The Integration of the Kübler Index and Vitrinite Reflectance as Thermal Calibration Parameters in the Numerical Modelling of the Atypical Petroleum System of the Pimenteiras Formation, Devonian of the Parnaíba Basin, NE—Brazil

ABSTRACT

The reflectance of vitrinite is a common thermal calibration parameter in sedimentary basins for evaluating and reconstructing thermal history. However, factors such as the scarcity of particles in Devonian and the high thermal stress reached in igneous sedimentary petroleum systems pose significant challenges to the modelling process. This study integrates organic petrography data, along with extensive X‐ray diffraction analysis (XRD), to improve the understanding of the Middle Devonian to Early Mississippian atypical petroleum system in the Parnaíba Basin, located in northeastern Brazil. The analytical program involved 54 samples for vitrinite reflectance (%Ro) and 207 samples for XRD, collected from three selected wells in the central part of the basin, covering the entire Middle Devonian to Early Mississippian sequence. Spectral decomposition on the 10Â peak ( d001 position) was conducted on the XRD data to differentiate between the authigenic smectite/illite (I/S) phase and the metamorphic detrital muscovite phase. The Kübler Index (KI), measured as full width at half maximum (FWHM ‐ ° ∆ 2θ ), was assessed in the illite and muscovite phases and converted to an equivalent %Ro for comparison with the results of calculated %Ro across three sets of 1D Petroleum System Models (PSM), which were originally calibrated using measured %Ro. The conversion ratio of smectite to illite was also calculated throughout these models. The Esquevin Index, calculated as the ratio of d 002/ d 001 reflections, was compared with KI along the profiles. The results indicate a strong correlation between KI (from the illite phase) and measured %Ro, thereby allowing KI to be used for thermal calibration. This finding suggests a progressive conversion of I/S (KI values ranging from 1.23° to 0.23° ∆2θ ) from the diagenesis zone through the anchizone to the epizone near igneous intrusions. The KI values for the muscovite phase range from 0.58° to 0.16° ∆ 2θ , supporting the detrital nature of this phase. While it does not correlate with calculated and measured %Ro, a progressive increase in metamorphism occurs along the wells, increasing both above and below igneous intrusions and suggesting muscovite recrystallisation related to the presence of magmatic intrusion. An exponential equation for the correlation between KI from illite phase and %Ro, based on the analysis of 37 pairs of KI versus measured Ro, is proposed as KI = 0.7379 Ro ^−0.604 ( r ²  = 0.788). These KI‐%Ro pairs of the authigenic illite phase were compared with literature data, showing a reasonable correlation with pairs obtained in regional metamorphic contexts, thus validating the use of KI as an additional thermal proxy in contact metamorphism in atypical petroleum systems.