INFLUENCE OF MATURATION ON THE PYROLYSIS PRODUCTS FROM COALS AND KEROGENS .2. MODELING

A methodology to determine the chemistry and kinetics of the multiple reactions during geological maturation was developed, with a special e mphasis on the representation of diagenesis and oil formation processe s. The methodology combines a unique macromolecular and kinetic model for hydrocarbon pyrolysis, the FG-DVC (functional group-devolatilizati on, vaporization, cross-linking) model, with a method of analysis base d on thermogravimetric analysis with Fourier transform infrared spectr oscopy (TG-FYTIR). TG-FTIR pyrolysis data from several natural maturat ion series of coals and kerogens were measured, systematic trends with the degree of maturation were identified, and empirical processes and reaction kinetics during maturation necessary to induce these trends were estimated. This approach eliminates potential inaccuracies when e xtrapolating kinetic parameters obtained from laboratory experiments t o geological conditions. The FG-DVC pyrolysis model was modified to in clude these maturation processes, with aqueous chemistry providing a g uide for such modifications. The resulting FG-DVC maturation model was then used to predict the maturation of several immature samples throu gh the well-known time/temperature history of the basin. The FG-DVC py rolysis model was subsequently used to predict the open-system pyrolys is decomposition of the predicted maturation residues, and the predict ions were compared to TG-FTIR data of the corresponding naturally matu red samples. For most of the series investigated, the model gave good predictions of the variations in oxygenated gas precursors, tar T-max, and extractable yield with maturation. Kinetics derived from open-sys tem pyrolysis for bridge breaking were found to be applicable during m aturation. However, faster kinetics were necessary to describe the rem oval of oxygenated gas precursors. In addition, the removal of methane and tar was found to be too slow during maturation when using open-sy stem pyrolysis kinetics. Artificial maturation experiments using confi ned pyrolysis were also performed for comparison. While the evolution rates, during subsequent pyrolysis of the maturation residues, of oxyg enated gas species are different from those obtained from samples natu rally matured, the yields compare favorably with model predictions. Th e trends for pyrolysis tar and methane from artificially matured sampl es are similar to those of natural samples but suggest different kinet ics.