Determination of maximum paleotemperatures of burial (MPTB) of sedimentary rocks from pyrolysis data on the associated organic matter: basic principles and practical application

Abstract

The Balazuc 1 borehole drilled in the frame of the “Géologie Profonde de la France” programme and the work carried out by various specialists provided the samples and the independent paleotemperature assessment data both needed to check a model aiming to the straightforward determination of maximum paleotemperatures of burial (MPTB) from Rock-Eval® pyrolysis data. Basically, the model supposes that laboratory (e.g., Rock-Eval®) pyrolysis resumes the successive elimination of kerogen moieties of increasing thermal stability where it had stopped during burial diagenesis. Then, the temperature Tmin determined at the onset of the S2 Rock-Eval® pyrolysis peak — supposed to correspond to a peculiar kerogen moiety which was just undergoing cracking when maximum burial depth was reached — might be used to calculate the corresponding MPTB, taking into account the different values of the experimental and natural thermal gradients. The good agreement between calculated MPTB-values and results of fission-track and fluid-inclusion studies carried out on Balazuc 1 borehole samples supports the validity of the model. According to fundamental considerations the application of this approach is limited to samples having at least reached the onset of the metagenetic stage. Other possible limitations and the causes of uncertainty on the obtained results are discussed. A detailed operating procedure is presented.