Patrick Landais

Comparison between natural and artificial maturation series of humic coals from the Mahakam delta, Indonesia

Abstract Type III (humic) organic matter from the Mahakam delta (Indonesia) was chosen to compare artificial and natural coal series. Powdered and concentrated immature organic matter was heated in sealed gold tubes for 24 hr at temperatures ranging from 250 to 550°C and under pressures ranging from 0.5 to 4 kb, with and without water. Both elemental and Rock-Eval analyses were used to characterize the products. A comparison between our results, published data and the natural model shows that, quantitatively, natural maturation is simulated better when pyrolysis is performed under confined conditions (no free volume, no diluting inert gas). Thus, pyrolysis in a medium swept by an inert gas, vacuum pyrolysis and some pyrolysis in sealed glass tubes must be considered to be poor simulation tools. The presence of water does not seem to have an essential effect. Allowing the hydrocarbons formed to reach a certain value of partial pressure seems to be important. Results are unchanged when external pressure varies from 0.5 to 4 kb.

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.

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.

Transmission micro-infrared spectroscopy: An efficient tool for microscale characterization of coal☆

Abstract Fourier transform infrared (FT-i.r.) micro-scale analysis of organic matter can be applied to evaluate the heterogeneity and to characterize the individual petrographic components of coal. Suitable sample preparation development was required to benefit from the potential advantages of this technique. Sections of coal (10–20 μm thick) have been prepared by slightly modifying the polished thin section preparation or by using a wire saw. Ultramicrotome sections (0.5–1.5 μm thick) were also designed. The micro-FT-i.r. spectra display good signal to noise ratio and a good individualization of bands. Advantages of the microspectroscopic technique over the KBr pellet global mode are: better spectral quality; the absence of adsorbed water; and better quantitative results. The limitations of the method are also discussed with special emphasis on the effect of coal mineral content. Applications of micro-FT-i.r. to characterize coal macerals and alteration phenomena are presented.

Chemical basis of fluorescence alteration of crude oils and kerogens—II. Fluorescence and infrared micro-spectrometric analysis of vitrinite and liptinite

Abstract Samples of pure vitrinite, sporinite and cutinite have been studied to understand the photo-chemical basis of their temporal fluorescence alteration (TFA), under constant u.v. irradiation, in air. TFA is the result of photo-oxidation reactions mainly affecting the extractable fraction; two different mechanisms compete: (1) Rapid oxidation releases hydrocarbons from the kerogen macromolecular structure and consumes aliphatic chains in the mobile phase. The dominant FTIR response is a rise of CO groups. Fluorescence declines and undergoes a red shift by oxygen and concentration quenching (negative TFA). (2) Depending on the mobile phase polarity, slow formation of a “loose molecular network” occurs through oxygen bridging (alkyl- and aryl-ethers, aromatics substitution by aliphatics and carbonyl groups). Fluorescence increases by dilution of fluorophores, sensitized fluorescence and phosphorescence (positive TFA). Natural samples exhibit intermediate behaviors depending on their own maturity and chemical composition. TFA spectra give valuable information both on the extractable fraction and oxidation state of the macerals.

Evidence of free but trapped hydrocarbons in coals

Abstract Pyrolysis experiments in a confined system (in sealed gold tubes under pressure) were performed on a lignite. The temperature range was 250–550 °C, under isothermal conditions (24 h). Solid and chloroform-extractable products of heat treatments were examined by various analytical techniques (e.g. thermovaporization, infrared spectrometry, pyrochromatography, liquid chromatography). A reference coal series was submitted to parallel analytical investigations. All samples came from the Mahakam Delta, Indonesia. Comparison between natural and artificial series leads to the conclusion that natural coals are able to retain appreciable quantities of free but trapped hydrocarbons, whereas artificially derived solid residues are not.

Chemical characterization of torbanites by transmission micro-FTIR spectroscopy: Origin and extent of compositional heterogeneities

Abstract Four Permian to Carboniferous torbanites of various geographical origins were examined by transmission micro-FTIR spectroscopy on doubly polished thin sections (10–25 μm). Several types of heterogeneities (different types of organic matrix; yellow and orange Botryococcus braunii colonies) were identified and chemically characterized. Important differences were noted between the organic constituents of the matrix and the algal bodies, regarding the intensity of OH, CO, and aromatic CC absorptions. The previous IR studies of torbanites on bulk samples therefore afforded substantially biased information on the composition of B. braunii fossil colonies, on their oil potential, and on the maturity of such kerogens. Micro-FTIR spectra indicate that the organic matrix corresponds neither to an extensive breaking up of colonies nor to humic substances. This matrix is highly heterogeneous; two types were identified in the Autun sample (chiefly corresponding to degraded algal and bacterial constituents, respectively). A precise characterization of the organic matrix was made difficult, however, in Pumpherston torbanite, due to intimate mixing with minerals. The co-occurrence of yellow and orange colonies, with contrasted micro-FTIR features, in Autun torbanite neither reflects radiolysis processes nor differences in maturation and/or source algae. A specific spatial relation was observed between these two types of algal bodies and the organo-mineral matrix, thus revealing differences in colony microenvironment after deposition. The orange colonies are likely derived, in agreement with their micro-FTIR spectra and their spatial correlation with the matrix, from sedimentological and/or matrix-catalysed diagenetic transformations of some yellow colonies. This first application of micro-FTIR to kerogens confirmed the utility of this nondestructive, in situ pin-point method. Although torbanites have been extensively studied, all the analytical methods so far used only provided bulk information. Further insight into torbanite composition, origin and evolution can be obtained via micro-FTIR spectroscopy.

Closed system pyrolysis : an efficient technique for simulating natural coal maturation

Abstract Results from open pyrolysis and confined pyrolysis experiments have been compared to data from a natural series of coal maturation. Residues and effluents have been studied using different analytical techniques. Results indicate that only the confined pyrolysis in gold tubes under pressure can really duplicate the natural model. It is demonstrated that in both natural evolution and confined pyrolysis, hydrogenation reactions are enhanced while in open pyrolysis experiments, recombination and unsaturation reactions are favoured.