Coking properties of perhydrous low-rank vitrains. Influence of pyrolysis conditions.

Compositional transformations occurring during natural coalification generally lead to increased coking potential of coals characterised in the resulting cokes by large sizes of molecular orientation domains (MOD) determined through transmission electron microscopy with 002 dark field mode. In this study, two sets of perhydrous low-rank vitrains (WJV and UCV) have been pyrolysed using an open-system with two heating rates in an attempt to increase their coking potential. Results show that, despite the high potentialities of such vitrains for producing hydrocarbons, i.e. a suspensive medium efficient for their cokefaction, each of the pyrolysis methods does not lead to solid residues chemically equivalent to natural coking coals, since the cokes from these residues are always made of smaller MOD than those obtained for coking coals. For comparison, a similar characterisation, carried out on a conventional vitrain (Fouthiaux) pyrolysed in a confined-system which prevents the release of hydrocarbons, leads also to non-coking coals. The formation of such MOD is likely due to the peculiar chemical composition of the precursors and/or the pyrolysis conditions. FTIR data show that perhydrous vitrains are characterised by a low degree of condensation of aromatic units with a very small concentration of aromatic rings of large size. Thermal treatment originates depolymerisation reactions in the vitrinite network with the formation of low molecular weight products which are not efficient to form large anisotropic domains. The oxygen, present in relatively high amount in some vitrains (UCV and Fouthiaux) might also act as a crosslinking agent preventing the formation of large MOD. Furthermore, while open-medium pyrolysis leads to an important effluent release, as shown by the rapid decrease of H/C ratio, hydrocarbon effluents are conversely retained within the coal matrix in the case of confined-medium pyrolysis. However, the latter pyrolysis induces secondary cracking reactions leading to the formation of lighter products. Therefore, the enhancement of coking properties has not been totally reached by our experiments insofar as they did not lead to oxygen-poor artificially matured coals, similar to natural coking coals

Organisation territoriale et socio-économique au Néolithique final dans la région du Grand-Pressigny: caractérisation des provenances des matériaux céramiques

National audienceIn and around Le Grand-Pressigny (Indre-et-Loire, France), a petrographic study was implementedon 92 ceramic samples from the Final Neolithic sites of Le Petit-Paulmy and Bergeresse (Abilly).Analyses showed that the sediments used were extracted from local sources: Claise and Creusealluvium, local substrate and loessic silts. Quantitatively, the study showed that almost all the ceramicmaterials in Bergeresse and more than half of those in Le Petit-Paulmy come from the valley of theCreuse. The mineralogical compositions of three samples from Le Petit-Paulmy, including one ofunusual form, indicate sources from an exogenous region, the Massif Central (perhaps the alluviumof the Loire or the Allier). These results show the important role played by the valley of the Creusein the territorial organisation of Le Grand-Pressigny. This study therefore makes an originalcontribution to the debate on flint exploitation in this region. These ceramic data are expected to offernew interpretative models concerning the socio-economic organisation of local Neolithic communities.Dans la région du Grand-Pressigny (Indre-et-Loire, France), une étude pétrographique a été conduite sur 92 échantillons céramiques du Néolithique final provenant des sites du Petit-Paulmy et de Bergeresse (Abilly), respectivement situés dans la vallée de la Claise et de la Creuse. Les analyses ont montré que les sédiments utilisés provenaient majoritairement de sources locales, correspondant aux alluvions de la Claise, de la Creuse, au substrat local et à des limons loessiques. Quantitativement, l'étude a montré que presque tous les matériaux céramiques de Bergeresse et plus de la moitié de ceux du Petit-Paulmy proviennent de la vallée de la Creuse. Les compositions minéralogiques de trois échantillons du Petit-Paulmy, dont un concernant une forme particulière, indiquent des provenances exogènes, du Massif central (alluvions de la Loire ou de l'Allier). Ces résultats montrent l'importance du rôle joué par la vallée de la Creuse dans l'organisation territoriale de la région du Grand-Pressigny. L'étude des céramiques apporte une contribution originale dans le débat sur l'organisation socioéconomique de l'exploitation du silex du Grand-Pressigny et devrait permettre de proposer de nouveaux modèles interprétatifs

Coal petrographic genetic units in deltaic-plain deposits of the Campanian Mesa Verde Group (New Mexico, USA)

The Campanian rocks of the Mesa Verde Group in the San Juan Basin, New Mexico, USA, form a littoral wedge composed of offshore, shoreface and onshore facies. This siliciclastic wedge was interpreted as a landward-to-seaward stepping of grouped genetic units, i.e. intermediate term cycles. The continental facies consist of coastal-plain deposits (coals, shales, and sandstones) represented by three main domains, progressively distant from the palaeoshoreline: the deltaic-plain, the intermediate-plain, and the alluvial-plain domains. In the deltaic-plain, grouped aggradational–retrogradational genetic units consist of preserved coal-shale doublets alternating with thick shales. Although previous works have identified genetic units in the shoreface facies, the existence of the latter has not been proven in the coastal-plain deposits. As coal-shale doublets may represent such genetic units, an organic petrography study was performed at high resolution on an 80-cm thick, 1.5-km long coal seam to demonstrate this hypothesis. In this work, the depositional environments of the initial peatlands were rebuilt utilizing two complementary methods: maceral and microlithotype analyses. The results reveal the existence of internal sequences in the coal seam, corresponding to recurrent patterns of a palaeogeographical model proposed for a littoral domain. The authors interpret this coal sequence as the record of successive progradational and aggradational events. Furthermore, a previous study of the organic content of the shales located above the coal demonstrated that the preservation of these facies is related to higher accommodation/peat production and accommodation/sedimentary supply ratios. Coal-shale doublets are then interpreted as elemental genetic units of the deltaic-plain formation. This study demonstrates that, contrary to original concepts, littoral coal seams are not just markers of stable shoreline conditions at a high time scale, but also record landscape migration

Changements climatiques et effets anthropiques au cours du dernier millénaire attestés par l'étude pétrographique de la matières organique (Annecy, Le Petit Lac; France)

L'étude pétrographique de la matière organique (MO) des sédiments du Petit Lac d'Annecy a permis d'obtenir un signal paléoenvironmental au travers du rapport MO authochtone / MO allochtone au cours du dernier millénaire. Le changement de régime du lac, en liaison avec l'anthropisation du bassin versant après l'an Mil, permet de lire ce rapport en terme d'évolution de la production du lac. Il met ainsi en évidence des varaitions d'ordre bi-séculaire du niveau trophique, sans doute liées aux fluctuations de même ordre enregistrées dans l'activité solaire. Des apports organiques de compositions spécifiques en provenance du bassin versant mettent en évidence des périodes d'érosion intense soit sous action anthropique soit sous l'action d'une plus grande pluviosité

Provenances des matériaux céramiques du Néolithique final du « Petit-Paulmy » et de « Bergeresse » (Abilly, Indre-et-Loire). Intérêt pour l’étude de l’organisation socio-économique et territoriale de la région du Grand-Pressigny

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