressure) and hydrous (water pressure) conditions (260–365°C during 72 h) at pressures ranging between 220 and 1300 bars. Comparisons of the results obtained in both systems also allowed the effect of the presence of water to be investigated. Increasing the external hydrostatic pressure in confined pyrolysis retards the oil potential consumption in the kerogen. However, the effect of pressure on the yields of extractable organic matter is limited. In hydrous pyrolysis, increasing water pressure significantly lowers the total yields of bitumen + expelled fraction and the consumption of the oil potential in the extracted solid residue is correspondingly reduced. The quantitative composition of the pyrolyzates is significantly affected, although, the C15 + n -alkane distributions do not show significant variations. The total yields of pyrolyzates are similar in confined pyrolysis and hydrous pyrolysis performed between 300 and 700 bars. However, during equivalent time-temperature-pressure experiments, the degradation rates of kerogen and asphaltenes is lower in hydrous pyrolysis than in confined pyrolysis. The very different effects of increasing effluent pressure (confined pyrolysis) and water pressure on oil generation indicate that the nature of the pressurizing medium is an important factor. Tentative explanations are given to elucidate these differences, as well as to estimate the importance of the effects of pressure in the natural environment.

The organic preservation of fossil arthropods: an experimental study

Modern arthropod cuticles consist of chitin fibres in a protein matrix, but those of fossil arthropods with an organic exoskeleton, particularly older than Tertiary, contain a dominant aliphatic component. This apparent contradiction was examined by subjecting modern cockroach, scorpion and shrimp cuticle to artificial maturation (350 °C/700 bars/24 h) following various chemical treatments, and analysing the products with pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). Analysis of artificially matured untreated cuticle yielded moieties related to phenols and alkylated substituents, pyridines, pyrroles and possibly indenes (derived from chitin). n-Alkyl amides, C16 and C18 fatty acids and alkane/alk-1-ene homologues ranging from C9 to C19 were also generated, the last indicating the presence of an n-alkyl component, similar in composition to that encountered in fossil arthropods. Similar pyrolysates were obtained from matured pure C16 and C18 fatty acids. Py–GC/MS of cuticles matured after lipid extraction and hydrolysis did not yield any aliphatic polymer. This provides direct experimental evidence that lipids incorporated from the cuticle were the source of aliphatic polymer. This process of in situ polymerization appears to account for most of the fossil record of terrestrial arthropods as well as marine arthropods that lacked a biomineralized exoskeleton.

Are time and temperature the only constraints to the simulation of organic matter maturation

Abstract Confined pyrolysis, hydrous pyrolysis and high pressure hydrous pyrolysis were performed with various samples (Toarcian shale and kerogen, Woodford shale and kerogen, Manakam coal) and under different experimental conditions, in order to test the effects of pyrolysis variables on the simulation of maturation of organic matter. Results show that: (i) experimental time and temperature are not exchangeable parameters as suggested by first order kinetic law models; (ii) increasing confining pressure (300–1300 bars) in gold cell experiments performed on Woodford samples has a limited influence on the reduction rate of the oil potential, the TSOM (Total Soluble Organic Matter) yield and the structure of the polars; (iii) the presence of excess liquid water (0–100 weight %) in confined pyrolysis experiments does not significantly influence organic matter maturation. However, comparisons with classic hydrous pyrolysis (200 weight % water) could indicate major differences in timing and composition of the TSOM (bitumen + expelled fraction) as well as of the solid residue. It appears that excess water is not necessary to simulate organic matter maturation when the system is sufficiently confined. Hydrous pyrolysis conditions (low partial pressure of products) delay Woodford kerogen conversion. In addition, results concerning the Mahakam coal suggest that the structure of the organic matter plays an important role concerning the water effect; (iv) increasing water pressure in hydrous conditions (220–1300 bars) drastically lowers and delays Woodford kerogen maturation. Quantitative analysis of the water produced during confined pyrolysis and comparison of the extent of aromatization in hydrous and confined pyrolysis suggests that two competing hydrogen transfer mechanisms occur during organic matter pyrolysis. This study shows that additional parameters must be defined when comparing the results of the experimental simulation of organic matter maturation.

Pyrolysis of organic matter in cold-seal pressure autoclaves. Experimental approach and applications

Abstract Comparison of natural series of type III coals and type II kerogens with experimental results have shown that the cold-seal autoclave system offers the possibility to simulate the maturation of organic matter. The experimental vessel as well as the temperature regulation and monitoring systems are described in some detail. Limitations of the technique for mass balance calculations and simulations on whole rocks are discussed and indicate necessary improvements of this system for organic geochemistry purposes.

Artificial coalification: Comparison of confined pyrolysis and hydrous pyrolysis

Abstract Two artificial coalification series were obtained by pyrolysing an immature coal in hydrous and confined systems. The solid residues as well as extractable liquid hydrocarbons were analysed. Both pyrolysis systems yield similar results that are generally consistent with data from natural coalification series. However, the amounts of total soluble organic matter are greater in artificial maturation than in natural maturation. This is attributed to the ability of natural coals to retain significant quantities of hydrocarbons. Emphasis is also placed on the amount of expelled hydrocarbons obtained by hydrous pyrolysis, which exceeds that observed in confined pyrolysis and suspected in the natural system. Detailed inspection of the analytical results indicates slight differences between hydrous and confined pyrolysis that are tentatively attributed to pressure effects.

Palaeoenvironmental assessment of Westphalian fluvio-lacustrine deposits of Lorraine (France) using a combination of organic geochemistry and sedimentology

Abstract The quality and the quantity of organic carbon preserved in modern and ancient terrestrial sediments is the result of a complex interactions of sedimentological factors, such as in-situ productivity, allochtonous supply of organic matter, oxidation processes, biodegradation and sediment accumulation rates. The study aims at utilizing multidisciplinary information in order to characterize the palaeoenvironment in Westphalian fluvio-lacustrine deposits of the Lorraine Coal Basin (NE France). In this basin, the lack of outcrops is the major difficulty. To compensate for this problem, sedimentology, and molecular geochemistry are combined in order to significantly improve the palaeoenvironmental assessment of the area. Geochemical analyses and sedimentology have been carried out on several core samples of the Saulcy drill hole to evaluate local palaeoenvironmental variability. The lithofacies associations in the Upper Carboniferous of the Lorraine basin suggest an alluvial plain environment without marine incursions. Sedimentological interpretations of the core, as well as comparison with sedimentological models, suggests the recognition of different depositional environments from braided and/or meandering rivers to swamp and lakes. The organic geochemistry is focussed on the recognition of aliphatic hydrocarbons and especially on molecular biomarkers, which provide valuable information about the original organic matter and the transformations that took place during sedimentation and diagenesis. In the Lorraine basin, the botanical variability is high and clearly related to the position of the vegetation communities in the fluvial system. The biomarkers (e.g., diterpanes, steranes) show variations, which can be related to the diversity of this ecosystem. In this study, a reinterpretation of the environmental zonation in the sterane ternary diagram adapted to a fluvio-lacustrine sedimentology is proposed. The correlation between sedimentological and geochemical interpretation permits characterization of various depositional environments. A palaeoenvironment model, relating sediment deposition conditions and organic matter characteristics is proposed.

Comparative behaviour of coal and maceral concentrates during artificial coalification

Abstract A series of experiments were performed in cold-seal autoclaves (confined-pyrolysis) to characterize the behaviour of individual macerais (vitrinite, inertinite, exinite) extracted from the same Lorraine Basin coal during artificial coalification. Rock-Eval pyrolysis, elemental analysis and weight loss determination provided the simple maturation indexes required. Emphasis was laid on the timing of hydrocarbon generation and on the possible similarities existing between inertinite and vitrinite of higher rank. Hydrocarbon potential and hydrocarbon yield were found to increase from inertinite to vitrinite and exinite, but similar mechanisms were involved during the thermal degradation of both inertinite and vitrinite.

Changes in the cross-link density of Paris Basin Toarcian kerogen during maturation

The volumetric swelling of three Paris Basin Toarcian kerogens by a set of 20 organic solvents was measured. There is no evidence of specific interactions (e.g. hydrogen bonds) between the kerogens and the liquids. Their interaction thermodynamics is described reasonably well by Regular Solution Theory. The cross-link density of the three kerogens is too high to permit accurate calculation of the number average molecular weight between cross links. Qualitatively, the swelling data demonstrate that the cross-link density increases with increasing maturation. Surprisingly, the solubility parameter (δ) of the kerogen is constant for the three samples of different maturity. Kerogen extractabilites have been measured and parallel the magnitude of solvent swelling, as expected.

Organic geochemistry in a sequence stratigraphic framework. The siliciclastic shelf environment of Cretaceous series, SE France

For a better understanding of siliciclastic shelf environments, correlation between sequence stratigraphy and organic geochemistry is used. Our study is focused on the Cretaceous deposits of Marcoule (Gard, France), particularly on a close-packed siltites layer (200-400 m thick), which is well characterized as a marine flooding facies of a single trangressive-regressive cycle. During the Uppermost Albian and the Lower Cenomanian, the stratigraphic data indicate a change in the depositional environment from offshore to shoreface. Organic geochemistry is used in order to characterize origin and variability of the organic matter in relation to the stratigraphic data. The study is carried out on core samples from 2 drill holes (MAR 203 and MAR 501). Analyses of the aliphatic and aromatic hydrocarbons were performed using GC-MS and focused on biomarker distributions. The biomarkers indicate a contribution of mixed terrestrial and marine organic matter. The changes in molecular signatures are related to variations in the source of organic matter (marine versus terrestrial), preservation conditions (largely influenced by clay and early diagenesis), environmental oxidation-reduction and acidic conditions as well as bioturbation. Various environmental zones, characterized by different molecular signatures, can be distinguished. Resin derived biomarkers can be assigned to higher plant material input and may reflect the evolution and diversity of Gymnospermae versus Angiospermae during the transgressive/regressive cycle. The relative sea-level variations are clearly correlated with the nature and the preservation of the organic matter. For example, the Pr/Ph and Pr/n-C 17 ratios as well as the regular steranes distributions underline the maximum flooding surface evidenced by other studies. We observe a good correlation between the organic data and sequence stratigraphy: changes in geochemical signatures reflect the 3rd order depositional cycles.