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An Analytical Workflow for Accurate Thermal Maturity Assessment in Unfavorable Sedimentary Successions


The determination of the thermal maturity of sedimentary successions is a key parameter to calibrate Petroleum System Modelling (PSM). The most common thermal maturity indicators are the vitrinite reflectance from optical microscopy analysis (%Ro) and the maximum pyrolysis temperature of generated hydrocarbons (HCs) from Rock-Eval analysis (Tmax). However, despite being robust methodologies, they present limitations and can be inadequate under certain conditions (e.g. lack of vitrinite, mud contaminations). This contribution provides an integrated methodological and procedural workflow developed recently to address the maturity evaluation in such unfavorable conditions. It consists in combining traditional maturity parameters with more recent parameters from Raman and Infrared (IR) spectroscopy analysis of the organic matter and from the quantitative color image analysis of palinomorphs. Other known alternative parameters derive from molecular analysis of extracted HCs (e.g. biomarkers) and X-Ray diffraction (XRD) of clay minerals (illitization of mixed layer illite/smectite). All the parameters presented here are particularly useful to study the sedimentary successions poor or without vitrinite macerals (e.g. older than Devonian, marine shales), because based on the analysis of the bulk kerogen and/or other organic and inorganic components. The choice of the best methodology depends on many factors and must be evaluated on a case-by-case, in fact: a) organic IR spectroscopic and HCs extract parameters are suitable for describing lower level of thermal maturity (up to 1.3 of %Ro) and are sensitive to kerogen heterogeneity and mud contaminations, respectively; b) digital quantitative analysis of palinomorphs cover the range from immature to mature stage of HCs generation (up to 1.8 of %Ro). The measurement of the index on a specific palynomorph group or taxon is necessary; c) Raman analysis of the organic phase and XRD of clays are adequate from low up to higher levels of maturity where only few parameters can be used. As proven in literature, they are also effective in overpressure conditions. The proposed workflow allows a more reliable estimation of thermal maturity as input for PSM, extending the applicability to unfavorable geological areas. Moreover, knowledge of sample collection and preparation procedures is fundamental to design an effective and economical analytical program.