Second order reactions as a prelude to gas generation at high maturity

The neoformation of a thermally stable pyrobitumen or recombined kerogen by interaction of first formed products and residual organic matter (OM) during maturation is postulated to be responsible for a late dry gas potential in some source rocks (Dieckmann, V., Ondrak, R., Cramer, B., Horsfield, B., 2006. Deep basin gas: New insights from kinetic modelling and isotopic fractionation in deep-formed gas precursors. Marine and Petroleum Geology 23, 183-199; Erdmann, M., Horsfield, B., 2006. Enhanced late gas generation potential of petroleum source rocks via recombination reactions: Evidence from the Norwegian North Sea. Geochimica et Cosmochimica Acta 70, 3943-3956). To investigate such bimolecular (second order) recombination reactions non-isothermal open system pyrolysis (Rock-Eval; bulk kinetics) and non-isothermal closed system micro-scale sealed vessel (MSSV) pyrolysis were performed on synthetic mixtures of three immature source rock samples stemming from different sedimentary environments. Indications for second order reactions under open system pyrolysis conditions are subtle. The TOC, S and S3 yields from mixed samples largely depend linearly on yields from their original source rock samples (Rock-Eval 6). Transformation ratio curves of mixed samples lie between those of the respective parent material (bulk kinetics). In contrast, closed system pyrolysis data clearly reveal deviation from linear behaviour, thereby signalling second order reactions during artificial maturation (MSSV to 440 °C). The strongly increased recovery of phenolic compounds in the pyrolysate of blended material points to the importance of phenolic/aromatic moieties in immature OM for second order recombination reactions during OM maturation.