A combined integrated multi-method approach for the assessment of the thermal maturity of sedimentary basins

The reliable assessment of thermal maturity of sedimentary successions is crucial for the evaluation of hydrocarbon generation/expulsion. Uncertainties in thermal maturity assessment up to 0.2-0.3% of vitrinite reflectance values can occur according to commonly adopted parameters (eg., vitrinite reflectance, Tmax, etc.) especially when exploring Lower Paleozoic shale gas targets that are devoid of vitrinite macerals. This level of uncertainties is not acceptable in thermal modelling and can negatively influence decisions on the development of prospects. This contribution provides a new methodological workflow to assess thermal maturity of organic matter dispersed in sediments and hosting sedimentary succession to largely reduce such uncertainties. The multi-method approach consists in combining optical analysis and Raman and FTIR spectroscopy of organic matter dispersed in sediments with X-ray diffraction of the inorganic fine-grained fraction of sedimentary successions. The four techniques have been systematically compared in diagenesis for the first time. Results are from an about 3000 m deep Cretaceous-Silurian section that shows a regular and highly resolved trend of increasing thermal maturity with depth on the base of spectroscopic and XRD data, and a general agreement among vitrinite reflectance, FTIR quantitative and qualitative indexes, pyrolysis and Raman indexes. Levels of thermal maturity of dispersed organic matter range from immature-early mature stages of hydrocarbon generation in the Cretaceous-Jurassic section to mid-late mature in the Triassic and Silurian units. Moreover, fine grained sedimentary rocks record an increasing trend of thermal maturity from early diagenesis in the Cretaceous-Jurassic section to late diagenesis for the Triassic and Silurian sections.