Origin and Evolution of Hydrocarbons in Early Toarcian Shales, Paris Basin, France

Abstract

The purpose of the study was to investigate the conditions of formation and evolution of hydrocarbons during burial and related diagenesis of sediments. Early Toarcian shales (Early Jurassic), in the Paris basin, were selected because all parameters except temperature and pressure (both related to burial of sediments) remain constant--age, nature of fossil organisms and clay minerals, and conditions of deposition (which were fairly homogeneous in the formation across the surveyed area). The amounts of the different organic constituents and some structural properties of the molecules reveal an orderly variation, depending on maximum depth of burial. At the beginning of burial, the transformation ratio of organic matter to hydrocarbons is low and changes little to a depth of 1,500 m, where the ratio increases markedly with increased depth. A detailed study shows that hydrocarbons present at shallow depth are directly inherited from original living matter or result from early transformation in sediment, without changing the general structure of the molecule (like molecules of steroid and triterpenoid types). When burial becomes sufficiently deep, these characteristic structures are diluted among newly formed hydrocarbons generated by thermal degradation of organic matter. Interpretation of the observations leads to the conclusion that burial (i.e., increase of pressure and especially of temperature) constitutes the determining factor in the evolution of organic matter. The temperature rise promotes the formation of petroleum compounds, particularly hydrocarbons, at the expense of kerogen. A general reaction scheme is proposed, based on hypothesis on the structure of kerogen, and on the observed relations of the various organic compounds.