Raymond Michels

The Lorraine coal basin is an intra-mountain Carboniferous basin located in north-eastern France and is the SW extension of the Saar-Nahe basin of Germany. Coal exploitation from the 19th century to 2004 was complemented from the 1990’s until recently by gas exploration (CBM and conventional) throughout the basin. Most recent exploration of la Française de l’Energie focused on the former coal exploitation sector in the east and revealed the presence of economic interesting CBM reserves. Scientific investigation by the Regalor project aimed to study the nature, origin, occurrence and abundance of gas. The detection of gas dissolved in the formation waters revealed at depth >800m the presence of a mixture dominated by methane and hydrogen (up to 18mol% at 1250m). In order to improve the interpretation of the observed gas, experimentation was conducted in the laboratory on coal using artificial maturation. Since gas generation from coal is a kinetic driven reaction and samples in the basin have experienced a minimum thermal maturity of vitrinite reflectance 0.70%, experiments were conducted at temperature between 300 to 470°C from 24h to 1 week in order to in order to cover the first stages of gas generation up to the gas window stage. The experiments designs and procedures used have proved their ability to provide geochemical data in accordance with kerogen evolution as observed in sedimentary basins. For each experiment, gas samples were collected and analyzed for their composition and C as well as H isotopes on individual constituents. From 300 to 350°C, gas is dominated by CO2 (87 to 64 mol%). At this stage methane generation corresponds to early stage. Gas generation with methane as dominant constituent (gas stage) occurs at T>350°C and represents 91mol% of hydrocarbons at 470°C. The carbon (‰PDB) and hydrogen (‰ SMOW) isotopic compositions of methane respectively evolve from -33.3 ‰/-211‰ at 300°C to a minimum of -37.7‰/-221‰ at 350°C and increased steadily up to -28.7‰/-143‰ at 450°C. The methane dissolved in the formation waters of the Folschviller 1A (FOLS1A) well presents isotopic characteristics between -43.8 and 39.9 ‰ PDB for -238 and -208 ‰SMOW. These values are in the range of the lightest observed in our experiments and obtained at 350°C (vitrinite reflectance equivalence of 1.4%). This suggests that the methane that accompanies hydrogen in the FOLS1A well originates from coal seams currently located at about 3000m depth. Molecular hydrogen was also generated from coal in our experiments. Preliminary data indicate that within the time/temperature range investigated up to 20mol% of hydrogen was generated in the hydrocarbon gas. Acknowledgments: This work was carried out by GeoRessources as part of the REGALOR project supported by the Grand-Est Region and the FEDER.

Co-authors: Salim ALLOUTI, Catherine LORGEOUX, Aurélien RANDI, Vitaliy PRIVALOV, Antoine FORCINAL, Fady NASSIF, Jacques PIRONON, Philippe de DONATO.